Universe - The Definitive Visual Guide (September 2012)

TELESCOPE ASTRONOMY 82 THE MILKY WAY 224 MONTHLY SKY GUIDE 426 SETTING UP A TELESCOPE 86 THE MILKY WAY 226 USING THE SKY GUIDES 428 ASTROPHOTOGRAPHY 88 STARS 232 JANUARY 430 ASTRONOMICAL OBSERVATORIES 90 THE LIFE CYCLES OF STARS 234 FEBRUARY 436 OBSERVING FROM SPACE 94 STAR FORMATION 238 MARCH 442 MAIN-SEQUENCE STARS 250 APRIL 448 GUIDE TO THE OLD STARS 254 MAY 454 UNIVERSE STELLAR END POINTS 266 JUNE 460 MULTIPLE STARS 274 JULY 466 THE SOLAR SYSTEM 98 VARIABLE STARS 282 AUGUST 472 THE HISTORY OF THE STAR CLUSTERS 288 SEPTEMBER 478 SOLAR SYSTEM 100 EXTRA-SOLAR PLANETS 296 OCTOBER 484 THE FAMILY OF THE SUN 102 490 THE SUN 104 BEYOND THE MIKY WAY NOVEMBER 496 MERCURY 110 TYPES OF GALAXY 300 DECEMBER VENUS 114 GALAXY EVOLUTION 302 502 EARTH 124 ACTIVE GALAXIES 306 GLOSSARY THE MOON 136 GALAXY CLUSTERS 320 INDEX 510 MARS 150 GALAXY SUPERCLUSTERS 326 ASTEROIDS 170 336 ACKNOWLEDGMENTS 526 JUPITER 178 THE NIGHT SATURN 188 SKY 344 URANUS 200 NEPTUNE 204 THE CONSTELLATIONS 346 THE KUIPER BELT AND THE THE HISTORY 348 OORT CLOUD 208 OF CONSTELLATIONS 354 COMETS 212 MAPPING THE SKY METEORS AND METEORITES 220 Guide to the Constellations

6 ABOUT THIS BOOK ABOUT THIS BOOK GUIDE TO THE UNIVERSE Universe is divided into three main sections.The This part of the book focuses on specific regions INTRODUCTION is an overview of the basic concepts of of space, starting from the Sun and then moving astronomy. GUIDE TO THE UNIVERSE looks, in turn, at outward to progressively more distant reaches the solar system, the Milky Way (our home galaxy), and of the universe. It is divided into three sections, the regions of space that lie beyond. Finally, THE NIGHT covering the solar system, the Milky Way, and features beyond the SKY is a guide to the sky for the amateur skywatcher. Milky Way. In each section, introductory pages describe features in a general way and explain the processes behind their formation.These pages are often followed by detailed profiles of actual features (such as individual stars), usually arranged in order of their distance from Earth. 150 MARS MARS 151 MARS MARS PROF LE INTRODUCTION 38 39 G av y m t on and rb s MARS IS THE OUTERMOST of the four ocky plane s Al o known AVE AGE D ST NCE F OM HE SUN R TA ION P R OD 64 65 C l s i l cy l s as the Red Planet because of ts us -red color it is named after 141 6 m l i n mi es 227 9 m l on km) 24 3 hou s This section is about the universe and astronomy as a 68 69 P a et ry m t on he Roman god of war Its varied su face features nclude deep whole. It is subdivided into three parts. what is the 1 0 01 The i t ry f he So a Sy t m canyons and the h ghest volcanoes in the solar system SUR ACE T MPE ATU E O BI AL P R OD ( ENG H F YEA ) universe? looks at different kinds of objects in the 1 2 03 The am y of he Sun Although Mars is now a dry planet a large body of evidence 195 F o 77 F ( 125 C to 5 C) 6 7 Ea th ays universe and the forces governing how they behave and ndicates that l quid wa er once flowed across its su face interact. the beginning and end of the universe covers the origin DI M TER 4 213 m es 6 80 km) MA S E RTH = 1) 0 1 and history of the universe, while the view from earth explains what G AV TY AT QUA OR ( ARTH = ) 0 8 we see when we look at the sky. VOL ME E RTH = 1) 0 5 S ZE OMPAR S N MARS NUMBER OF M ONS 2 ARTH ORBIT ax s of o at on OBS RVA ION t ts 60 f om M rs s v s b e to he na ed e e t s Ma s has an el ipt cal orbit so t ts clo est ve t c l br g te t wh n t ts c o es to E r h app o ch to the Sun perihe ion) it rec ives wat r i e w i h is pp ox ma e y on e ev ry wo ye rs 45 pe cent more solar r diat on th n at the con en r ted It hen as an a e age ma n t de of 2 0 at c ld ower d ec i n l t udes o sun ght wat r ce arthe t po nt aphel on) This means that the s uth p l r 60 s i l p e ent u face temperatu e can vary rom 195 F eg on at e ua or ( 125 C) at the w n er po e to 77 F (25 C) e pos d o ATMOSPHERE dur ng the summer At 25 2 the current ax al s nl g t is wat r i e AND WEATHER ce f ee bu ds up at c l er ilt of Mars s im lar to that of Ea th and ike no th p le Ear h Mars experi n es changes n sea ons as eq a or wat r ce Ma s has a very hin he north po e and hen the south po e points ece v s mo e con en r ted owa d the Sun du ing he cour e of its orb t s nl g t t an a ound n r h atmo phere which ex rts Throughout ts his ory Ma s s xial t lt has at 0 t t po ar eg on 45 n ave age pre sure on the urf ce of about 6 mi l bars luctu ted g eat y due o var ous actors 0 6 pe cent of he ncluding Jupi er s g avi at on l pu l These a mospher c p essu e on luctu tions have aused igni ic nt changes in Ear h) The a mosphe e is c imate When Mars is heav ly i ted the poles mo t y ca bon dioxide and it a e mo e exposed to 5 ppears pink bec u e ine he Sun caus ng w ter 25 ce o vapori e and HANGES N AX AL T LT ce f ee part cles of i on oxide du t re SAND DUN S bu ld up round the Wa er ce d st bu ion d r ng a eq at r co der ower ati udes Ma t an wi ter n t e or he n su pended in it Th n clouds of Lo k ng down in o a sma l imp ct c a er At a l sser t lt wa er ce emi phe e va i s w th he ax al becomes concen rat d rozen arbon diox de and wa er in a s ut ern u la d rea c l ed Noa his at the colder poles lt The ra s uce t w i e ar as Te ra NASAs M rs Re onn i san e O b t r hown he e re re ent h n ce hat ice are pre ent at high al i udes ca tu ed t ese r p l ng sa d unes The m l s du ing he summer whe eas nd clouds al o fo m on h gh du es w re s ul ted by Ma t an wi ds and he hi k w i e ice ema ns peaks in the ummer Mars is a a e hown he e in en anc d o or The ima e cold dry plane — he ave age is bout 0 6 mi es 1 km) ac oss surf ce t mpera ure s 81 F ( 63 C —where it never ra ns but in he w nter clouds at the po ar a is i s f om NORTHERN NORTH RN egions cause ground f osts Mars has high y dynam c weather sy tems v r i al y SPR NG W N ER 25 2 EQUINOX SO STI E n the southern spr ng and ummer warmer winds f om he south blow ERIH LION SP N AND ORB T into the nor hern hem sphere st rr ng up loc l louds of dust that can 28 mi i n Ma s s or it s m es ( 07 ig ly e cen r c reach 3 000 t (1 000 m) in he ght and la t or weeks The h gh lev l winds m l on km) ompa ed to t at of ar h wh ch means hat c n al o cr ate power ul du t torms hat co er va t areas of the p anet (see ts is an e om he Sun 28 WHAT S THE UN VERSE? M TTER 29 APHEL ON ar es mo e ur ng a M r i n below) Ma s a so h s ow evel preva l ng winds which ha e sandb as ed i s 155 m l on ear A Ma t an day s 42 mi es 249 m nu es l nger han an E r h ay urf ce for centu ies reat ng d st nct ve andforms ( ee photog aph above) mi i n km) MATTER EM TY P CE UC E S CHEM CAL ELEMENTS NIELS BOHR Sn xy en ar on m no ide ATMOSPHER C Mo t o an t m s mp y A gh y b und a l Atoms a e not ll he s me hey c n old d f e ent n m ers f pr tons nd t a e ases 0 4%) COMPOSI ION t epoo s e to s f ix r t ns p r l ) neu r ns nd e ec r ns A ubs an e m de of a oms of ust ne ty e s Da i h ph s c s N e s B hr ( 885 The h n tmos he e f a d e e t o s a e a l h wn nd i ne t o s go d) 1 62) w s t e f r t o pr po e t at rg n 1 6%) Mars s dom na ed by ca bon h re m ch a g r han c l ed a chem ca e ement and s g ven an a omic numb r equ l to he e e t on in n t m mo e wi h n i r gen (2 %) d ox de w th i y moun s EXAM NED AT THE TINIE T SCALE the univer e s t er e l i e eai e number f pr tons nd thus l c rons n s at ms Ex mpl s a e d s r te o b ts He ug e t d th t Ma s p ns on of n ro en and a gon and ma ter is compo ed of fundamental pa ti les some to e who e t m hyd ogen wi h an at m c number f 1 a l h dro en a oms he e o bi s h ve i ed e e gy ev ls ts a is ve y o her g ses and some ra es of which go erned by var ous orces g oup con a n ne pr ton and o e l ct on) h l um a omic a d th t a oms mit r ab o b 24 63 h urs of wa er v por 4 27 e e t l b e ts toge her o fo m toms and ons In addi ion o ST UC URE F A CA BON A OM numb r 2) and c rbon number ) A oge her he e e e gy in i ed am un s ( qu n a ) NORTHE N c rb n di x de ( 5 %) a i t n 34 7 the e well unders ood types of ma ter other forms At he e t r f an t m i t e a e 0 at ra y ccu r ng e emen s The a oms a e ec ons mo e be we n or i s SUMMER exi t Mo t of the unive se s mass cons s s of h s n c e s h ch o t i s p o ns of a y l m nt a e a l the ame s ze and Bo r s o b ts re o ay c l ed SOL TICE Ma s or i s Sun NORTHE N S a e a d i e 40 3 dark ma ter whose exact natu e s t ll unknown a d n u r ns E e r ns m ve c uc a ly co t in t e ame con gu at on n 687 Ea th d ys ALL T e B g a g 48 1 a o nd w h n t o r g o s of l ct ons wh ch is u iq e o th t or i a ; t ey a e EQU NOX c l e sh ls u r u d ng he e ement nd i es t sp ci c hem cal ub t uc u es Ou o t e a k e s 54 5 n c e s he h l s p ear p op r i s T e ni er e once o e ec on STRUCTURE sma l p oba ly he un 04 7 f z y ec u e e e r ns co s s ed a most nt r ly of he h ls so d i on co e do ot ov in l gh e t e emen s yd ogen nd Ma s s a small p anet about half the s ze of Earth and far her way d fn dp t s he ium Mo t of he oth rs THE SOLAR SYSTEM rom the Sun I s ize and di tance mean that it has cooled more apid y man le f VOLUT ON OF A STORM SY TEM THE SOLAR SYSTEM in l ding uch common h n Earth and its once molten iron core s p obably now olid ts s i ate ock WHAT IS MATTER? UT R L CT ON HE L ones s oxy en a bon ela ive y ow den ity compa ed to the other t rre tr al p anets nd ca es eg n in h ch o r and ron ha e l rg ly h t he core may al o conta n a li h er lem nt such s ulfur in he MARS NTER OR M t er s an th ng t at po s s es ma s th t is a yt ing a ec ed by gr v ty Most ec on o b t b en cr a ed in s a s orm of i on su f de The small core is sur ounded by a thick mant e Mars h s a is nct ru t nd s ar e pl s ons compo ed of sol d il ca e rock The mant e was a sour e of volcan c man le and c re he ma ter on E r h s made of toms a d ons E s w ere n the un v r e h w ver I N R E E TR N S E L ac iv ty n he past but t s now nert Data gathe ed by he Mars co e is mu h mal er n Re on w t n wh ch Global Surveyor space raft has rev aled that the ocky cru t s bout p opo t on to E r h s a d ma ter xi t under a ast an e of con i i ns and akes a ar e y of f rms f om tw e e t o s o b t 50 mi es (80 km) th ck n the southern h mi ph re whe eas it is only has p ob bly o id f ed about 22 m les (35 km) thi k in the nor hern hem sphere Mars has he t in i te s e l r med um see p 2 8) to he m t er n n in t ly d nse b a k ol s ame otal and area as Earth ince t has no liqu d water on i s urf ce ( ee p 26 ) Not ll f th s ma ter s made of toms but l m t er s m de of s me k nd of p r ic e Cer a n yp s of p r ic e a e SCARRED SURFA E Th s mosa c of V k ng Or i er f ndame tal th t is hey a e ot made of ma ler ima es sh ws Ma s s d st nc red co o at on a d ev als he va t s b un s T e mo t common pa t c es w th n ord n ry ex ent f the V l es Mar ne s a sy tem of v l eys mo e th n ma ter re qu rks nd e ec ro s wh ch make up a oms 2 500 mi es 4 00 km) ong and ons a d orm a l v s b e m t er Mo t of he 1 On June 30 1999 a st rm 2 A g ant t rb le t c oud of 3 E pand ng ap dly he st rm 4 Six h urs f er t e i st mage was aken t e to m was t ll un ve se s m t er howe er is ot or in ry ma ter H D OG N ys em eve op d ver he no th o ange b own dus w s a sed by swi l d ver he whi e i e ap g th r ng s ren th A o o l ss as t but ark ma t r ( ee p 27) ock c ust ol r re ion o Ma s h gh su f ce win s (c nt r op) 0 F ( 1 C) t a oms pe ha s compo ed pa t y of UM NOUS M TT R h ve u t 1 r t n ne tr nos heo e i al WIMPs h se l m na e ga c o ds (w ak y in e ac i g ma s ve n n e s l a s ac a e nd 1 e c r n in ade f r i a y m t er a i ges el pa t c e ) r bo h om o ed f a o s a d i ns P OP RT ES F E EME TS l me ts a y m r e l in ATOMS AND IONS h i p op t es s s own A U I UM y t e f u e am l s h re A oms a e ompo ed of f ndamen a pa t c es c l ed qu rks nd he e p o e i s a e A o id m t l t 7 F e e mi ed y he 2 ° ) I s t ms l me ts i e e t h ve 3 p o o s t mcs u t r s 4 ne ro s a d 13 l c r ns n 3 he s le t ons The qu r s are b und in g oups of h ee by g uons wh ch △ THE SOLAR SYSTEM This section is about the Sun re ma s e s pa t c es of or e T e ua k rou s fo m p r i les a l d and the many bodies in orbit around it. It covers the nine p o ons a d n u rons Th se a e c us e ed in a comp ct eg on at t e planets one by one and then looks at asteroids, comets, en er of he a om c l ed t e uc eus Most o the est f an a om s and meteors, as well as the artwork main image illustrations remote regions on the margins of planet’s shows planet show atmospheric m ty s ace but mo ing a ound w th n th s s ace a e e ec r ns The e of the solar system. For most interior as it appears planets, profiles of individual structure composition for ar y a eg t ve e ec r c l ch rge a d h ve a v ry l w ma s ea ly ll surface features or moons from space each planet are also included. M G NG THE TOM he mass n an a om s in he ro ons nd neu ro s A oms a way con a n T i im ge f g d a oms n qual umbe s of p oto s (po t ve y ch rg d) and e e t ons neg t ve y color-coded panel a g d o g ee ca b n a o s ha ged) nd so a e el c r ca y n ut al f th y lo e or ga n e ec ro s hey contains references wa ma e by a c n i g b come ch rg d ar cl s c l ed i ns to other relevant t n e ng m c o c pe em t d A SO PT ON A D MI S ON S L UR ROM E sections co i g p o on p o on he l c r ns n t ms c n e st A um ng r wn n d f r n en r y t t s y A y l ow b t e THE MILKY WAY ▷ s l d t 7 °F 1 C) qu d a 7 °F 2 ° ) The subject of this section is l c on t ow m v ng e we n e e gy a es ts t ms h ve 6 t a oms a e 35 the Milky Way and the stars, n r ysae h y c n e he a so b r em t p o o s 6 18 p o o s 44 r 46 nebulae, and planets that it a ke s o qu n a of n r y n u r ns a d 16 n u ons nd 35 contains. Pages such as those he e e e g pa k t a e e e ro s n 3 s e s l c r ns n 4 he s shown here describe how particular types of features u l us od m n are formed. nu e s a ed p o o s CHEMICAL COMPOUNDS h o de on e e t on t i h l c on a s b c to EM S ION en r y t te ed u rk Mo t ma t r n the u i er e con i ts f unbo nd at ms ow r n r y s a e l on A SOR T ON r en u rk r io s of a f w ch mic l e emen s but a ig i i ant mo nt l c on a ed o xi s as comp unds co ta n ng a oms of more han one i h re eg sae e c ed ec on e ement oi ed by chem ca bonds Compou ds oc ur i n r he l c a ge 1 ) ee t n n om g h gh n o j cts uch s pl ne s and a t ro ds n vi g ne y p o on o gan ms a d n the nt r t l ar med um n on c NTRODUCT ON NTRODUCT ON c mp unds uch as a ts a oms t ade e e t ons and u eus E E TR N he re u t ng ch rg d ons re bo ded n c us mp y b u qu k E e t o s h ve a by e e t ic l f rc s and a ra ged in a ON C C MPOU D h ll n ur n ne a ve ha ge r g d ry t l i e ruc u e n ov l nt om ou ds f h s ne t n rtn an a ma s mo e p o on e cr n n ON CH RGE 1) NS DE A N UT ON t an a h u and c mp unds uch as wa er t e t ms re y e c n i t f he o t r he l P t ns nd n u on a e e ch m de t me sm l e t an h ld in t uc ur s ca ed mo e u es by t e o s o tw or m re ON Z T ON f h ee ua s o nd y g u ns a p o on r n u r n ha ing of le t ons b twe n hem Two h mi a e em n s TOM O e wa an t m ay ec me a o i ve on T e q a ks ip e we n ed or mo e d nt c l a oms an a so c m ine y i a y a an ed n a N UT AL NO CH RGE) s b t e e e t o s a s r i g e e gy om a g e n nd b ue f rms ut e e t ng o d s uc re T is 238 STAR FORMATION i h e e gy ho n and s a es t be g h re s l a s o e of a h c l r j c ed a ng w h i s ha ge om t e a om o orm m l cu es of er a n l m nts x mp e s s lt o i m c l r de STAR FORMATION TRIGGERS TO STAR FORMATION △ WHAT IS THE UNIVERSE? 24 27 Ce es al b ec s STARS ARE FORMED by he grav tational co lap e of cool GA A This section begins by looking at some basic questions about the size and shape 55 The i s s ars dense interstellar clouds These clouds are composed ma nly Clouds of nter te lar ma eri l need a rigger to st rt Arn of the universe. It goes on to explain concepts such as matter and radiation, the 228 T e in e s e ar med um of molecular hydrogen (see p 228) A cloud has o be of a h m co lap ing s nce h y are held up by he r own glx motion of objects in space, and the relationship between time and space. 232 33 S a s cer ain mass for gravi ational collapse to occur and a tr gger pr ssure and hat of n ernal magne ic f elds Such a h ve 234 37 T e l fe yc es f s a s is needed for the collapse to start s nce the clouds are held rigger m ght be as simple as he gr vit tional ug f om om up by the r own internal p e sure La ger clouds fragment as a pas ing tar or it might be a hock wave cau ed by he t r c us e s 88 89 they collapse forming sibl ng pro os ars that initia ly lie close b ast f om a supernova or he col i ion of two or more ROM ga axies In pir l g lax es uch as the Mi ky Way densi y S oc together some so close they are gravitat onally bound The mater al hea s up as S AR-FORM NG REG ON waves move through he dust and gas n the g lac ic disk upe n the n bu a CW 120 in he sou he n M l y see p 227) As the waves pass they empor ri y inc ease he i it collapses unt l in some clouds the temperature and pressu e at the r centers Way an exp nd ng bub le of on z d as is he loc l den ity of nte ste lar m ter al c u ing it to nw c us ng t e ur ou di g m te i l to c l ap e n o co lap e Once he waves have pa sed he r shape c n become so great that nuclear fusion begins and a star is born d nse c umps in wh ch new s ars w ll e orn be picked out by he t ai s of br ght young ta s STELLAR NURSERIES BOK GLOBU E ST Sma l co l c ouds o dust As we l as being among the most beaut ful objec s in he unive se s ar orm ng a d gas nown as Bok Wh g o ul s a e the o i i s f c ou n bu ae on ain a comb nat on of aw ma eri ls that makes tar bir h poss ble ome f the M lky Way s st r orm ng ne g ower ma s st rs re io ran These clouds of hyd ogen mo ecules hel um and dust can be m ss ve ystems B k l bu e pa r you g st r usu hund eds of l ght ye rs across or sm ll r nd vidual c ouds known s Bok s e ar EGGS c us e s tar see globules A though hey m y l e undi turbed for mi l ons of yea s di turbances ST LLAR EGGS dus Wi h n he ev po at ng can t igger these nebulae to o l pse and fr gment into sma ler c ouds f om g seo s lo u es (EG S) of he Ea le Neb la which st rs are ormed Remnants f om he s ar orm ng nebulae w ll su round nt rs e l r mat r al s c l ap ing o fo m s a s 8 THE B G NNING AND END OF THE UN VERSE THE BIG BANG 49 the tars and the s el ar winds produced by the new s ars can n turn cau e the e remnan s o col apse If the c ouds a e pa t of a arger comp ex th s c n become a THE BIG BANG HE I ST M CR SE OND EXP ORI G S ACE great s el ar nur ery M ss ve tars have re at vely short l ves and they c n be born V OL he me ne n t i pa e a d he n xt Y un h ws ome v n s d r ng h f st RECREAT NG THE EARLY UN VERSE l ve and d e as a upernova whi e the r ormi i r s c nd 1 m l o th f a e o d or 1 27 0 e o d ) a er he i Ba g Ov r At he Eu op an Ce t e fo N c e r le s mass ve ib ings are st ll o ming w th h s e od t e u i e s s t mp a u e Re ea ch a o nown s CERN wh c 8 31 M t r TIME SPACE ENERGY AND MATTER a e a l thought o have come r p ed om a ou 10 ( en i on p r i le hy i i s a e un av ing he ORMA ION IN ACTION The shock wave rom the upernova es l 4 37 a i t n in o ex stence 13 7 bi l on years ago n he event ca led the B g i on r l n de ee ) o a m re in r d t i s of he ar y u i er e by Wi h n the n bu a NGC 24 7 e t rs may p ow through nea by in ers el ar tu n 4 45 x a d ng p ce Bang In ts f r t moments the un ver e was inf ni ely dense 0 C en r l n de ee ) T e im l e m s i g pa t c es o e her n pa i le a va io s st ges of orm t on At he matt r tr gger ng yet more s ar b rth unimaginab y hot and conta ned pu e energy But with n a t ny e rs o he i me e of h ob e v b e cc l r t rs nd s a ch ng or r ces h f t o t e U i e e 58 9 f act on of a econd vast numbe s of und ment l n v se t s i t e a p o im te f o her un ame t l p r i l s In ower e t l es a ve y you g t r th t is M p i g ee s a e 3 9 par ic es had appea ed cr ated out of energy as s o i a d am t r f he p t o t e do ng o hey xp o e t e on i ue ts n v se we an u en y o s r e of m t er nd t e f rc s hat on r l b ea i g r e f i s su ro nd ng b r h h ir n e ac ons CERN c e t s s h ve e en r c ea ed ond i ns i e h se oco n f gas On he f r r ght a wal of ho t y a er he B g ang by c e t ng the un ver e coo ed Wi hin a few hundr d thous nd years p a mas on a n ng f ee q a ks a d g uons b ght g s lows as t s vap ra ed by TO these par ic es had comb ned o form he f r t a oms L RA H GH NE GY RO ON OL S ON he ene gy of ma y newly o m d ot As c n t s ma e o t n d by d t c o a t e L ge ad n con o de a C RN t e y l w i es h w t e a hs f a i es ta s D rk anes of ust a the c nt r hey od ed r m he o i on f l a h g en gy r o s bec IN THE BEGINNING h de p r s f the n bu a th t are orm hei p ob bly o m ng n w s ars The Big Bang was ot an ex lo on n sp ce but n xpan on of s ace whi h happ ned e er w ere Ph s c s s do not kn w w at h ppe ed in t e i st n t nt a t r the Big Ba g kn wn s the P an k ra but t HE P ANCK RA the e d f th s pe i d hey be eve hat No u en t e ry f gr v ty s l t f om he o her or es of n tu e hy cs an e c i e w at a p ned n fo owed by the t ong nu l ar or e s e he n v r e d r g p 30) M ny be i ve t is e ent r gg red hs i e i f a ion a hor but r p d xpa s on f in at on d d ccur t he ps o xp a n why I ME ER 3 10 f 10 m 3 t1 m 0 m 2ml s1 0k ) 0 m ( 20 m e / 00 km) 0 m 20 00 m e /1 i on m) 0 m 6 0 m l on m e 1 b l n km) J L E DREYER b l n t l n ° ) 0 K 1 b l n t l o ° /1 l on l on C) 0 K 1 8 i on r on F 1 m l o t l o °C) 0 K 1 00 r on F 1 0 0 t l n ° ) the u iv r e se ms o smoo h nd f at EM E A URE 0 K 1 00 r i n r i n ° (1 00 i on i on C) 0 K ( 8 b on r i n F 10 Dan sh– ri h st onomer oh Lou s Em l D eyer (18 2–19 D r ng in l t on a a ta t c amou t of THE N L T ON ERA THE UA K ERA E ARA I N OF T E E EC ROW AK FO CE comp led the N w Gene al ea t e e d o t e q a k e a t e e e t o ea f r e Ca a og of N bul e and C u m ss e er y ame in o ex s en e n P r of he n v se xp n ed r m So e i es a l d he e c r we k e a t s p r od aw e a a ed to he ec om gn ic o e a d t e we k of S ars rom wh ch nebul e b l o s o t m s sm l r ha a v st um e s o q a k a d a t q a k p i s o m ng om t r c i n s e p 3 ) F om h n on he o c s of a u e gal xies get heir NGC num t ndem w th an e ual ut ne at ve p o on o om t i g b tw en he e e gy nd h n a n h l i g b ck o e e gy G on and nd h s c l aw we e as ey re ow e p r e c d the time of omp la ion it w s e of a a b e a d a f o b l i ld o h r mo e x t c p r c e a so p ea ed known if a l the nebu ous o amount f gr vi a onal we e wi hin the M lky W y stud ed the proper ene gy By the e d f n u a ty IE A h n r d b i n h o a y c o e o d A un ed m l n h o a y c o e o d y c o ec nd 1 e t s c nd 1 to e o d 1 em o e o d 1 i o e ond 1 n n s c nd 1 m r s co d mot ons of many in at on m t er h d t he a t 0 scns 0 se o ds 0 se o ds 0 se o ds 10 e o ds 10 s c nds 0 scns and con luded 0 e o ds 0 s co ds the sp ral Hg sb s n nebul e now beg n o pp ar f me ( y o h t a) known to be spi al ga ax es A en i o th f a o t s c nd qu k a i u rk RE ZE O T AND AN I I AT ON we e ik ly 0 e o ds u rk q a k to be mo e u rk a t c e a t p t c e a rs n l d n qu r s dis ant HE G AND U I I D TH ORY RA an q a k p o on n i u r s we e s l c n t n ly rm n and objec s u i g h s e a ma er nd ne y we e n i e t no e u n ng o ne gy o ea h y e of a i le he em e a u e w u d e e t a l d op o he om e e y n e c an e b e Th e of he pr o nt w e e he a t c s o e out h y ou d o l n er o m r m h ba k r u d p ol u d me t l o c s o na u e w re i l n i d f e e gy Mo t ee p t c e an an p r c es f e ch y e we e a i l an h l t d l a i g X o on a ma l e due f a t l s s q a ks nd n i u r s f o e ut t he nd f t e q a k s r ng u ea f ce q a k a i u rk ra n t ad f b i g a n h a ed s me eg n QUA KS B COM NG THE M LKY WAY o m ng nd r up g to o m h a i r p r c es B UND NTO H AV ER a n i tn AR I L S BY G UONS su e o ce G a d U i ed l c ow ak ea nu e r o ce e e r ma n t c o ce Fre o ce rv a o a f re i gs o on yp he c ) 10 3 E ONDS 1 3 SE ONDS 1 1 SE ONDS l on Wb s n g vt n SE AR T ON OF OR ES AR I LE S UP h po e i a ) MOR M T ER T AN AN IM TT R de a ng a k a tq a k ar Ph s i t be e e ha at h ex e d n l h gh em e a u es X b son p e en j s a t r he ig a g he ur u d me t l o c s bo t 0 s c nd a t r he i Ba g t e X os n On of e p r c es ou h t ha e x s ed q a k a d a t u rk we e u i ed T en a t e u i e se o l d t e f ces n v r e s ho g t o h ve e n a o p h p t a) d r n t e e r y m m n s o t e B g ang X os n fr ig rm nryad s pa a ed r f o e o t t he i e i t r a s h wn h re f u da en l p r c e a d a t p r c es wa a v ry gh m s pa i e t e X b s n dcy i me i t y r u n ng o h se w r co i u l y o m d f om n r y n q ak ( o g w th s o n a t p t c e he n i X p du s e e gy s he me t s p r c e a t p t c e a rs h ch h n m t b s n) he X o on nd s a t a t le e e p r ce a d IN L T ON nd w r an h a ed a k to n r y Am ng u s a l an de a e i t o he p r c e a d ni a i e ) h se a i es w re o e t a s i ex t o ay a t p t c s q a ks n qu ks e c r ns NTRODUCT ON In B g Ba g w t o t n l t n ha a e n w i e y s c n t u n s o ma er r s f r e a r er a d p s r ns a i l c o s) A e u a i y q ak at l s T e ei cu eq ak a dt er o t e X b s n a d ts n p r c e s hat s ac d eg ns f he u ve e c u d n v r h ve n pa i es a t ua s) nd o o s s ch s wh n he d c yed h y r d c d a t y a t u rk l o s s e pp 0 31 Ot e p r c es ay p e o de n e of a i es v r n i a i es be o e so i i a in e s y a d a e b en r s nt at o on e e i t r a e t a is b ut b l o an on pa c es o a h pa c es nd n p r c e meet a d o d t ct pe h p so e g a i ns b l n a t p r c s W en e e we e a er co v r ng h r omb ed m t r t mp r t re n a i n he r p op s s h p t e c l g v t c r y n pa i e ) a d a n h a ed a e du of a i es e a n d i to u e ne g ( h t n ) i gs o o s so y o h t c l w i h a d i s p s u a d t a t e e g ve se o ll t at u ob e v b e u v r e is m a t m ss o o er a t l s t e m t e c r e t y n h un e se de v d f om a ny om g ne us pa h of h o i i a un e se T e e f c of n a i n s i e e p nd g i h e ce s f a i es e t v r a w i k ed p e e a er he e pa s on s s r a e WR NK ED SMOO HER VE Y SMOO H EX REM LY S OOTH AND F AT ap e r sm o h a d f at △ THE BEGINNING AND THE NIGHT SKY detailed END OF THE UNIVERSE chart 2 THE VIEW FROM AR H THE CELEST AL SPHERE 63 The universe is thought to This section is an have originated in an event atlas of the night sky. THE CELESTIAL SPHERE MAG NA Y GL BE DA LY SKY MOVEMENTS known as the Big Bang. This It is divided into text T e c es a sp e e s p r ly section describes the Big describes ma i a y w th a p c ic As he Ea th s ins a l el s i l ob e ts move cr ss he sky a hough he m vemen s of Bang in detail and looks at features of s a e b t no ec e s ze how the universe came to FOR CENTURIES humans h ve known ne e p nd u a to A t o om r u e e ac y he s a s and p an ts b come v s b e nly a ni ht For an ob e ver n m d a i ud s t rs be the way it is now, as well interest c p cpa e pa e d f ed p n s a d c r es n as how it might end. Ce s a c c es 4 67 fE r h o bt ts u a e as e e e c s or n po ar r g ons of he ce e t al ph re de c ibe a d i y c rc e a ound t e or h r sou h r u d S n) d s r i g or e e m n ng that s ars l e at d ff rent dis ances f om E r hs x s s he os i ns f t r and e es i l po e The Sun Moon p an ts and he rem in ng s a s r se a ong he ea t rn 368 THE CONSTELLATIONS E r h o b t 24 t ed t 23 c e t al oh r ee ta o j cs s h re h r zon swe p n an a c c oss he s y nd set n the we t Th s m t on has a i t to t e Ea th However when record ng a s a e x d to he ap ng h s y 3 8 53 the po it ons of tars n the at s p e es u a e a d ou h ( or ob e ve s in he N th rn Hemi phe e) r to t e or h So th rn Hemi phe e) U ng e s y u d s 4 8 29 xs f p e r o mo e n pn he ower he ob er er s a i ude t e eep r the i t S a s ha e xed po i ons on t e ph re ANDROM DA con te at on Pega us w ere it ma ked ie d of vi w and to con en ra e he Th op o te r c on sky t s convenient to o th e s i of a h s p n o the p t e n o th ir mo ement EQ AT R AL N GHT Andromeda the n vel of he orse The s ar s wo ig t The m ll c mp nion ga ax es pr ep a s wi h re t p ec s on on e F om he q a or lm s t e pret nd that th y are a l tuck to he n ide of oe i s ve y s de ea day see p 6 ) The e th t 6 00 M wh l of he e e i l ph re I E R NK NG 9 names Alph ra z and Si rah a e M32 nd M110 re d f ic lt o ee mo i e l ab ve ust R GH E T S A S both de i ed r m an A ab c te m hrou h a sma l el sco e in a ph re hat sur ounds Ear h The idea of this at sN rh l he a z α) 2 1 oe l ne s un nd Mo n lw ys move c n be e n f r ome f he M ach β) 2 1 th t me ns he or e s avel Gamma γ) Andr me ae k own n e on he ce e t a sp ere o he me d r n one ght he sphere a so he ps st onomers to und rs and er od of epe t on di ers om S n s l w ob c es n y a EN I E a o a A maak or Almach see p 277) of hat f t e t rs ma l a t f he ph re nd ome ae how heir lo at on on Ear h the ime of BB E I T ON A d SPECIF C FEATURES s a oub e t r of ont a t ng olo s night nd the t me of year a fect what trlw I H ST N S Y AT 0 PM On a c ear n ght the f r hest t is t cons s s of an or nge gi nt s ar of no c ob r ov mber they see n he night sky om s n et two parts.The first pos i le to s e w th the na ed ye magn t de 2 3 and a a nt r blue of U LY S BLE I CUM OL R ST RS b c r ssas 0N 7S is bout 2 5 mi ion l ght ye rs compan on nd t is as y een de t r in h po a r g o s M DN G T Th s ce eb a ed ons e la on of he w ich is he di t nce to the t rough a sma l t le cope am ort ern sk es de ic s the d ught r THE SKY AS A SPHERE en l r a hs f he e e t l s h re z n ha re aw g ow f the myth cal Que n Ca s ope a Andromeda Ga axy ( ee p ng o th o e e c be p f ct a t m dn h bs u e s a s q i ox who is ep es nted by a nei hbor ng To an ob er er on Ea th t e t rs ppe r to r t p nt r l s a o nd h no h o s r e sve N r h P e ou d ons e l t on The he d of he pr nce s pp 312 3 3) a hu e pi al of 2h f re ) r ou h e e t a p le a mi n ht s w i h E r h o a es s mark d by A phe atz or Si rah ) s ars im lar o ur own ga axy m ve s ow y c os the n ght ky Th ir mo ion u ng o e n ht as u o sc ed A pha (α) And omedae w ich is he Al o kn wn as M31 his PERSEUS at s h wn b t is ng b e v r s ew (the constellations) is a guide to the 88 regions t r at the n ar st co ner of t e ga axy s ans s ve al d amet rs 0˚ h is au ed by Ea th s o at on a th ugh i m ght pn x o u e p o og ph ob e v r s e e o e un se s qua e of ega us n ano her ad ac nt a t r un e i b c r d nt e ons e l t on Long ago Alp er tz was se m hat he s y s pi ni g ro nd our YEARLY SKY MOVEMENTS ob c r d n he a t yt eS n eg rded as b ing s ar d wi h the of a fu l moon nd i s high we t b t e S n in the mid nor he n ky on pl net o he ob e ver he sky c n be 65 ima in d s t e n i e f a sp ere kn wn s at s 6 00 AM z n t at o a on an f ll e en ngs The n ked 51 the el s i l phe e o whi h he s a s a e f xed and e a i e o whi h he Ea th As Ea th o b ts he Sun the S n eems o m ve ag i st he ba kg ound EXP OR NG S ACE eye s es it s a a nt ϕ AR STOTLE S SPHERES pa ch t lo ks el nga ed ro a es Th s s he e has ea u es e at d to of t rs As he Sun m ves n o a eg on of he s y ts l re wa hes ut into which astronomers divide the sky. It contains ra her han sp r l ωξ bec use t s t ted at a the ea sp ere of he E rth I has n rth f i ter ght rom t at p rt so a y ar or o her bj ct he e emp ra i y s eep ang e towa d 60 the Ea th When γ1 A mach and s uth p les wh ch ie on ts ur a e be omes d f cu t to v ew f om nywhe e on Ea th Ea th s o b t a so Unt l he 1 th c n ury d he i ea o a p e e of lo king at M31 4˚ ce e i l s he e s r oun i g Ea th xd th ough a di e t y abo e Ear h s Nor h and So th E rh Ea h s mea s th t the art f the e es al phe e on the o po i e s de to Ea th was n t j st a on en e t f c on s rs te e cope l w NGC 891 υ M 10 q a or 8 τ ν M1 Po es and t has n qu tor t e e e t al f om he Sun hat s the p rt i i le n he mid le of he n ght NGC 752 M32 equ to ) whi h ts d r c ly t eS na dpa e ch ng s T e vi i le p rt f the ky at or ex m le mid i ht in une many p op e b l e ed i had a μθ abo e Ea th s e ua or The a e n t i ed n he ce e t al ph re s l ke a ce s a sp e e b t Se t mb r D cemb r and M r h s ig i i an ly d f re t at ea t ph s c l r a i y Su h be e s illustrated profiles of all the constellations, arranged ce e t al e s on of a g obe mo e a o nd n r c o e to a r u r p th f r ob er ers t eq a or al or da e b ck to a mo el o the mid l t udes n E r h un un v r e de e op d y the m gn f ca ion β m st be us d o TRIANGULUM the p s t ons f s a s and c le t ee l tc E r h a No h rn Gr ek ph os ph r Ar s o e gi e he w de t Mi a h Hem p e e s π ga a ie can be eco ded on E r h a No h rn s mme so t e ( 84 22 bc) nd e a o a ed 0˚ Hem p e e s ( u e2 ) δ it ust s c t es n E r h ee t le u t r a un s w n er o t e by t e a t on m r P o emy ε Al he ha e he r po t ons of a tu e i le n he e e t l mt n ( e em er 1 2 ) and ong t de on a lo e ph e c nc n ic (ad 85 1 5) Ar to e p ac d w h E r hs q a o Ea th t t on ry t t e according to their position in the sky with the most THE B UE SNOWBALL 54 h m sp e e i b e a hs x s un v r e s c nt r s r oun ed by When s en t r ugh a sm ll f m q a o at f o a on m d i ht n he s ve a t an p r nt co ce t ic w n e so t e EFFECTS OF LATITUDE u um al sp e es o whi h the ars l ne s t l sc pe NGC 7662 ppe rs e un x An o s rv r on Ear h can v ew at b st on y ha f of he c l s i l fspit f Sun and Mo n w re t a hed as a b u sh d k ts t uc u e sp ere t any ns a t ( s uming a c ou l ss ky and un bs ru t d L b a one f wo is r ug t out n y on CCD hor z n) The ot er h lf s ob cu ed by Ea th s u k In act or an po ts f n e e t on J NE AND Pt l my su po ed hat he ph r s A I TO E I N MO EL F T E UN V R E ηζ be w en e e t l qu or D CEMB R SK S ro a ed t d f e e t s ee s a ound S a s e f ed o h o t r p e e Wo i g n a d im ges uch s th s o e a de i tc At p o i e p n s Ea th so ro uc ng he ob e v d t e t e s h r s r u d a t c r y a u n J p er northerly ones first and the southernmost last.The o Ea h s r it an b e v r on e a t so bt mot on of he c l s i l b di s Ma s he un e us M c ry n t e Mo n e u t r s es x c y ob er er at i her f Ea th s p les a pe i ic h l of t e e e t al phe e is lw ys o th ou h e e t l o e i s o p s te a es f h m s h re s b e P SCES ov rh ad wh e he o her h f is n ver i i le For o s rv rs t ot er l t udes E r h s ee i l oe e ow E th So h Po e he e e t a s he e r m eq a o at a mi n g t m d gh on he ro a i n on in a ly b in s new pa ts o the c l s i l s he e in o vi w nd id s o he s MOT ON AT OR H PO E umm r s s i e n r hc e ta p e c e t a me d an he ne THE ANDROMEDA GA AXY 4 This m ans f r ex m le t at o er t e ou se of a n ght n bs rv r at a a i ude of 0 N At he o es l c l s al o 0° g t a c n on On y the n er p r s of M31 a e or 60 S an s e up to th ee qu r e s of t e e e t al ph re f r at ea t ome f the ime; o j c s s em o c c e he a g e o d c na on br g t en ugh o be se n wi h sma l ( 5) a oe s a po t n in t ume ts CCD mag s su h as t is c l s a e ua r br ng ut he f ll x ent f t e sp al arms Be ow M31 on h s ima e l es and an ob e ver t t e qu tor an s e ve y po nt on the e es al phe e at ome t me W c l s al o e d r c y o er second part (the monthly M 10 w i e M32 is n i s up er im o t ce s a po e h ad T e mo on s c u t r CELEST AL COORDINATES c o kw e at h No t Po e Y S N c o kw e at h so h Us ng the e es al phe e con ept s ron m rs an r co d nd f nd the os t ons H E c c mpo r E rh r a MOT ON AT M D L T TU E of t rs nd th r ce e t al bj c s To d f ne an ob e t s po i on s ro omers se A A At m d a t des m s s a s a s s em o coo d na es s mi ar o l t tu e and l ng tu e n E r h T e oo di a es o a KY r s in h ea t c o s he a e ca ed de l na i n nd r ght sc ns on Dec i a ion s m a ur d n deg e s and t h NTRODUCT ON t s a w ys W s y o l q e y a d s t n he a c m nu es 60 a c m nu es 1 d g ee/1 ) no th or ou h of t e e e t al qua or NTRODUCT ON sky guide) is a month-by- THE CONSTELLATIONS ▷ T sbe we t So e ( r um o a ) h G ee t l t sn v r o j csn v r i eo s tb t so t is qu va e t to at ude R gh as en i n he equ v l nt of ong ude is he 5° qu t r sbe N c r e t e c es a po e l S E an le of n ob ect o the e st f the e es al me id an The me id an s a l ne o ts S i OBS RV R AT QU TOR BS RV R AT N RTH OLE O SE VER T MID AT UDE om t m s MOT ON AT QU TOR pa ing hr ugh bo h e e t al p l s and a po nt on wh F r a p r on n he qu t r o t i ob e v r he o t e n or h s b e v r a p r of he sbe At he q a or t r and m Ea h s o a on r n s a l a f f he e e t a s h re e e t a s h re s lw ys o h r c l s a ob e s t e e e t al qua or c l ed he f r t po nt o Ar es or R CO D NG A ST R S PO I ION clsa i t p nt f A es e n l ng e f i ht month guide, containing a Each constellation profile is pa s o t e c l s al p e e s l a s v s b e a d he i b e pa t s e er i b e o i on f a pe r o r e v r c l y n v rna equ nox p int s e p 65) An ob ec s r ght T e m as r me t f a s r s os on e ua r e u n x o n ) s he r in r s en on summary of the highlights illustrated with a chart, two A i to ew or ome me e ch o t e n h f s n ve v s le nd a th r t t o b i gs b ev r W t e e st m ve v r e d a ce s on can be t ted n deg ees nd a c m nu es o t e c l s al p e e s s own e e i h a c ns n me s r me ts 1 h ur r 5 ) for each month, detailed star locator maps, and one or h day he e e i l p es re he e e i l qu t r s on t er a t i to ew or ome a d t en a l e t a ly or n ours nd min tes One hour s e ui a ent o T s s a ha a d c i a on f a ou 45 charts, and charts showing more photographs. A more o on he o i on e ob e er ho z n f he i e ea h d y b e v rs N a d s t n t e we t s me mes w t en 45 an a r g t the positions of the planets. detailed guide to the section n ozn E can be found on pp.348–49. a 15 b cau e 24 hou s make a who e c r le a c n i n o a ou 1 h ur r 15 △ THE VIEW FROM EARTH THE N GHT SKY HEAD TO TOE 2 This section presents a simple model for making sense of the An rom da s one of he r g nal re k changing appearance of the sky. It also contains practical advice on looking at the sky with the naked eye, telescopes, and binoculars. c ns e l t ons ts r gh e t s a s r p es nt he p i ce s s he d (α) h r pe v s β) and er e t f ot γ)

THEMED PANELS ◁ EXPLORING SPACE ABOUT THIS BOOK 7 This type of feature is used to EXPLORING SPACE describe the study of space, CONTRIBUTORS either from Earth’s surface or Three types of color-coded ARISTOTLE’S SPHERES from spacecraft. Individual Martin Rees General editor panels are used to present a panels describe particular Robert Dinwiddie more detailed focus on Until the 17th century ad, the idea of a sphere of discoveries or investigations. “fixed” What is the Universe? celestial sphere surrounding Earth stars The Beginning and End was not just a convenient fiction— of the Universe selected subjects.These many people believed it had a The View From Earth The Solar System panels appear both on physical reality. Such beliefs JOHANNES KEPLER Philip Eales The Milky Way explanatory pages and date back to a model of the David Hughes Exploring Space in feature profiles. MYTHS AND STi ORIESd l d b th The German astronomer Johannes The Solar System Kepler (1571–1630) discovered Iain Nicolson Glossary ASTROLOGY AND THE ECLIPTIC the laws of planetary motion. His Ian Ridpath The Night Sky first law states that planets orbit the MYTHS AND Astrology is the study of the positions and movements Sun in elliptical paths.The next ◁ BIOGRAPHY STORIES ▷ of the Sun, Moon, and planets in the sky in the belief states that the closer a planet comes Profiles of notable As well as being that these influence human affairs. At one time, when to the Sun, the faster it moves, astronomers and studied scientifically, astronomy was applied mainly to devising calendars, while his third law describes the pioneers of spaceflight, objects in the night astronomy and astrology were intertwined, but their as well as a brief sky have featured in aims and methods have now diverged. Astrologers pay link between a summary of their myths, superstitions, planet’s distance achievements, appear and folklore, which little attention to constellations, but measure from the Sun in this type of panel. form the subject of the positions of the Sun and planets in and its orbital this type of panel. sections of the ecliptic that they call period. “Aries” and “Taurus,” for example. Newton used However, these sections no longer Kepler’s match the constellations of laws to Aries,Taurus, and so on. STARGAZER name or astronomical catalog EMISSION NEBULA Robin Scagell number of feature (features The View from Earth without a popular name are Carina Nebula identified by number) Giles Sparrow Exploring Space CATALOG NUMBER Beyond the Milky Way 246 STAR-FORMI G NEBULAE 247 NGC 3372 Pam Spence The Milky Way EMIS ION NE ULA r vea ed hat the m te ial of which EMISS ON NEBULA EMISS ON N BULA th y a e ompo ed s in onst nt DISTANCE FROM SUN Carole Stott The Solar System IC 2944 m tion This m y be cau ed by DR 21 Carina Nebula r dia ion f om the loo e c us er of 8,000 light-years Kevin Tildsley The Milky Way A AL G NUMB R m s ive young st rs embedded n C AT LOG UMBER AT LOG N MBER MAGNITUDE 1 C 2 44 2944 The s ars ul r vio et rad a ion DR 1 NGC 3 72 s g adua ly ero ing the globu es and CARINA I TA CE F OM SUN t s pos ib e hat th s could pre ent I TAN E F OM SUN IS AN E F OM UN 900 i ht y ars th m from ol aps ng o orm s ars 6 00 i ht y ars 8 00 i ht ye rs In add t on to ra ia ion he ta s al o M GN TUD 1 M GN TU E 4 5 emit s rong s el ar winds th t end out YGNUS m te ial t h gh elo it es cau ing CA INA CENTAURUS hea ing and ero ion of nt rs el ar The bi th of ome of the M lky m te ial The la gest Bok lobu e n Way s mo t ma s ve ta s has been A so kn wn as the Eta (η) C rin e PROBING THE NEBULA B tween he onst l at ons C ux and IC 2 44 ( elow) is about 1 4 ight d s over d wi hin DR 21 a gi nt Nebu a th s is one of the la gest nd An in r red mage r vea s t e ta s l i g Cen aurus ies he b ight busy s ar ye rs ac oss w th a mass about 15 mol cul r loud spann ng bout b igh est nebu ae to be d sc vered It wi h n he ne ula s d nse du t and g s The forming nebu a C 2944 Th s n bula t m s hat of the Sun 80 ight yea s nfr red images h s a d ameter of more th n 200 l ght open lu te s T ump er 14 and T ump er 16 is m de up of dust and gas th t s h ve eve led an nerge ic ears s re ching up o 300 l ght yea s are i ib e to he le t and op of he image i lumina ed by a loo e lus er g oup of newborn s ars ear ng f i s fa nter ou er fi aments a e of ma s ve oung ta s IC part he gas nd du t round nc uded W th n ts hea t a d hea ing te es opes rev al th t po t ons of he 2944 s perhaps b st known hem One s ar alone is up i s dust and gas is an int re t ng C rina Nebula a e moving at ve y for the m ny Bok lobu es that 1 0 000 t mes as b ight as oo of young st rs These inc ude high spe ds—up to 22 000 mph are vi wed in s lhoue te ag inst he un This s ar is e ec ing xamp es of the mo t ma s ve t rs (828 000 km/h — n va ying d rec ions i s b ckdrop Bok g obules a e hot te l r ma er al in o he known wi h a spe tr l type of O3 Co i ions of in er t l ar c ouds at the e thought to be cool op que ur ounding mo ecu ar see pp 232–33) T is type of s ar w s spe ds h at ma er al to su h hi h reg ons of mo ecul r ma er al loud ugge ting t may i st di cov red in he Ca ina Nebu a temper tu es that t m ts high ener y that wi l even ua ly col ap e o h ve a p anet fo m ng nd he nebu a remains he c os st X rays and the ent re C rina N bula form ta s How ver stud es of d sk a ound it oca ion of O3 st rs to Ear h A so is a sou ce of exte ded X ay m ss on the globu es in IC 2944 have wi hin the C rina Nebula a e hree The movem nt of hese c ouds of G GANT C EMBRYOS Wol –R yet s ars wi h sp ct al type ma er al s hought to be due to the COSM C CONSTRUCT ON THACKERAY S GLOBULES his n ra ed m ge re ea s a l tch WN see pp 254–55) Th se st rs a e s rong st l ar w nds em t ed by the Th s fa se co or mage compo ed of our The Bok g ob les n IC 29 4 w re f g gan ic n wborn ta s hown he e b l eved to e evo ved O3 s ars with ma s ve st rs w th n bombard ng he sep ra e ima es ta en in d f e ent n ra ed f r t obs rv d n 19 0 y ou h n g een n op i al ig t he su r und ng ery la ge at s of mass e ec ion One sur ounding m te ial nd ac ele a ing w ve eng hs re ea s more han 300 new orn Af i an as ro omer A D Tha ke ay mo e ul r c oud s paq e of he be t known fe tur s w th n the it o i s high ve oci ies st rs s at e ed th ou hout he RCW 49 Th s gl bu e as r cen ly een Car na Nebu a is the b ue uperg ant nebu a he o dest ta s of he neb la ap ear shown o e wo ve l pp ng c ouds tar E a η) Car nae ( ee p 262) EROD NG TOWER M SS ON NE ULA in t e en er n l e gas i amen s ap ear n mbedded with n p rt of the nebu a A ower o cool yd ogen g s and du t th ee gr en a d us y te dr ls re sh wn n pi k EMIS ION NEBULA known s the K yhole l ght ea s lo g xt nds rom the C r na RCW 49 locator map shows table of summary selected features Nebu a R cent Nebu a in h s a se co or Hu b e mage The constellation in which information (varies are described in Tr fid Nebula ob erv t ons made C TA OG N MBERS feature can be found between sections) double-page wi h nf ared t w r is b ing rod d y the e er y r m RCW 49 GUM 9 and its position within feature profiles A AL G NUMB R hot yo ng st rs ne rby the constellation M0 D S AN E FR M UN GALACT C NEIGHBORS 14 00 i ht y ars Da k dust an s are s hou t ed aga nst I TA CE F OM SUN g owi g as and s a s n t is v ew of t e 600 i ht y ars CAR NA EXPLOR NG S ACE And omeda Ga a y and i s two c ose M GN TU E 6 3 com an ons the dwa f e l p i al ga a ies One of he most produ t ve egions SPITZER TELESCOPE M32 (u per e t) a d M1 0 bo t m) SAGIT ARIUS of s ar fo ma ion to have been found n he M lky Way RCW 49 pans a L unched n Augu t 2003 the Th s em s ion nebula s one of the d s ance of bout 350 ight ye rs It is Sp t er te es ope s one of the hought that ov r 2 200 st rs re ide a gest nfr red te es opes put in o youngest et di cove ed It was f r t wi hin RCW 49 ut b cause of the o bit It has be n ve y ucces ful n nebu a s dense a eas of dust and gas p obing the den e du t nd g s hat ca led the Tr f d Nebula by En l sh he ta s re hidden f om view t i s n he nte s el ar m dium and op ic l wa el ngths of l ght However h s eve led fe tu es and de a ls as ronomer John Her chel be ause of he nf ared te e cope onbo rd the wi hin s ar forming c ouds hat Sp tz r pace ra t (s e pan l igh ) h ve n ver been seen befo e As i s hree lob d app aran e when een has ecen ly rev al d the pr sence of Sp t er obse ves in in ra ed i s up to 00 newly fo med s ars St rs ns ruments a e coo ed almost o throu h h s 18 h c ntury te e cope ha e be n obs rved at eve y tage bso ute zero to ensu e hat the r of heir ea ly evo ut on n th s ar a own h at does not in er ere w th The nebu a s a egion of in er te l r making it a r ma kab e ource of he ob erv t ons A ol r hi ld da a for s udying s ar fo ma ion p ot cts he el scope f om the Sun dust and gas be ng l um nated by st rs and deve opment One urpr s ng p el m nary obs rva ion sugge ts hat INS DE SP TZ R forming w th n t t spans a di tan e most of the s ars ha e a cre ion di ks The S i ze c aft a ound hem Th s is a far h gher has a 34 n ( 5 of around 50 ight ye rs The young at o han would usu l y be expec ed cm) e es ope Det i ed ob erv t ons of wo of the and hr e su er st r lu ter at ts cen er NGC 514 d sks r veal hat they a e composed co l d r ce s ng 312 313 of ex ct y what is equi ed n a in t um nts was ormed on y bout p anet fo m ng ys em The e re he ar hest and fa nt st pot nti l 100 000 yea s ago The p anet fo m ng d sks ev r ob erv d This d sco ery suppor s the theo y Tri id s lob s he br gh est hat pl net orming di ks are a n tur l pa t of a tar s e olu ion t l o of wh ch s actu l y a ugges s that olar ys ems ike our own re robab y not are in the mu t ple sy tem a e c ea ed M l y Way see pp 296 99) by d rk i aments ying in and round he right nebula The whole a ea THE M LKY WAY is su rounded by a b ue 239 THE M LKY WAY re le tion nebu a par icu ar y consp cuous in the upper part wh re dust par ic es di per e i ht T C CO LI IONS HEART OF THE RI ID f st rs s cr at d wh n wo The ma n mage spa ni g bout 0 ght s co ide He e s ock wav s yea s ev als et i s of t e NGC 6 14 st r p l d ut r g er ng s ar c us er and he f l m nts o dust we v ng on n the nt r te ar ma er al th ou h he T i id Ne ula A w der i w (a ove shows he fu l b ea th of he neb la OLD TO NEW △ FEATURE PROFILES wa es and ma er al r m a Throughout the Guide to the Universe, introductory pages va bl st s re d ut t ro gh are often followed by rs e l r med um t i ge ing profiles of a selection of a fo mat on specific objects. For AR CLUSTERS themed panel (see above) n they h ve fo med rom the ragmen at on of a ingle col aps ng mo ecular young s ars are oft n clu tered toge h r Many ta s a e o med o c o e o the r bors that they are g avi at onal y bound and some are even c ose enough to Sb SP RAL GALAXY er ma eri l It is unusual or a star not to be in a mu tip e ystem such as a bina y Andromeda Galaxy s e pp 274–75) and in this re pect he Sun is uncommon S ars w thin a c ust r y have a im l r chemical compo it on al hough ince succes ive genera ions of AT LOG N MBE S M31 NGC 2 4 may be produced by a s ngle nebu a c uste s may con ain ta s of di fe ent ages I TAN E p 288–89) Remnants of dust and gas f om he in t al cloud wi l inger and the 2 5 m l on i ht y ars g ains of en re lect he s arl ght pr dominan ly n the shor er b ue waveleng hs IAM T R 50 0 0 l gh yea s Thus oung star c uste s re o ten urrounded by M GN TU E 3 4 ANDROM DA T S AR ORMA ION dis inc ive blue ef ect on nebulae Young st rs are hot a t onome s tar l st rs bl e) and tar and br ght and any nearby inte ste lar m ter al w ll The Andromeda G laxy (M31) is thou ht hat M31 and the c ose t ma or al xy o the M lky oth r sp ral nebu ae g eg ons p nk) bound n NGC be hea ed by new s ars h at produc ng red emi s on Way and the la ge t member of he might be so ar sy tems n he As the g la y s gas c l i es nebulae S ars ndiv du l mot ons w ll even ual y Local Group of gal xies I s di k is pro ess of fo ma ion wh le othe s e i te ga a t c m di m hr ugh cause a young tar clu ter to d ss pate though twi e as wide as our ga axy s gue sed cor ect y hat th y were he ga axy s t av l ng he mul ip e s el ar sys ems may remain g avi at on l y ndependent s st ms of m ny ta s g pr ssu e t i gers i le t but M31 s b igh ness and s ze mean it It was in the ea ly 20th cen ury hat has been s udied for onger than any Edwin Hubb e see p 4 ) eve led the g s ar c us er orma on bound and m y move through a ga axy toge her other g laxy Fi st id nt f ed as a i t e t ue n ture of M31 t a tro e huge y c oud by Per i n a t onomer Al Sufi ncre sing e tima es of the s ze of WARD THE MAIN SEQUENCE example, the introduction to ( ee p 421) n a ound ad 964 it was the un ver e see panel oppos t ) s typ cal a star-formation (left) is for cen ur es as umed to be a nebu a As ronomers now know t at M31 pi al ga axy as it l aps ng f agmen s of nebulae cont nue to hrink the r mat er o le ces nd followed by profiles of BEYOND THE MILKY WAY ▷ at a simi ar d st nce o o her obje ts ike the M lky Way is a huge ga axy ppears For example actual star-forming regions This section looks at features in the sky Improved te e copes a tend d by a c us er of smal er d sp te i s hu e ize it ac s o o m p o ost rs These s el ar ledg ings re ease a g eat de l of energy as in the Milky Way (above). found beyond our own galaxy, re ea ed hat th s nebu a ike m ny orb t ng al xies wh ch cca iona ly ppears o be l ss m ss ve including other galaxies and othe s h d a sp ral tru ture Some f ll nw rd un er M31 s g av ty han the Mi ky Way w th a par e con inue o col apse und r h ir own g avi y However hey a e not asi y een galaxy clusters and superclusters, and are to n ap rt h lo of dark ma ter Desp te th s the largest known structures in GALAXY CORE s rophy ic s s al ula e hat M31 s se they a e gene al y sur ound d by he remn nts of he c oud from wh ch h y Th s X ay m ge of t e De pi e b ing int ns vely s udi d ent al bl ck ho e h s he ma s of 30 ce t al a ea of M31 shows the Andromeda G laxy s i l holds mi l on Suns a mo t ten t mes more ed The h at nd pres ure en rated w thin protos ars ac s ag inst the grav ty of nume ous p in X ay many mys er es nd t may not be han the Mi ky Way s cen ral b ack so rc s and a di f se c oud ho e The huge ma s of M31 s b ack ma s oppos ng the col apse Eventua ly mat er at he cen ers of he pro o ta s of g s n ra ge) wh ch ho e is su pri ing becau e a ga axy s is ei g ea ed by sho k b ack hole s hought gene al y o ge s o hot nd dense hat BEYOND THE M LKY WAY waves r m su er ova ef ect he mass of ts pa ent ga axy BEYOND THE M LKY WAY ex lo i ns Fu the mo e s udies t d f er nt nuc ear us on st rts and a st r s wa elen ths have re ea ed d sru tion EXPLORING SPACE n he g l xy s d sk po s bly cau ed by born At this s age s ars are very n encoun er w th one of ts s te l te NTERGALACT C D STANCE al xies n the pa t few mi l on ears uns able They lose ma s by The study of M31 pl yed a key ro e n he d sc very M31 and the Mi ky Way are hat ga axi s xi t eyond our own Al hough he expe l ng s rong s el ar w nds mov ng ow rd ach o her and hey pect a of al xies ugges ed hey shone w th the hould co l de a d beg n o coa e ce ight of coun le s s ars no one could m asu e heir 6) which a e o ten d rect d in two n round 5 b l ion years mmense di t nce In 1923 Edwin oppos ng je s chann led by a Hubble ( ee p 45) p oved that M31 C N RAL BLACK HOLE ay outs de our g l xy He ound the e s d sk of dust and gas that o ms his X r y mage of a m ll rea of M31 s rue di tance of M31 by c lcu a ing nd a ound their equa o s G adual y ore s ows i s cen r l b ack h le as a b ue he um nos t es of i s Cephe d ot—it s coo and i ac i e ompa ed to he v ri ble s ars see pp 2 2–83) nd er At he bal n e between g avi y and al xy s o her X ray ou ces ye l w d ts) el t ng th ir t ue b igh ness o heir app rent magni ude s not pr ssure begins to qu li e and cts re er he s ars set le down on to he t e ame m in equence ( ee pp 234–37) s ar at s b ig te t THE M LKY WAY C phe d va ab e V1 t i s f i te t po ar ADOLESCENT STAR the universe. gas e s T au i ( bov ) is he p ot ty e of a ty e of ado e cent t r th t ac re on is s i l u de go ng gr v ta i nal d sk con r ct ons Th se s ars h ve ex ens ve a cr t on d scs a d v ol nt s e lar w nds oming f om he r po es le t) 332 333 WEST NORTH WEST STAR MAGNITUDES 0 LOOK NG NORTH SCULPTOR STAR MOT ON 20 N Ec p c JANUARY | NORTHERN LATITUDES No h S uh locator shows artwork of 23 NORT EGASUS EN 40 N constellation figure depicted X in context by constellation HW ARIES TH 60 N THE GREEK ALPHABET EST PISCES W SOU TRIANGU UM M33 EST 45 Mi CETU PHO M31 P SCES S ORNAX NDROMEDA POINTS OF REFERENCE Deneb LACERTA Ho zons 0 N 0 N 20 N Zen hs Va ab e M29 CYGNUS M39 HOROLOG UM Astronomers use a convention for sa naming some stars in which Greek LYRA letters are assigned to stars according to DEEP-SKY OBJECTS Ve a CASS ER DANUS their brightness.These letters appear on some of the charts in this book. THE CONSTELLAT ONS 369 Ga a y M57 M52 OPE M 4 PERSE AURUS 2 h 2h CEPHEUS o en t r clu t r NGC 752 THE L ZA D to be a ecu i r 4th m gni ude 0h NGC Ca e la PERSEUS ads ver an a ea l rger han a fu l va ia le s ar has g ven ts name to a CEPHEUS NGC 884 AUR GA S n a d c n be den i ied wi h Lacerta c ass of al xi s w th a t ve nuc ei A M 03 DORADO RET CULUM cu ars but a sma l t l scope s ca l d BL Lac ob ec s or b aza s US LYNX LE AD d d o e olve ts ndi idu l s ars THE IZ RD G bu a 69 DES th magn tu e nd a nter A BL L c o je t is a ty e of uas r c us e NGC 76 2 which s popu ar y I E R NK NG 8 th t shoo s j ts of g s f om i s cen er CAMELOPARDAL S n CAELUM P ne a y wn s the Bl e Sn w all s one di ec ly ow rd the Ea th B cau e HY SOUTH ebu a he e s est lan t ry nebu ae to R GH E T S AR we ee the e je s of gas h ad n th se LOOK NG SOUTH t fy and it an be ound th ough l ha α) 3 8 BL Lac ob ec s t nd o ook s ar ke 50˚ β Open A de a OR ON Be a x M42 a l te e cope ANDROMEDA 9 c us e R ge JANUARY | NORTHERN LATITUDES EN I E ac r ae α4 M38 AUR GA M LEPUS COLUMBA LMC D ue NGC M36 n bu a BB E I T ON L c DRACO o ar s 5 7 43 D se M92 URSA I HE T N SK AT 0 PM ebu a HERCULES M NOR ep em er Oc o er 2 11 U LY I BLE 23h 0 N 33 S 15 6 M7 Be e g use Can pus 0h P CTOR BL 10 M35 Open α alpha ν nu 1 c us e β beta ξ xi γ gamma ο omicron 3 LACERTA P ne a y NI δ delta π pi 87 ebu a GEM ε epsilon ρ rho L ce ta con i ts of a z gz g f fa nt CYGNUS NOC S r us DEEP-SKY OBJECTS G bu a ζ zeta σ sigma t rs n he n rt ern sky s ueez d 0˚ PEGASUS M8 ROS M41 c us e η eta τ tau b tween And om da and C gnus l ke θ theta υ upsilon ψλ a i ard be ween roc s t is one of he M3 MO M47 Adha a PUPPIS ι iota ϕ phi κ ev n c ns el a io s nv nt d by MCAAJNOIRS CAR NA κ kappa χ chi ι ο ohan es H ve ius s e p 384) ur ng Ho zo s 60 N 0 N POINTS OF REFERENCE Cast r M50 λ lambda ψ psi he l te 17 h c ntu y Po lux μ mu ϖ omega N C 662 LACERTA NOR Ga a y Th s con te l t on ont ins M44 no ob ec s of no e or ama eur Ma THE Pro n MI s ronome s l hough BL L cer ae M D y s e p 325) wh ch was o ce thou ht CANIS ANDROMEDA BOOTES BIGER M46 σ THE TR ANG E CBOORROENAAL S P M93 Tr angulum UMRSAAOR CANCER 8 VELA M I E R NK NG 8 α 3h 0 N Z n hs E L TC M67 Va ab e az R GH E T S AR A sa B ta β 3 0 40˚ PERSEUS PEGASUS h M PYX S EN T E T i ng li De embe 5 Da e OBSERVATION TIMES CVENNEAST C MILNEOOR HYDR 23 45 BB E I T ON T i J nua y ANDROMEDA J nua y 5 THEAST I HE T N SKY T 0 PM F b ua y N vem er D cem er R γβ 0 N 40 N 20 N Ec p c F b ua y 5 NORTH I Reg l s ANTL A LEO U LY I I LE δ THE N GHT SKY EAST M3 SOU THE N GHT SKY 0 N 52 S M53 MB64ERCEONMCAES ˚0 M33 M dn gh S nda d M87 LEO SEXTANS 6α pm me 0m EAST STAR MAGNITUDES ANDROMEDA TRIANGULUM 9pm HS A D STORIES 8pm RO C RESCUE ay gh 0 av ng me o ding o Gr ek my ho ogy Th s sma l nor he n co s el a ion AR ES P SCES romeda was c ain d o a ock s to be fou d yi g b tween 2˚ am he se sho e and of er d s a And omeda and Ar es It con i ts M dn gh f ce to a sea mons er n of t le m re th n a t i ng e of ement f r he boa t ul ess of hr e t rs Tr angu um s one of the pm mother Queen Cas i peia ons e la i ns known o he anc ent 0pm G eek hero Pe se s l ing Gr eks w o v su l zed t as the pm e f er s ay ng Med sa he Ni e de ta or t e s and of S ci y gon not ced the ma den s EAST ht He es onded by swoop ng SPEC FIC FEATURES n n is win ed and ls nd T i ngul m con ai s the th rd l rg st THE RIA GLE ng the ea mon t r He th n member of our Lo al Group of ked Andromeda o a ety and g l xi s M33 or he Tr ang lum △ MONTHLY SKY GUIDE chart on this page lines on chart show r ed her Ga axy see p 311) In phy i al t rms This section includes two charts for each shows view looking reference points M33 s ab ut one hi d the s ze of month of the year, for observers in northern north, with view to for observers at S L IN D ST ESS he Andromeda Ga axy or M31 ( ee and southern latitudes. The section is south on facing page different latitudes em sh a i t R be s ad ed he l ng pp 3 2 13) and is much f in er described in more detail on pp.428–29. e e as s to is 7 h c nt r de i t on f med s d ama c r sc e by e se s f om The sp r l ga axy M 3 a pea s i y on he o k s a a ge pa e p t h of ky t is im lar n i e o a u l moon w en v ewed t rough b nocu a s r a m ll T IANGU UM AND MARS 2 M33 54 THE N GHT SKY e es ope on a da k le r nig t o ee T is ma e f t e t ree t rs hat he c ou s of p n i h gas n t e arms f M33 he sp r l arms a a ge te e cope is ma e up t e sh pe of r an u um a so how up n CCD im ges f t is pi a ga axy n eded M33 lo ks l ke a st r i h on nc u es he p an t Mars p ss ng n t e Lo al G o p t is r se ted lm st ong expo ure pho og aphs h ough e ghb r ng P s es a e n to he Ea h The e s i t e l e of ote in t e ons e la i n a art rom 6 T i nguli Th s ye l w s ar has a magn tu e of 5 2 and has a 7 h magn tude omp nion hat can be de ec ed hro gh a m ll el sc pe www.ebook77.com

THE PACIFIC OCEAN This view of Earth, taken from the Space Shuttle, is dominated by the Pacific Ocean. Above the water are clouds of water vapor and a volcanic ash plume, a reminder of the continuing geological activity of the planet’s interior.

A SHORT TOUR OF THE UNIVERSE the night sky has always evoked mystery KENNEDY SPACE CENTER and wonder. Since antiquity, astronomers Many of humankind’s have tried to understand the patterns of first ventures into space the “fixed stars,” and the motions of the were set in motion on the Moon and planets.The motive was partly launch pads of Kennedy a practical one, but there has always been Space Center. This remains a more “poetic” motivation, too—a the busiest launch and quest to understand our place in nature. landing site of the US Modern science reveals a cosmos far space program, and it is vaster and more varied than our ancestors also the main base for the could have envisioned. Space Shuttle. No continents on Earth remain to be THE FLORIDA COAST discovered.The exploratory challenge has The islands and reefs of now broadened to the cosmos. Humans the Florida Keys are seen have walked on the Moon; uncrewed here from Earth orbit. The spacecraft have beamed back views of all reefs are partly made of the planets; and some people now living living organisms, in the may one day walk on Mars. form of corals. To date, life has not been found The stars, fixed in the “vault of anywhere other than on heaven,” were a mystery to the ancients. Earth, but the search for They are still unattainably remote, but we alien life will be perhaps know that many of them are shining even the most fascinating more brightly than the Sun.Within the quest of the 21st century. last decade, we have learned something remarkable that was long suspected: many stars are, like our Sun, encircled by orbiting planets.The number of known planetary systems already runs into hundreds—there could, all together, be a billion in our galaxy. Could some of these planets resemble the Earth, and harbor life? Even intelligent life? All the stars visible to the unaided eye are part of our home galaxy—a structure so vast that light takes a hundred thousand years to cross it. But this galaxy, the Milky Way, is just one of billions visible through large telescopes.These galaxies are hurtling away from each other, as though they had all originated in a “big bang” 13 or 14 billion years ago. But we don’t know what banged, nor why it banged. The beauty of the night sky is a common experience of people from all cultures; indeed, it is something that we share with all generations since prehistoric times. Our modern perception of the “cosmic environment” is even grander. Astronomers are now setting Earth in a cosmic context.They seek to understand how the cosmos developed its intricate complexity—how the first galaxies, stars, and planets formed and how, on at least one planet, atoms assembled into creatures able to ponder their origins.This book sets humanity’s concept of the cosmos in its historic context, and presents the latest discoveries and theories. It is a beautiful “field guide” to our cosmic habitat: it should enlighten and delight anyone who has looked up at the stars with wonder, and wished to understand them better. Martin Rees THE MOON 1.3 light-seconds from Earth The Earth is seen here rising above the horizon of its satellite, the Moon. Our home planet’s delicate biosphere contrasts with the sterile moonscape on which the Apollo astronauts left their footprints.

The Sun OUR LOCAL STAR The Sun dominates the solar system. Our chief source of heat and light, it also holds Earth and the rest of the planets in their orbits. This ultraviolet image reveals the dynamic activity in the ultra-hot corona above the Sun’s visible surface. A SOLAR FLARE The Sun usually appears to the unaided eye as a bright but featureless disk. However, during a total solar eclipse, when light from the disk is blocked out by the Moon, violent flares in the outer layers of the atmosphere can be seen more clearly. ULTRA-HOT CORONA The gas in the Sun’s corona is heated to several million degrees, causing it to emit X-rays, which show up in this image taken by the Japanese YOHKOH satellite. The dark areas are regions of low-density gas that emit a stream of particles, known as the solar wind, into space. PROMINENCES In the corona, electrified gas called plasma forms into huge clouds known as prominences, flowing through the Sun’s magnetic field. As the prominence in this image erupts, it hurls plasma out of the Sun’s atmosphere and into space. SUNSPOTS ON THE SOLAR SURFACE 8 light-minutes from Earth These regions, cooler and darker than the rest of the Sun’s surface, are sustained by strong magnetic fields. Some sunspots are large enough to engulf the Earth. Sunspot numbers vary in cycles that take about 11 years to complete, and peaks in the cycle coincide with disturbances, such as aurorae, in our own atmosphere.





CANYONS ON MARS 4 light-minutes from Earth Mars is one of the solar system’s four inner rocky planets. This image (with exaggerated vertical scale) shows part of the Valles Marineris, a vast canyon system. SATURN AND ITS RINGS 71 light-minutes from Earth There are rings of dust and ice particles in nearly circular orbits around all four of the giant gas planets, but those around Saturn are especially beautiful. This close-up was taken by the Cassini spacecraft. IO 34 light-minutes from Earth Jupiter has 64 known moons—and there are almost certainly others yet to be discovered. Io, Jupiter’s innermost moon, is seen here moving in front of the turbulent face of the planet. 433 EROS 3.8 light-minutes from Earth A vast number of asteroids are in independent orbit around the Sun. Eros is marked by the impact of much smaller bodies. This image was taken by the NEAR–Shoemaker craft from only 60 miles (100 km) above the surface. The planets JUPITER’S GREAT RED SPOT 34 light-minutes from Earth The gas giant Jupiter is more massive than all the other planets in the solar system combined. Its mysterious swirling vortex, the Great Red Spot, has been known since the 17th century, but our knowledge of Jupiter improved greatly when the planet was visited by uncrewed spacecraft in the 1970s. This image of the Great Red Spot was taken in 1979 by Voyager 1, using filters that exaggerate its colors.

Stars and galaxies THE CENTER OF OUR GALAXY 25,000 light-years from Earth The center of our galaxy, the Milky Way, is thought to harbor a black hole as heavy as 3 million Suns. This image reveals flare-ups in X- ray activity close to the event horizon, the point of no return for any objects or light that approach the black hole. CENTAURUS A 15 million light-years from Earth Not all galaxies exist in isolation; occasionally, they interact. Centaurus A is far more “active” than our own galaxy. It has an even bigger black hole than the Milky Way’s, and its gravity may have captured and “cannibalized” a smaller neighbor. THE WHIRLPOOL GALAXY 31 million light-years from Earth The Whirlpool is involved in another case of galaxy interaction. A spinning, disk- like galaxy, viewed face-on, its spiral structure may have been induced by the gravitational pull of a smaller satellite galaxy (at the top of this picture). THE ORION NEBULA 1,500 light-years from Earth The Orion Nebula is a vast cloud of glowing dusty gas within the Milky Way, inside which new stars are forming. The nebula contains bright blue stars much younger than the Sun, and some protostars whose nuclear power sources have not yet ignited.



The limits of time and space GALAXY SUPERCLUSTERS This image, generated by plotting the positions of 15,000 galaxies, depicts the main “topographic” features of our cosmic environment out to 700 million light-years from Earth. The yellow blobs are superclusters of galaxies, which are interspersed with black voids. LARGE-SCALE STRUCTURES This view of the sky, in infrared light, shows how galaxies outside the Milky Way are distributed in clusters and filamentary structures. The galaxies are color-coded according to brightness, with bright ones in blue and faint ones in red. DISTANT CLUSTER OF GALAXIES This massive cluster of galaxies is one of the most distant known to astronomers, some 8.5 billion light-years from Earth. Superimposed on the optical picture is an X-ray image revealing hot gas (shown in purple) that pervades the cluster. DWARF GALAXIES BURSTING INTO LIFE 9 billion light-years from Earth Tiny young galaxies brimming with stars in the process of formation, some 9 billion light-years away, are seen in this image taken at near-infrared wavelengths by the Hubble Space Telescope. They stand out in the image because energy from the new stars has caused oxygen in the gas around them to light up like a neon sign. This phase of rapid star birth is thought to represent an important stage in the formation of dwarf galaxies, the most numerous type of galaxy in the universe.





INTRODUCTION

20 INTRODUCTION

“There are grounds for cautious optimism that we may now be near the end of the search for the ultimate laws of nature.” Stephen Hawking THE UNIVERSE IS ALL OF EXISTENCE— all of space and time and all the matter and energy within it.The universe is unknowably vast, and ever since it formed, it has been expanding, carrying distant regions apart at speeds up to, and in some cases possibly exceeding, the speed of light.The universe encompasses everything from the smallest atom to the largest galaxy cluster, and yet it seems that all are governed by the same basic laws. All visible matter (which is only a small percentage of the total matter) is built from the same subatomic blocks, and the same fundamental forces govern all interactions between these elements. Knowledge of these cosmic operating principles—from general relativity to quantum physics—informs cosmology, the study of the universe as an entity. Cosmologists hope to answer questions such as “How big is the universe?”, “How old is it?”, and “How does it work, on the grandest scale?”. BOW SHOCK AROUND A STAR This mysterious image from the Orion Nebula shows how matter and radiation interact on a stellar scale. A star surrounded by gas and dust has met a fierce wind of particles blowing from a bright young star (out of frame). Around the star, a crescent-shaped gaseous bow shock has built up, like water flowing past the prow of a boat. WHAT IS THE UNIVERSE?

22 WHAT IS THE UNIVERSE? THE SCALE OF THE UNIVERSE galaxy NGC 147 the Andromeda galaxy NGC 185 Galaxy, 2.65 million light- EVERYTHING IN THE UNIVERSE is part of something larger. years from the Milky Way Celestial objects 24–27 The scale of the Earth and its moon may be relatively easy for Expanding space 44–45 the human mind to grasp, but the nearest star is unimaginably Andromeda I The fate of the universe 58–59 remote, and the farthest galaxies are billions of times more The family of the Sun 102–103 distant yet. Cosmologists, who study of the size and structure Andromeda II Andromeda III Triangulum Galaxy The Milky Way 226–29 of the universe, use mathematical models to build a picture of Beyond the Milky Way 300–39 the universe’s vast scale. the stellar neighborhood lies in the Orion Arm of the Milky Way, some 26,000 light-years from its center galactic nucleus THE SIZE OF THE UNIVERSE Cosmologists may never determine exactly how big the universe is. It could be infinite. Alternatively, it might have a finite volume, but even a finite universe would have no center or boundaries and would curve in on itself. So, paradoxically, an object traveling off in one direction would eventually reappear from the opposite direction.What is certain is that the universe is expanding and has been doing so since its origins in the Big Bang, 13.7 billion years ago (see p.48). By Alpha studying the patterns of radiation left from Centauri the Big Bang, cosmologists can estimate the minimum size of the universe, Sun should it turn out to be finite. Some parts must be separated by at least tens of billions of light-years. Since a light-year is the distance that light travels in a year, Sirius (5.878 trillion miles, 5,000 light-years THE MILKY WAY or 9.46 trillion km), The solar system and its stellar the universe is neighbors are a tiny part of the Milky Way galaxy, a disk of 200 bewilderingly big. billion stars and some enormous clouds of gas and dust. The Milky orbit of 5 light- Way is over 100,000 light-years Pluto years across and has a supermassive asteroid black hole at its central nucleus. belt THE STELLAR NEIGHBORHOOD VIEW FROM EARTH Sun The closest star system to the Sun, The Milky Way galaxy is a Earth Alpha Centauri, lies 4.35 light- complex 3-D structure, but years, or 25 trillion miles (40 trillion from our position within it, Earth km), away. Within 20 light-years it appears as a 2-D band of the Sun are 79 star systems across the sky (above). 1 light-hour containing 106 stars. The total includes binary stars—two stars THE SOLAR SYSTEM within the same system. These binary stars include Sirius, the The Earth–Moon system is part brightest star in the sky. Most of of the solar system, comprising the rest are small, dim, red stars. our local star, the Sun, and all INTRODUCTION the Moon 0.5 light-seconds the objects that orbit it, including moves comets 1.6 light-years away. around the THE EARTH AND MOON Neptune, the outermost planet, Earth in is on average 2.8 billion miles a slightly Earth has a diameter of 7,930 (4.5 billion km) from the Sun. elliptical orbit miles (12,760 km), while the diameter of the Moon’s orbit DISTANT OBJECTS around Earth is about 480,000 The red patches in this Hubble miles (770,000 km). A space probe Space Telescope false-color image sent to the Moon takes around are some of the most remote objects two to three days to get there. ever detected. The light from them began its journey toward us about 13 billion years ago.

THE SCALE OF THE UNIVERSE 23 THE LOCAL GROUP THE LOCAL SUPERCLUSTER DISTANT GALAXY CLUSTER OF GALAXIES The vast galaxy cluster Abell 2218 (left) The Local Group of galaxies, is visible from Earth even though it is The Milky Way is one of a cluster together with some nearby galaxy more than 2 billion light-years away. of galaxies, called the Local Group, clusters, such as the giant Virgo that occupies a region 10 million Cluster, is contained within a LARGE-SCALE STRUCTURE light-years across. It contains around vast structure called the Virgo 50 known galaxies, only one of Supercluster. It is 100 million Galaxy superclusters clump into which—the Andromeda Galaxy— light-years across and (if dwarf knots or extend as filaments that is bigger than the Milky Way. Most galaxies are included) contains can be billions of light-years others are small (dwarf) galaxies. tens of thousands of galaxies. long, with large voids separating them. However, at the largest Ursa Minor scale, the density of galaxies, dwarf galaxy and thus all visible matter, in the universe is uniform. Milky Way 250,000 light-years Leo A 10 million light-years THE OBSERVABLE UNIVERSE Although the universe has no edges and may be infinite, the part of it that scientists have knowledge of is bounded and finite. Called the observable universe, it is the 100 million spherical region around Earth from which light has light-years had time to reach us since the universe began.The boundary that separates this region from the rest of the universe is called the cosmic light horizon. Light reaching Earth from an object very close to this horizon must have been traveling for most of the age of the universe, that is, approximately 13.7 billion years.This light must have traveled a distance of around 13.7 billion light-years to reach Earth. Such a distance can be defined as region Planet X a “lookback” or “light-travel- observable from time” distance between both planets Earth and the distant Earth object. However, the true distance is much greater, because since the light arriving at Earth left the object, the object has been carried farther away by the universe’s INTRODUCTION expansion (see p.45). OVERLAPPING observable observable FROM HOME PLANET TO SUPERCLUSTERS OBSERVABLE UNIVERSES universe for Earth universe The universe has a hierarchy of structures. Earth Earth and Planet X—an imaginary for Planet X is part of the solar system, nested in the Milky planet with intelligent life, located Way, which in turn is part of the Local Group. tens of billions of light-years away— cosmic light horizon The Local Group is just part of one of millions of would have different observable for Earth (edge of galaxy superclusters that extend in sheets and universes. These may overlap, as observable universe) filaments throughout the observable universe. shown here, or they may not.

24 WHAT IS THE UNIVERSE? CELESTIAL OBJECTS The family of the Sun 102–103 THE UNIVERSE CONSISTS of energy, space, and matter. Some of Stars 232–33 the matter drifts through space as single atoms or simple gas molecules. Other matter clumps into islands of material, from The life cycles of stars 234–37 dust motes to giant suns, or implodes to form black holes. Extra-solar planets 296–97 Gravity binds all of these objects into the great clouds and disks Types of galaxy 302–303 of material known as galaxies. Galaxies in turn fall into clusters Galaxy clusters 326–27 and finally form the biggest celestial objects of all—superclusters. Galaxy superclusters 336–39 DARK NEBULA GAS, DUST, AND PARTICLES STAR-FORMING NEBULA A globule of dust and dense gas, Barnard 68 The Carina Nebula, a giant cloud of gas, is an example of a dark nebula. The thick Much of the ordinary matter of the universe exists as a thin and is a prominent feature of the sky in the dust obscures the rich star field behind it. tenuous gas within and around galaxies and as an even thinner gas Southern Hemisphere and is visible to the between galaxies.The gas is made mainly of hydrogen and helium naked eye. Different colors in this image atoms, but some clouds inside galaxies contain atoms of heavier represent temperature variations in the gas. chemical elements and simple molecules. Mixed in with the galactic gas clouds is dust—tiny solid particles of carbon or substances such as silicates (compounds of silicon and oxygen).Within galaxies, the gas and dust make up what is called the interstellar medium.Visible clumps of this medium, many of them the sites of star formation, are called nebulae. Some, called emission nebulae, produce a brilliant glow as their constituent atoms absorb radiant energy from stars and reradiate it as light. In contrast, dark nebulae are visible only as smudges that block out starlight. Particles of matter also exist in space in the form of cosmic rays—highly energetic subatomic particles traveling at high speed through the cosmos. INTRODUCTION GLOWING GAS This ocean of glowing gas is an active region of star formation in the Omega Nebula, an emission nebula. Clouds of gas and dust may give birth to stars and planets, but they are also cast off by dying stars, eventually to be recycled into the next stellar generation.



26 WHAT IS THE UNIVERSE? GALAXIES The solar system occupies just a tiny part of an enormous, disk- shaped structure of stars, gas, and dust called the Milky Way galaxy. Until around a hundred years ago, our galaxy was thought to comprise the whole universe; few people QUASAR imagined that anything might exist Some, if not most, outside of the Milky Way.Today, we galaxies are thought know that just the observable part to have been quasars of our universe contains more than earlier in their life. 100 billion separate galaxies.They Quasars are extremely luminous galaxies vary in size from dwarf galaxies, a powered by matter few hundred light-years across and falling into a massive, containing a few million stars, to central black hole. giants spanning several hundred thousand light-years and containing several trillion stars. As well as stars, galaxies contain clouds of gas, dust, and dark matter (see opposite), all held together by gravity.They come in five shapes: spiral, barred spiral, elliptical (spherical to football-shaped), lenticular (lens-shaped), and irregular. Astronomers identify galaxies by their number in one of several databases of celestial objects. For example, NGC 1530 indicates galaxy 1530 in a database called the New General Catalog (NGC). SPIRAL GALAXY BARRED SPIRAL This image taken by In a barred the Spitzer Space spiral galaxy, Telescope shows a such as NGC 1530, nearby spiral galaxy above, the spiral called M81. The sensor arms radiate from the captured infrared ends of the central radiation, rather than barlike structure, rather visible light, and the than from the nucleus. image highlights dust in the galaxy’s nucleus and spiral arms. galactic nucleus, or core spiral arm BLACK HOLES hot gas bubble INTRODUCTION A black hole is a region of space containing, at its center, some matter squeezed into a point of infinite density, called a singularity.Within a spherical region around the singularity, the gravitational pull is so great that nothing, not even light, can escape. Black holes can therefore be detected only from the behavior of material around them; those discovered so far typically have a disk of gas and dust spinning around the hole, throwing off hot, high-speed jets of material or emitting radiation (such as X-rays) as matter falls into the hole. There are two main types of black hole—supermassive and stellar. Supermassive black holes, which can have a mass equivalent to billions of suns, exist in the centers of most galaxies, including our own.Their exact origin is not yet understood, but they may be a byproduct of the process of galaxy formation. Stellar black holes form from the collapsed remains of exploded supergiant stars (see p.267), and may be very common in all galaxies. STELLAR BLACK HOLE GALACTIC BLACK HOLE spinning disk of The black hole SS 433 is situated in the A huge bubble of hot gas rises from a disk of dust dust and gas center of this false-color X-ray image. spinning around what is thought to be a supermassive It is detectable because it is sucking in black hole in the center of a nearby galaxy, NGC 4438. location of matter from a nearby star and blasting black hole out material and X-ray radiation, visible here as two bright yellow lobes.

CELESTIAL OBJECTS 27 GALAXY CLUSTERS Galaxies are bound by gravity to form clusters of about 20 to several thousand. Clusters vary from 3 to 30 million light-years across. Some have a concentrated central core and a well-defined spherical structure; others are irregular in shape and structure. The cluster of galaxies that contains our own galaxy is called the Local Group.The neighboring Virgo Cluster is a large, irregular HICKSON COMPACT GROUP cluster of several hundred galaxies, This cluster includes a face-on spiral lying 50 million light-years away. galaxy in the center of the image, Chains of a dozen or so galaxy two closer oblique spirals, and an clusters are linked loosely by elliptical galaxy at lower right. gravity and make up superclusters, which can be up to 200 million light-years in extent. Superclusters in turn are arranged in broad sheets and filaments, separated by voids of about 100 million light- years across.The sheets and voids form a network that permeates the entire observable universe. RICH CLUSTER One of the most massive galaxy clusters known, Abell 1689 is thought to contain hundreds of galaxies (colored gold here). DARK MATTER AND DARK ENERGY There is far more matter in the universe than is contained in stars and other visible objects.The invisible mass is called “dark matter.” Its composition is unknown. Some might take the form of MACHOs (massive compact halo objects)—dark, planetlike bodies—or WIMPs (weakly interacting massive particles)—exotic subatomic entities that scarcely interact with ordinary matter. Evidence for dark matter includes the motion of galaxies in clusters.They move faster than can be explained by the gravity of visible matter—there must be further mass present. Even if all the dark matter deduced from observations is included, the density of the universe is not sufficient to satisfy theories of its evolution.To find a solution, cosmologists have proposed the existence of “dark energy,” a force that counteracts DARK-MATTER DISTRIBUTION. gravity and causes the universe This image from a computer simulation to expand faster (see p.58).The shows the way in which dark matter exact nature of dark energy is (red clumps and filaments) must be still speculative. distributed within the galaxy superclusters in our local universe. DISTORTED GALAXY EXPLORING SPACE INTRODUCTION Nicknamed “the Tadpole,” this galaxy lies 420 million light-years away. Like any galaxy, it is a vast, spinning THE SEARCH FOR DARK MATTER wheel of matter bound together by gravity. In clusters, gravity can also rip galaxies apart. The streamer of stars To find dark matter, scientists are investigating emerging from this galaxy is thought to have been torn some of the several forms it could take. out by the gravity of a smaller galaxy passing close by. Underground detectors search for evasive particles, such as WIMPs and neutrinos. Neutrinos are so tiny, they were once thought to be massless, but they do have a minute mass.There are so many neutrinos in the cosmos that their combined mass could account for 1–2 percent of the universe’s dark matter.WIMPs, if detected, could account for far more. NEUTRINO DETECTOR Neutrinos are extremely difficult to detect. This instrument is full of oil during operation. The numerous photomultiplier tubes detect flashes of light as neutrinos collide with the oil.

28 WHAT IS THE UNIVERSE? MATTER EMPTY SPACE EXAMINED AT THE TINIEST SCALE, the universe’s Most of an atom is empty— matter is composed of fundamental particles, some the protons, neutrons, of which, governed by various forces, group and electrons are all shown together to form atoms and ions. In addition to here much larger than these well-understood types of matter, other forms their real size relative exist. Most of the universe’s mass consists of this to the whole atom “dark matter,” whose exact nature is still unknown. 24–27 Celestial objects STRUCTURE OF A CARBON ATOM Radiation 34–37 At the center of an atom is the nucleus, which contains protons Space and time 40–43 and neutrons. Electrons move The Big Bang 48–51 around within two regions, called shells, surrounding the Out of the darkness 54–55 nucleus. The shells appear The Sun 104–107 fuzzy because electrons do not move in defined paths. WHAT IS MATTER? OUTER ELECTRON SHELL Region in which four Matter is anything that possesses mass—that is, anything affected by gravity. Most electrons orbit matter on Earth is made of atoms and ions. Elsewhere in the universe, however, INNER ELECTRON SHELL Region within which matter exists under a vast range of conditions and takes a variety of forms, from two electrons orbit thin interstellar medium (see p.228) to the matter in infinitely dense black holes (see p.267). Not all of this matter is made of atoms, but all matter is made of some kind of particle. Certain types of particle are fundamental—that is, they are not made of smaller sub-units.The most common particles within ordinary matter are quarks and electrons, which make up atoms and ions and form all visible matter. Most of the universe’s matter, however, is not ordinary matter, but dark matter (see p.27), LUMINOUS MATTER perhaps composed partly of These illuminated gas clouds neutrinos, theoretical WIMPs in interstellar space are (weakly interacting massive made of ordinary matter, particles), or both. composed of atoms and ions. ATOMS AND IONS Atoms are composed of fundamental particles called quarks and electrons.The quarks are bound in groups of three by gluons, which are massless particles of force.The quark groups form particles called protons and neutrons.These are clustered in a compact region at the center of the atom called the nucleus. Most of the rest of an atom is empty space, but moving around within this space are electrons.These carry a negative electrical charge and have a very low mass—nearly all IMAGING THE ATOM the mass in an atom is in the protons and neutrons. Atoms always contain This image of gold atoms on equal numbers of protons (positively charged) and electrons (negatively a grid of green carbon atoms charged) and so are electrically neutral. If they lose or gain electrons, they was made by a scanning- become charged particles called ions. tunneling microscope. emitted ABSORPTION AND EMISSION photon The electrons in atoms can exist incoming photon in different energy states. By electron at low moving between energy states, energy state they can either absorb or emit packets, or quanta, of energy. nucleus These energy packets are nucleus called photons. electron falls back to EMISSION electron at high red quark lower energy state energy state gluon green quark ABSORPTION electron raised to incoming high- ejected electron higher energy state energy photon (charge -1 ) INTRODUCTION inner-shell electron nucleus nucleus electron in empty blue quark outer shell shell neutron neutron proton proton ATOM ION (CHARGE +1) (NEUTRAL, NO CHARGE) IONIZATION One way an atom may become a positive ion is by the electron’s absorbing energy from a high-energy photon and, as a result, being ejected, along with its charge, from the atom.

MATTER 29 NUCLEUS CHEMICAL ELEMENTS NIELS BOHR A tightly bound ball Atoms are not all the same—they can hold different numbers of protons, Danish physicist Niels Bohr (1885– of six protons (purple) neutrons, and electrons. A substance made of atoms of just one type is 1962) was the first to propose that and six neutrons (gold) electrons in an atom move within called a chemical element, and is given an atomic number equal to the discrete “orbits.” He suggested that number of protons, and thus electrons, in its atoms. Examples are these orbits have fixed energy levels hydrogen, with an atomic number of 1 (all hydrogen atoms and that atoms emit or absorb contain one proton and one electron), helium (atomic energy in fixed amounts (“quanta”) number 2), and carbon (number 6). Altogether, there as electrons move between orbits. are 90 naturally occurring elements.The atoms Bohr’s orbits are today called of any element are all the same size and, crucially, contain the same configuration orbitals; they are of electrons, which is unique to that substructures element and gives it specific chemical of electron properties.The universe once shells. consisted almost entirely of the lightest elements, hydrogen and helium. Most of the others, including such common ones as oxygen, carbon, and iron, have largely been created in stars and star explosions. HYDROGEN ALUMINUM A colorless gas at 70°F (21°C). Its atoms A solid metal at 70°F have just 1 proton (21°C). Its atoms and 1 electron in have 13 protons, a single shell. 14 neutrons, and 13 electrons in 3 shells. PROPERTIES OF ELEMENTS Elements vary markedly in their properties, as shown by the four examples here. These properties are determined by the elements’ different atomic structures. SULFUR BROMINE A fuming brown A yellow, brittle liquid at 70°F (21°C). solid at 70°F (21°C). Its atoms have 35 Its atoms have 16 protons, 44 or 46 protons, 16–18 neutrons, and 35 neutrons, and 16 electrons in 4 shells. electrons in 3 shells. sodium ion CHEMICAL COMPOUNDS chloride ion Most matter in the universe consists of unbound atoms or ions of a few chemical elements, but a significant amount exists as compounds, containing atoms of more than one element joined by chemical bonds. Compounds occur in objects such as planets and asteroids, in living organisms, and in the interstellar medium. In ionic INTRODUCTION compounds, such as salts, atoms trade electrons, and ELECTRON the resulting charged ions are bonded Electrons have a by electrical forces, and arranged in a IONIC COMPOUND negative charge rigid, crystalline structure. In covalent Compounds of this and a mass more INSIDE A NEUTRON than a thousand compounds, such as water, the atoms are type consist of the Protons and neutrons are each made times smaller than held in structures called molecules by the ions of two or more of three quarks, bound by gluons. a proton or neutron sharing of electrons between them.Two chemical elements, The quarks flip between “red,” or more identical atoms can also combine typically arranged in a “green,” and “blue” forms, but repeating solid structure. This there is always one of each color. to form molecules of certain elements. example is salt, sodium chloride.

30 WHAT IS THE UNIVERSE? STATES OF MATTER Ordinary matter exists in four states, called solid, liquid, gas, and plasma. These differ in the energy of the matter’s particles (molecules, atoms, or ions) and in the particles’ freedom to move relative to one another. Substances can transfer between states—by losing or gaining heat energy, for instance.The constituents of a solid are locked by strong bonds and can hardly move, whereas in a liquid they are bound only by weak bonds and can move freely. In a gas, the particles are bound very weakly and move with greater freedom, occasionally colliding. A gas becomes a plasma when it is so hot that collisions start to knock electrons out of its atoms. A plasma therefore consists of ions and electrons moving extremely energetically. Because stars are made of plasma, it is the most common state of ordinary matter in the universe; the gaseous state is the second most common. SOLID, LIQUID, AND GAS On Earth, water can sometimes be found as a liquid, in solid form (ice or snow), and as a gas (water vapor), all in close proximity. FORCES INSIDE MATTER The bonds that link the constituents of solids, liquids, gases, and plasma are based on the electromagnetic (EM) force.This force attracts particles of unlike electrical charge and repels like charges. It is one of three forces that control the small-scale structure of matter.The others are the strong nuclear force, composed of fundamental and residual parts; and the weak nuclear force or interaction.Together with a fourth PLASMA Plasma exists naturally force, gravity, these are the fundamental forces of nature.The EM, weak, and strong in stars but can also be artificially created. In a neutron forces are mediated by force-carrier particles, plasma ball, electricity is which belong to a group of particles called the induced to flow from a charged metal ball through red down quark bosons.The EM force, as well as binding atoms a gas to the surface of a glass sphere, creating fundamental in solids and liquids, also holds electrons within plasma streamers. strong nuclear atoms.The strong force holds together force protons, neutrons, and atomic nuclei.The proton gluon, weak force brings about radioactive decay ++ STEVEN WEINBERG the force and other nuclear interactions. pion, the American physicist Steven particle proton force-carrier Weinberg (b. 1933) is best known particle for his theory that two of the green up RESIDUAL STRONG residual fundamental forces—the weak quark NUCLEAR FORCE strong interaction and the electromagnetic This force binds the protons nuclear force force—are unified, or work in an blue and neutrons together in neutron identical way, at extremely high atomic nuclei. It is carried energy levels, such as those existing FUNDAMENTAL STRONG NUCLEAR FORCE down by particles called pions. just after the Big Bang (see p.48). quark Pions are generated from Weinberg’s so-called electroweak Also known as the color force, this force energy created when nucleons try theory was confirmed by particle to move apart. This energy arises as accelerator experiments in 1973. binds quarks within protons and neutrons. a byproduct of the fundamental strong He and his colleagues received the force. Once generated, pions are 1979 Nobel prize for physics. It controls the quarks’ “color” property, and as it exchanged back and forth between the nucleons, creating a binding force. operates, the quarks constantly change “color” by exchanging virtual gluons (the force-carrier particles). electromagnetic – force electrical charge neutrino neutrino electron transformed + W + boson into negatively − exchanged charged electron weak nuclear force between neutron neutron and neutrino neutron transformed into INTRODUCTION photon, the force- down up quark down quark positively carrier particle quark transformed charged proton into up quark proton down + quark ELECTROMAGNETIC FORCE up quark Within an atom, the electromagnetic (EM) force holds the electrons within the shells surrounding down quark the nucleus. It attracts the negatively electrically charged electrons toward the positively charged WEAK INTERACTION, OR WEAK NUCLEAR FORCE nucleus and keeps electrons apart. The force carrier for the EM force is the photon. This force governs radioactive decay, among other interactions. Its force- carrier particles are W+, W–, and Z0 bosons. Here, a W+ boson controls the changing of a neutrino into an electron and the transformation of a down quark into an up quark, converting a neutron into a proton.

MATTER 31 PARTICLE PHYSICS CLASSIFICATION OF PARTICLES For some decades, physicists have directed Physicists distinguish composite particles, which have an internal structure, from research toward a better understanding of matter fundamental particles, which do not. They also divide particles into fermions and and the four fundamental forces. Part of the bosons. Fermions (leptons, quarks, and baryons) are the building blocks of matter. purpose has been to clarify what happened in Bosons (gauge bosons and mesons) are primarily force-carrier particles. the early universe, shortly after the Big Bang. Research is centered on smashing particles FUNDAMENTAL PARTICLES COMPOSITE PARTICLES together in devices called particle accelerators. These experiments have identified hundreds Leptons and quarks form matter, while gauge bosons carry Also known as hadrons, of different particles (most of them highly forces. Quarks feel the strong nuclear force, but leptons do not. these are composed of unstable), which differ in their masses, electric quarks, antiquarks, or charges, other properties such as “spin,” and LEPTONS QUARKS both, bound by gluons. in the fundamental forces they are subject to. Known particles, and their interactions, electron, up, charge +2⁄3 BARYONS are currently explained by a theory called charge –1 the standard model of particle physics.This Relatively large-mass explains the properties of most of the particles neutrino, charge 0 down, charge –1⁄3 particles containing 3 quarks. (see table, right). One exception is the graviton, a hypothetical particle thought to carry the Six different leptons exist, There are 6 “flavors” of quark, proton, 1 down force of gravity.The graviton does not fit into but the 2 above are the only but only 2 occur in ordinary and 2 up quarks, the scheme, because the best theory of gravity stable ones and are those matter: “up” and “down”. Each charge +1 that occur in ordinary matter. can exist in any of 3 “colors.” (general relativity, see neutron, 1 up pp.42–43) is incompatible GAUGE BOSONS and 2 down with aspects of the standard model. New These are force-carrier particles. Some shown are hypothetical. quarks, charge 0 theories such as string theory (see panel, below) photon W + intermediate MESONS attempt to unite gravity gluon vector boson with particle physics. Particles containing a quark X-boson Higgs and an antiquark. SPRAY OF PARTICLES (hypothetical) boson This image from a detector within a (hypothetical) positive pion, 1 up particle accelerator shows a spray graviton quark, 1 anti-down of light particles shooting to the (hypothetical) quark, charge +1 right, following collision of two higher-mass particles on the left. Hundreds of other baryons and mesons exist. ANTIPARTICLES EXOTIC PARTICLES Most particles have an antimatter equivalent that has the same Further particles have been hypothesized that mass, but whose charge and other properties are opposite. do not have a place in this particle classification. positron anti-up quarks, They include magnetic (antielectron), charge –2⁄3 monopoles and WIMPs charge +1 (weakly interacting antineutrino massive particles). antiproton,1 anti- antineutron, 1 anti- down and 2 anti-up up and 2 anti-down quarks, charge –1 quarks, charge 0 NUCLEAR FISSIONhydrogen nucleus (single proton) EXPLORING SPACE hydrogen nuclei fuse, AND FUSION STRING THEORY and one is converted into a neutron Twentieth-century physicists learned that atomic nuclei are For decades, physicists have sought not immutable but can break up or join together. In nature, a “theory of everything” (see neutrino Quantum gravity, p.43) that will unify the four fundamental forces hydrogen positron unstable atomic nuclei can spontaneously disassemble, giving of nature and provide an underlying nucleus off particles and energy, measured as radioactivity. Similarly, scheme for how particles are in the artificial process of nuclear fission, large nuclei are constructed. A leading contender is string theory, which proposes deuterium nucleus intentionally split into smaller parts, with huge energy that each fundamental particle (1 proton, 1 neutron) release. On a cosmic scale, a more important phenomenon consists of a tiny vibrating filament called a string.The vibrational gamma-ray helium-3 nucleus is nuclear fusion. In this process, atomic nuclei join, forming modes, or frequencies, photon of these strings lend (2 protons, 1 neutron) a larger nucleus and releasing energy. Fusion powers stars particles their varied properties. Although addition of and has created the atoms of all chemical elements heavier it sounds bizarre, another proton than beryllium.The most common fusion reaction in stars many leading releases energy joins hydrogen nuclei (protons) into helium nuclei. In this physicists are enthusiastic and other fusion reactions, the products of the reaction adherents of string theory. LOW-FREQUENCY have a slightly lower mass than the combined mass of the STRING fusion of helium-3 hydrogen reactants.The lost mass converts into huge amounts of nuclei forms stable nucleus energy, in accordance helium-4 and with Einstein’s famous releases excess equation E=mc2 that protons links energy (E), mass hydrogen helium-4 nucleus (m), and the speed of VIBRATING STRINGS INTRODUCTION nucleus (2 protons, light (c) (see p.41). A string is closed, 2 neutrons) like a loop, or open, like a hair. The two FUSION REACTION IN THE SUN THE HEAT OF FUSION closed strings shown HIGH-FREQUENCY In stars the size of the Sun or smaller, the dominant All solar energy comes from here are vibrating STRING energy-producing fusion process is called the proton– nuclear fusion in the Sun’s at different resonant proton chain. This chain of high-energy collisions fuses core. The energy gradually frequencies, just hydrogen nuclei (free protons), via several intermediate migrates to the Sun’s surface as the strings on stages, into helium-4 nuclei. Energy is released in the form and into space through heat a guitar have rates of gamma-ray photons and in the movement energy of the transfer by convection, at which they prefer helium nuclei. Positrons and neutrinos are also produced. conduction, and radiation. to vibrate.

NEUTRINO OBSERVATORY High-energy processes in the Universe produce neutrinos—fast-moving particles that rarely interact with matter. To detect them, scientists created the IceCube Neutrino Observatory in Antarctica. Eighty-six holes drilled in the ice contain over 5,000 optical sensors. In the dark, clear ice the sensors record faint flashes of light as neutrinos crash into the ice molecules.



34 WHAT IS THE UNIVERSE? RADIATION 28–31 Matter RADIATION IS ENERGY IN THE FORM of waves or particles Light and gravity 42 that are emitted from a source and can travel through space and some types of matter. Electromagnetic (EM) radiation Beyond visible light 91 includes light, X-rays, and infrared radiation. Particulate Observing from space 94–95 radiation consists of fast-moving charged particles such as cosmic rays and particles emitted in radioactive decay. Stars 232–33 EM radiation is vastly more significant in astronomy. electric amplitude ELECTROMAGNETIC field RADIATION strength magnetic field strength wavelength Energy in the form of EM radiation is one of the two main components of the universe, the other being matter (see p.28).This HOW WAVES TRAVEL type of radiation is produced by the motion of electrically charged An EM wave consists of particles, such as electrons. A moving charge gives rise to a magnetic oscillating electrical and field. If the motion is constant, then the magnetic field varies and in magnetic fields arranged turn produces an electric field. By interacting with each other, the two perpendicular to each other, fields sustain one another and move through space, transferring energy. and carrying energy forward. As well as visible light, EM radiation includes radio waves, microwaves, infrared (heat) radiation, ultraviolet radiation, X-rays, and gamma rays. All these phenomena travel through space at the same speed—called the velocity of light.This speed is very nearly 186,000 miles (300,000 km) per second or 670 million mph (1 billion km/h). WAVELIKE BEHAVIOR PARTICLE-LIKE BEHAVIOR In most situations, EM radiation acts as a wave—a disturbance moving EM radiation behaves mainly like a energy from one place to another. It has properties such as wavelength wave, but it can also be considered to consist of tiny packages or (the distance between two successive peaks of the wave) and frequency “quanta” of energy called photons. Photons have no mass but carry a (the number of waves passing a given point each second).This wave- fixed amount of energy.The energy in a photon depends on its wavelength— like nature is shown by experiments such as the double-slit test (see the shorter the wavelength, the more energetic the photon. For example, photons of blue (short- below), in which light waves diffract (spread out) after passing through wavelength) light are more energetic than photons of red (long-wavelength) light. A classic demonstration of light’s a slit and also interfere with particle-like properties is provided by a phenomenon called the photoelectric effect (see below). If a blue light shines on a metal INTERFERING WAVES each other as their peaks and surface, it causes electrons to be ejected from the metal, whereas even a very bright red light has no such effect. The slit experiment troughs overlap. The forms ultra-high-energy is analogous to of EM radiation differ only ultraviolet photon disturbing two points in wavelength, but this affects on the surface of a other properties, such as liquid. The ripples penetrating power and ability interfere to corrugate the liquid’s surface. to ionize atoms (see p.28). light waves along red paths combine to cast bright band on screen pattern of light falling on screen low-energy high-energy photon of photon of red light blue light low-energy electron slit INTRODUCTION light source gold foil electron ejected at SLIT EXPERIMENT light waves forming RED LIGHT BLUE LIGHT higher energy If light shines on a card containing two interference pattern When red light shines on a Blue light shining on the same slits, diffraction spreads the light waves out metal surface, no electrons surface causes electrons to ULTRAVIOLET LIGHT like ripples emanating in arcs from each slit. are ejected, even if the light be ejected, because the blue Shining ultraviolet light on The two wave trains then interfere to produce is intensely bright. photons are more energetic. the metal surface causes a light and dark pattern on the screen. electrons to be ejected at very high energy.

RADIATION 35 ANALYZING LIGHT The radiation output of celestial objects is a mixture of wavelengths.When passed through a prism, the light is split into its component wavelengths, giving a record called a spectrum. A star’s spectrum usually contains dark lines called absorption lines, caused by photons being absorbed at certain wavelengths by atoms in the star’s atmosphere.They can be used to establish what chemical elements are present.The spectrum of a nebula can also reveal its composition.When heated by radiation from a nearby star, its atoms emit their own light.The resulting spectrum, called an emission radiating star absorption by spectrum, consists of a series of nebula bright lines characteristic of different elements. radiation remaining after absorption prism emission by heated gas direct radiation SPECTRUM WITH AN from star ABSORPTION LINE When a star is viewed SPECTRUM WITH through a cooler gas, dark lines appear in the AN EMISSION LINE spectrum. These are caused by atoms in the A gas that has been heated gas absorbing energy at specific wavelengths. by energy from a local star very hot reemits radiation at specific INTENSITY blue star EMISSION NEBULA wavelengths. When viewed hot yellow star, such This nebula glows as its gas is CONTINUOUS SPECTRUM obliquely, this produces an as the Sun heated by nearby stars. The emitted A hot, dense gas such as a star emission-line spectrum. cooler red star light consists of photons of a few specific produces a continuous light Earth wavelengths. These photons were emitted spectrum from its surface, with RADIATION FROM by the gas’s atoms as their electrons all different light wavelengths HOT OBJECTS WAVELENGTH settled to lower energy states. (colors) represented. Not only is the total wavefronts radiation greater from bunched up hotter objects, but the RED SHIFT AND BLUE SHIFT wavelength of peak intensity is also shorter LOUIS DE BROGLIE The spectrum of radiation received by an observer shifts if the (toward the blue end source of the radiation is moving relative to the observer—and of the light spectrum). The French physicist Louis de celestial objects are always moving. Astronomers can detect the Astronomers can calculate Broglie (1892–1987) received the shifts by measuring the position of spectral lines, which occur at the temperature of stars Nobel prize in 1929. He found characteristic places. Observers watching an object moving away by measuring the peak that particles of matter, such see its spectral lines shifted toward longer wavelengths (a red of the star’s spectrum. as electrons, have wavelike shift). For an approaching object, the lines are shifted to the properties.The dual nature of galaxy receding matter and light (each has both shorter wavelengths (a blue shift).The greater from observer 1 particle-like and wavelike and approaching properties) the relative velocity between source and observer 2 is called wave- observer, the greater the shift. Distant particle duality. galaxies show large red shifts, wavefront of indicating they are receding at emitted radiation enormous speeds; these are called cosmological red shifts. wavefronts spread out SHIFTING WAVELENGTHS OBSERVER 1 OBSERVER 2 INTRODUCTION Shifts occur because of a phenomenon called BLUE-SHIFTED SPECTRUM LINE the Doppler effect. The wavefronts of light RED-SHIFTED from a receding object are stretched out, SPECTRUM LINE increasing their wavelength, while those of an approaching object are squashed up.

36 WHAT IS THE UNIVERSE? ACROSS THE SPECTRUM Celestial objects can emit radiation across the EM spectrum, from radio waves through visible light to gamma rays. Some complex objects, such as galaxies and supernova remnants, shine at nearly all these wavelengths. Cool objects tend to radiate photons with less energy and are therefore only visible at longer wavelengths.Toward the gamma-ray end of the spectrum, photons are increasingly powerful. High-energy X-rays and gamma rays originate only from extremely hot sources, such as the gas of galaxy clusters (see p.327) or violent events, such as the swallowing of matter by black holes (see p.267).To detect all this radiation and form images, astronomers need a range of instruments—each type of radiation has different properties and must be collected and focused in a particular way. Radiation at many wavelengths does not penetrate to Earth’s surface, and is detectable only by orbiting observatories above the atmosphere. RADIO WAVES TELESCOPE ARRAY parabolic dish reflects all primary INFRARED MOUNTAINTOP TELESCOPE Radio waves can be many meters long. To create incoming radio waves to reflector Little infrared radiation from space reaches sea sharp images from such long waves, astronomers the subreflector dish level on Earth, but some penetrates down to the collect and focus them using telescopes with huge height of mountaintops. Some infrared telescopes, dish antennae. They might use a single dish or an receiver Sun shield such as NASA’s Spitzer Space Telescope (see entire array. The Very p.247), have been launched into space, but most Large Array (right), in subreflector focuses the infrared astronomy is conducted from mountain New Mexico, is the radio waves onto receiver observatories. This one, the United Kingdom world’s largest array. Infrared Telescope (UKIRT) is at 13,760 ft (4,194 m) It consists of 27 MICROWAVES SPACE PROBE in Hawaii. Like optical telescopes, it uses mirrors dishes, each 82 ft Most microwaves are absorbed by Earth’s atmosphere, to collect and focus the radiation. With a 12.5-ft (25 m) across, moving so microwave observatories must be placed in space. (3.8-m) mirror, UKIRT achieves great brightness on a Y-shaped rail Launched in 2001, the Wilkinson Microwave Anisotropy and resolution. It can pick up dim galaxies, brown network. Their data Probe (WMAP, above) is a NASA spacecraft with a dwarfs, nebulae, and interstellar molecules combines to form a goal to map the cosmic microwave background radiation glowing in the infrared, and it can peer into star- single, fine-detailed (see p.54) across the whole sky. This is the oldest image, the dishes electromagnetic radiation in the universe, released forming nebulae to image effectively forming soon after the Big Bang. The probe the young stars one giant, 16-mile was placed in a stable orbit shining within. (27-km) antenna. around the Sun 900,000 miles (1.5 million km) from Earth. red denotes a fractionally higher temperature RADIO WAVES GALAXY blue denotes INFRARED In this map of the Andromeda Galaxy produced by a radio telescope, a slightly lower red and yellow indicate the highest-intensity radio-wave emissions. temperature GALACTIC CENTER To produce such an image, a radio dish must scan an area of sky. As This infrared image penetrates to the central it points at each location in the sky in turn, the telescope gradually MICROWAVES UNIVERSE region of the Milky Way galaxy, which in visible builds a picture by recording the radio intensity at each location. The The lack of microwave sources in the nearby universe light is hidden behind thick clouds of dust. The core resolution is low because radio waves are so long. Radio emissions is fortunate, because it reduces difficulties in observing are produced by hydrogen clouds in galaxies, or by synchrotron the cosmic background radiation, which reaches Earth at of the galaxy is at upper left. The reddening of radiation from active galaxies (see p.320) and black holes (see p.267). the stars in that area and along the galactic microwave wavelengths. The pattern of microwaves plane is caused by dust scattering. from the whole sky, as measured by WMAP, is here projected onto two hemispheres. RADIO WAVES 10 m 1 m 10 cm MICROWAVES INFRARED 10 μm 1 km WAVELENGTH 100 m 1 cm 1 mm 100 μm INTRODUCTION 60 miles opaque atmosphere at RADIO WINDOW HEIGHT IN EARTH’S ATMOSPHERE (100 km) long radio wavelengths ATMOSPHERIC ABSORPTION Radiation with wavelengths between Only certain types of EM radiation—visible 1 cm and 11 m (0.4 in–36 ft) passes light and some radio waves—can pass readily through the atmosphere. through Earth’s atmosphere. Others are This part of the spectrum, absorbed to varying extents, and can which includes some only be detected from space or at radio waves and some 30 miles high altitudes. Gray areas microwaves, is called (50 km) indicate the altitude at which transparent the “radio window.” opaque different wavelengths are atmosphere at shorter absorbed. atmosphere radio wavelengths 0

RADIATION 37 EXPLORING SPACE IMAGES FROM INVISIBLE RADIATION Astronomers have developed telescopes that can gather information from EM radiation other than visible light, but they still face a problem of how to visualize the invisible.The 1 High-energy 2 Low-energy 3 Image in infrared most popular technique uses computers to create “false- (short-wavelength) (longer-wavelength) radiation, taken by color” images—pictures that show the object in particular X-ray image from X-ray image from the Spitzer Space Chandra Observatory. Chandra Observatory. Telescope. wavelengths of radiation, sometimes (see p.273) show radiation in visible light, COMBINED IMAGE The false-color images are color-coded, but often just using infrared, and two different wavelengths of combined with a Hubble image of the remnant in visible light. varying intensities of a single color. X-rays, revealing the temperature of different These images of Kepler’s Supernova regions and the overall structure. solar telescope solar nested grazing array body panel incidence mirrors VISIBLE LIGHT OPTICAL TELESCOPE sunshade Optical telescopes with the largest mirrors door achieve the brightest, sharpest images and the greatest power (see p.82). They range from ULTRAVIOLET X-RAYS those of amateur astronomers, such as this example with an 8.5-in (21.5-cm) mirror, to ORBITING OBSERVATORY ORBITING OBSERVATORY those of large observatories, with mirrors NASA’s Extreme Ultraviolet Explorer X-rays are highly energetic and up to 33 ft (10 m) wide. Other telescopes satellite (above) surveyed sources of so powerful that they pass through include the 98-ft (30-m) Thirty Meter Telescope extreme (very-short wavelength) ultraviolet conventional mirrors. To focus X-rays, and the 128-ft (39-m) European Extremely Large radiation during the 1990s. Ultraviolet light telescopes such as the Chandra X-ray Telescope (E-ELT). originates from hot sources such as white Observatory (above) use a nest of curved dwarfs, neutron stars, and Seyfert “grazing incidence” mirrors of polished galaxies (see p.320). metal. X-rays glance off these mirrors, like GAMMA-RAY ricocheting bullets, toward the focal point. ORBITING TELESCOPE Gamma rays are the most energetic EM waves, emitted by the most violent cosmic events. The Fermi Gamma-Ray Space Telescope (above) was launched in 2008 to study gamma rays from phenomena such as supernovae, black holes, pulsars, and gamma-ray bursts. point source Milky Way GAMMA-RAY SKY Gamma rays are too powerful to focus, VISIBLE LIGHT GALAXY ULTRAVIOLET GALAXY X-RAY GALAXY so sharp images are impossible. This The spiral galaxy M90, which lies 30 million light-years away, is shown here as it appears This image of spiral galaxy M74 is a composite The orange-pink regions in this Chandra view of the sky shows the Milky Way to human eyes through a large telescope. This galaxy is similar in size to the Milky Way. The of visible light and ultraviolet images. The Observatory image of two colliding galaxies as a bright band. Point sources may be image was taken at Kitt Peak National Observatory in Arizona. high-energy ultraviolet emission is in blue (called the Antennae, see p.317) are X-ray- neutron stars or hypernovae (see p.55). and white and picks out extremely hot, emitting “superbubbles” of hot gas. The The image comes from the Energetic young stars in the spiral arms and point X-ray sources (bright spots) are Gamma Ray Experiment Telescope in the galactic nucleus. black holes and neutron stars. (EGRET) on the Compton Observatory. VISIBLE ULTRAVIOLET X-RAYS 1 nm 0.1 nm GAMMA RAYS 0.001 nm 0.0001 nm 0.00001 nm 1 μm 100 nm 10 nm 0.01 nm transparent OPTICAL WINDOW opaque atmosphere atmosphere Wavelengths of radiation between 300 and 1,100 INTRODUCTION nm (nanometers) pass easily through the atmosphere (the visible light spectrum extends from 400 to 700 nanometers).

38 WHAT IS THE UNIVERSE? GRAVITY, MOTION, AND ORBITS Space and time 40–41 GRAVITY IS THE ATTRACTIVE FORCE that exists between every DISKS AND RINGS Planetary motion 68–69 object in the universe, the force that both holds stars and galaxies The disk- and ringlike structures common Observing from space 94–95 together and causes a pin to drop. Gravity is weaker than nature’s in celestial objects are maintained by The family of the Sun 102–103 other fundamental forces, but because it acts over great distances, gravity. Examples include Saturn’s and between all bodies possessing mass, it has played a major part in rings (pictured), spiral galaxies, and the disks around black holes. Every shaping the universe. Gravity is also crucial in determining orbits and creating particle in Saturn’s rings is held in orbit through gravitational phenomena such as planetary rings and black holes. interactions with billions of other particles and with Saturn itself. NEWTONIAN GRAVITY NEWTON’S LAWS OF MOTION The scientific study of gravity began with Galileo Galilei’s From his studies of gravity and the motions of heavenly bodies, and demonstration, in about 1590, that objects of different weight again extending concepts first developed by Galileo, Newton formulated fall to the ground at exactly the same, accelerating rate. In his three laws of motion. Before Galileo and Newton, people thought 1665 or 1666, Isaac Newton realized that whatever force that an object in motion could continue moving only if a force acted causes objects to fall might extend into space and be on it. In his first law of motion, Newton contradicted this idea: he stated responsible for holding the Moon in its orbit. By analyzing the that an object remains in uniform motion or at rest unless a net force acts motions of several heavenly objects, Newton formulated his on it (a net force is the sum of all forces acting on an object). Newton’s law of universal gravitation. It stated that every body in the second law states that a net force acting on an object causes it to accelerate universe exerts an attractive force (gravity) on every other (change its velocity) at a rate that is directly proportional to the magnitude body and described how this force varies with the masses of of the force. It also states that the smaller the mass of an object, the higher the the bodies and their separation.To this day, Newton’s law acceleration it experiences for a given force. Newton’s third law states that for remains applicable for understanding and predicting the every action there is an equal and opposite reaction—for example, Earth’s movements of most astronomical objects. gravitational pull on the Moon is matched by the pull of the Moon on Earth. F F 1 Two bodies, each FIRST LAW OF MOTION m The first law states that m of mass m, attract an object remains in a constant, uniform motion altered motion each other with state of rest or moves at a constant speed in a force F distance = 1 4F 4F 2 Doubling the mass of each straight line unless acted force 2m 2m body, while maintaining their on by a net force. low mass, high acceleration separation, quadruples the gravitational force to 4F SECOND LAW OF MOTION When an object of low distance = 1 F F mass and one of greater high mass, slow acceleration 2m 2m mass are subjected to a force of the same distance = 2 magnitude, the low-mass object accelerates at a 3 Doubling the separation between the bodies higher rate. reduces the force by a factor of 4, back to F MASS AND DISTANCE THIRD LAW OF MOTION action: backward reaction Any two bodies are attracted by To every action there is blast of fuel a force of gravity proportional to an equal and opposite ISAAC NEWTON the mass of one multiplied by the reaction. The forward mass of the other. The force is thrust of a rocket is a The English mathematician and also inversely proportional to the reaction to the backward physicist Isaac Newton (1642–1727) square of their separation. blast of combusted fuel. was one of the greatest-ever scientific intellects. As well as his WEIGHT AND FREE FALL discoveries in the fields of gravity and motion, he co-discovered the The size of the gravitational force acting on an object is called mathematical technique of calculus. In 1705, Newton became the first its weight. An object’s rest mass (measured in pounds or scientist to be kilograms) is constant, while its weight (measured in knighted for his newtons) varies according to the local strength of gravity. A work. mass of 2.2 lb (1 kg) weighs 9.8 newtons on Earth, but only 1.65 newtons on the Moon.Weight can be measured, and the feeling of weight experienced, only when the gravity producing INTRODUCTION it is resisted by a second, opposing, force. A person standing on Earth feels weight not so much from the pull of gravity as from the opposing push of the ground on his or her feet. In contrast, a person orbiting Earth is actually falling toward Earth under gravity. WEIGHTLESSNESS Such a person is in “free fall” Astronauts in training must and experiences apparent frequently experience apparent weightlessness.This is due not to weightlessness. Here, a plane lack of gravity but to the absence is plunging sharply from high of a force opposing gravity. altitude, putting the trainee astronauts inside into free fall.

39 COMMON CENTER OF MASS In an orbital system of two bodies, the smaller body does not simply smaller body pivot: center orbit the larger one. Instead, both (smaller star of rotation of revolve around the joint center of or planet) both bodies mass. In the Earth–Moon system, this point is located deep inside SHAPES OF ORBITS Earth. For two bodies of more equal mass, it is located in space When one object is in orbit around another object of higher mass, it is in free between the two fall toward the larger body. It experiences a constant gravitational acceleration objects. toward the larger object that deflects what would otherwise be its straight- line motion into a curved trajectory.The direction of its motion, and the common center smaller orbit of direction of acceleration both constantly change, producing its curved of mass larger body path. All closed orbits in nature have the shape of an ellipse (a stretched circle).The degree to which an ellipse varies from a perfect circle is larger orbit of called its eccentricity. Many solar system orbits (such as the Moon’s around smaller body Earth) are not very eccentric—that is, they are almost circular. Others, such as larger body Pluto’s orbit around the Sun, are much more eccentric and highly elongated. (massive star) Some celestial bodies follow open, non-returning orbits, along curves with shapes called parabolas and hyperbolas. path planet would take from COMPACT, ROTATING BODIES point B if there was no gravity path planet would take from Stars, pulsars, galaxies, and planets all rotate, governed by the law point A if there was no gravity of conservation of angular momentum. An object’s angular C momentum is related to its rotational energy, which depends on the B planet following elliptical orbit distribution of mass in the object and on how fast it spins.The around star angular momentum of any rotating object stays constant, so if A gravity causes the object to contract, its spin rate increases to make up for the redistribution of mass. Compact, fast-rotating objects acceleration toward therefore tend to form from diffuse, slowly-rotating ones. star due to pull of gravity periapsis comet from apoapsis (point at ANGULAR MOMENTUM INTRODUCTION (point of deep space which orbiting object When an ice skater draws her limbs in, closest is farthest from the her spin rate soars. Similarly, a rotating approach) hyperbolic path orbit’s focus) cloud of gas spins faster as it contracts. star focus of orbits ORBITING BODIES paths of skater’s Shown here are two elliptical orbits of different eccentricities and a hyperbolic limbs as she spins path. Any orbit results from the combined effect of an object’s tendency to move at planet following fast, with a compact constant speed in a straight line and the gravitational pull of the body it orbits. a more eccentric body shape (elongated) elliptical orbit paths of skater’s limbs as she spins slowly, with a less compact body shape

40 WHAT IS THE UNIVERSE? SPACE AND TIME 34–37 Radiation MOST PEOPLE SHARE SOME COMMON-SENSE NOTIONS about direction of 38–39 Gravity, motion, and orbits Observer 2’s the world. One is that time passes at the same rate for everyone. motion Expanding space 44–45 The family of the Sun 102–103 Another is that the length of a rigid object does not change. path of In fact, such ideas, which once formed a bedrock for the laws Observer 1’s ball, as seen of physics, are an illusion: they apply only to the restricted by Observer 2 range of situations with which people are most familiar. In fact, time and space are not absolute, but stretch and warp depending on relative viewpoint.What is more, the presence of matter distorts both space and time to produce the force of gravity. PROBLEMS IN NEWTON’S UNIVERSE Problems with the Newtonian view of space and time (see p.38) first surfaced toward the end of the 19th century. Up to that time, scientists assumed that the positions and motions of objects in space should all be measurable relative to some non-moving, absolute “frame of reference,” which they thought was filled with an invisible medium called “the ether.” However, in 1887, an experiment to measure Earth’s motion through this ether, by detecting variations in the velocity of light sent through it in different directions, produced some unexpected results. First, it failed to confirm the existence of the ether. Second, it indicated that light always travels at the same speed relative to an observer, whatever that observer’s own movements.This finding suggested that light does not follow the same rules of relative motion that govern everyday objects such as cars and bullets. If a person were to chase a bullet at half of the bullet’s speed, the rate at which the bullet moved away from him or her would CONSTANT SPEED OF LIGHT halve. However, if a person were to chase a Light leaves both the ceiling lights and the headlights of the moving cars at the same speed relative to its source. light wave at half the speed of light, the wave Paradoxically, light from both sources reaches an observer standing in the tunnel at, again, exactly the same speed. would continue moving away from him or her at exactly the same velocity. Observer 1 ALBERT EINSTEIN SPECIAL RELATIVITY path of Observer 1’s ball, as seen The work of German-born In 1905, Albert Einstein rejected the idea that there by Observer 1 mathematician and physicist Albert Einstein made him the most famous is any absolute or “preferred” frame of reference in scientist of the 20th century. Although he won the Nobel prize the universe. In other words, everything is relative. He for his work on the particle-like properties of light (see p.34), he is also rejected the idea that time is absolute, suggesting that VIEWPOINT ONE more famous for his theories of it need not pass at the same rate everywhere.To replace the special relativity (1905) and general relativity (1915).These theories old ideas, he formed the special theory of relativity, called From Observer 1’s point of view, “special” because it is restricted to frames of reference in the green ball within his or her introduced a constant, unchanging motion (because they are not being own frame of reference travels revolutionary accelerated by a force, see p.38). He based the entire theory on up and down. If Observer 1 looks new way of two principles.The first principle, called the principle of across at the red ball in a frame thinking of reference in relative motion, about space, time, mass, relativity, states that the same laws of physics apply equally in the ball seems to follow an arc. energy, and gravity. all constantly moving frames of reference.The second principle states that the speed of light is constant and INTRODUCTION independent of the motion of the observer or source of light. Einstein MOVING FRAMES OF REFERENCE recognized that this second principle Here we see two travelers—effectively two conflicts with accepted notions about moving reference frames—passing each other. how velocities add together; further, Each tosses a ball up. By the principle of relativity, the laws of physics apply in each reference frame, that combining it with the first so each traveler observes the behavior of the direction of principle seems to lead to perplexing, two balls as predicted by those laws. Although Observer 1’s nonintuitive results. He perceived, the two travelers see different motions for each motion however, that human intuition about ball, neither traveler’s point of view is superior to time and space could be wrong. the other’s—both are equally valid, and there is no preferred frame of reference.

SPACE AND TIME 41 path of EFFECTS OF SPECIAL RELATIVITY spacecraft traveling at 90 percent of Observer 2’s the speed of light relative to Earth ball, as The results that flow from the principles of special relativity are remarkable. Using seen by thought experiments, Einstein showed that for the speed of light to be the same in 10 MINUTES Observer 2 all reference frames, measures of space and time in one frame must be transformed ELAPSED to those in another.These transformations show that when an object moves at high observer 20 MINUTES speed relative to an observer, the observer sees less of its length—an effect on Earth ELAPSED TIME DILATION called Lorentz contraction. Also, time for such an object appears to run more slowly—an effect called time dilation. So measurements Special relativity predicts that of time and space vary between moving reference frames. an Earthbound observer sees Einstein also showed that an object gains mass when its time slow down onboard a energy increases and loses it when its energy decreases. spacecraft traveling at close This led him to realize that mass and energy have an to the speed of light relative equivalence, which he expressed in the famous to Earth. At 90 percent of equation E (energy) = m (mass) x c2 (the speed light-speed, the passage of of light squared). time is halved—a clock on the spaceship advances only path of Observer 2’s ball, 10 minutes while more than as seen by Observer 1 20 minutes pass on Earth. MASS IS ENERGY 10 MINUTES observer on To Einstein’s ultimate ELAPSED spacecraft dismay, one of the first applications of his 20 MINUTES ELAPSED equation E=mc² was the SYMMETRICAL EFFECTS development of atomic Relativistic effects occur bombs. In such bombs, symmetrically, because there is the loss of tiny amounts no absolute frame of reference. of mass in nuclear For the spacecraft pilot, time reactions releases vast on Earth passes more slowly. amounts of energy. More than 20 minutes pass on the spacecraft while the pilot Observer 2 watches a clock advance only 10 minutes on Earth. passage through cone of passage through future space-time of an space-time of an space-time object that stays at the same VIEWPOINT TWO object moving point in space To Observer 2, the red ball within light would move from place to place his or her own reference frame appears to move vertically. The through space-time green ball, which is in another frame of reference in relative along the side of the cone motion, seems to follow an arc-shaped path. TIME EXPLORING SPACE SPACE-TIME MEASURING STRETCHED TIME A further implication of special Special relativity’s prediction that relativity is that space and time are time can stretch has been proved to be true by mounting atomic closely linked.When two events clocks in jet airliners and monitoring their timekeeping occur in separate places, the space SPACE compared with Earth-bound clocks. Here, American physicist between them is ambiguous, because Harold Lyons explains an early experiment of this type, with observers traveling at different each 2-D plane SPACE the help of a graphic. Relativistic velocities measure different distances. represents 3-D space time dilation has some practical The time passing between the events object in the present consequences.The atomic clocks also depends on each observer’s at its starting point INTRODUCTION in global positioning system in space (GPS) satellites run about 7.2 microseconds a day slower than motion. However, a mathematical method can be devised FOUR DIMENSIONS Earth-bound atomic clocks, for measuring the separation of events, involving a In this representation of space- so their data must be adjusted combination of space and time, that gives values that all time, time moves upward into to maintain accuracy. observers can agree on.This led to the idea that events the future, while the three in the universe should no longer be described in three spatial dimensions are reduced spatial dimensions, but rather in a four-dimensional to two-dimensional planes. The world, incorporating time, called space-time. In this cone represents the effective system, the separation between any two events is limits of space-time for any described by a value called a space-time interval. object—its boundary is defined by the speed of light.

42 WHAT IS THE UNIVERSE? ACCELERATING MOTION apparent position of galaxy to observers on Earth, who Having completed his study of relativity in the special case of assume light has traveled in a straight line reference frames in uniform motion (inertial reference frames), Einstein turned his attention to changing, or accelerated motion. In particular, he examined the link between gravity and acceleration.This led him to formulate a proposition called the principle of equivalence, which describes gravity and acceleration as different perspectives of the same thing. Specifically, Einstein stated that it was impossible for any experiment to tell the difference between being at rest in a uniform gravitational field and being accelerated. He illustrated this idea using thought experiments involving scientists sealed into boxes under various conditions of acceleration and gravity. Starting from the principle of equivalence, by 1915 Einstein had gone on to develop his most complex and major masterpiece, the general sealed box undergoing theory of relativity, which uniform acceleration provided a new description of gravity. person is true position weighed down of galaxy sealed box in a uniform gravitational field, caused by planet’s gravity ball falls to the floor rocket engine accelerates box and imparts the same force as the planet’s gravity ball falls to GRAVITY AND ACCELERATION orbiting planet follows the floor elliptical path because FEEL THE SAME space-time is curved in person is A person in a sealed box at rest the vicinity of the Sun weighed down on the surface of a planet with planet’s mass creates a strong gravitational field, and two-dimensional rubber sheet gravitational field a person within a similar box represents four-dimensional in deep space that is being space-time—dents in the sheet accelerated by a rocket, could represent distortions of space-time not distinguish between the two situations. LIGHT AND GRAVITY sealed box undergoing uniform By visualizing experiments in accelerating reference frames and using acceleration upward the principle of equivalence to transpose them to gravitational situations, Einstein postulated that light, despite having no mass, should follow a curved path in a gravitational field. Although he had no direct evidence that this was true, he convinced himself that it must be (by 1919, it had been shown to be true by astronomical observations). Developing the idea further, Einstein theorized sealed box in a uniform beam of light that gravitational effects might be caused by large masses or gravitational field, caused bends downward concentrations of energy causing a local distortion in by planet’s gravity THOUGHT EXPERIMENT the shape of four-dimensional space-time—that WITH LIGHT If a light beam is fired across is, that gravity might be a purely light source a box that is accelerating geometrical consequence of the upward, within the box the light would appear to effect of mass on space-time. If curve downward. By the equivalence principle, in INTRODUCTION so, light curves around a large beam of light an identical experiment mass because of the warping of curves through carried out on a box in a space-time caused by the mass. gravitational field gravitational field, a light ray should follow the same Similarly, a planet in orbit downward curve. around a star, such as the Earth around the Sun, follows a curved trajectory not because of a pull of the star on the planet, but because space-time is warped in the vicinity of the star, and the shortest path for the planet to take through this distorted massive planet creates region of space-time is a curved one. gravitational field

43 DENTED SPACE-TIME GENERAL RELATIVITY AT WORK object with Space-time can be visualized as large mass a rubber sheet in which massive Einstein encapsulated his theory of how mass distorts space-time in his set objects make dents. In this view, of “field equations.” Physicists have used these equations to find that it is in the warped planets orbit the Sun because they strongest gravitational fields—where massive, dense objects distort space-time space-time roll around the dent it produces. most strongly—that reality departs farthest from that predicted by Newton (see Similarly, light passing by a p.38). For instance, Mercury is so close to the Sun that it always moves in a PINCHED SPACE massive object has its straight- strong gravitational field (or in strongly curved space-time). Its orbit is distorted Instead of a two-dimensional line path deflected by following in a way that Newton could not account for, but which general relativity explains sheet, four-dimensional the local curvature of space-time. perfectly (see p.110). General relativity also provides a framework for models of the space-time can also be Remember, however, that it is universe’s structure, development, and eventual fate. It predicts that the universe visualized as a three- 4-dimensional space-time, not dimensional volume that just space, that is warped. must be either expanding or contracting. Before the introduction is narrowed or “pinched in” of general relativity, space and time were thought of only around large masses. white dwarf star as an arena in which events took place. After general relativity, physicists realized that space and time are dynamic entities that can be affected by mass, forces, and energy. relatively weak gravity moderately deep intense gravity gravitational well close to star WHITE DWARF STAR intense relatively weak A white dwarf is a very dense, planet- gravity gravity sized star that can be thought of as relatively weak deep, steep event horizon, producing a smaller but deeper dent gravity gravitational well beyond which in space-time than does a nothing, not even star like the Sun. massive, dense light, can break neutron star free of the gravitational field NEUTRON STAR extremely intense gravity A neutron star is an exceedingly dense gravitational well of infinite depth, with stellar remnant that makes a very deep steepness (gravity) increasing to infinity dent in space-time. A neutron star singularity at the center of significantly deflects light passing the black hole by, but cannot capture it. distortion of space-time BLACK HOLE caused by the Sun’s In a black hole, all the mass is mass deflects light from concentrated into an infinitely distant galaxy dense point at the center, called a singularity. A singularity space-time around the Sun is warped by the Sun’s produces an infinite distortion mass, creating a so-called a “gravitational well” in space-time—a bottomless gravitational well. Any light that passes a boundary called the “event horizon” near the entrance to this well cannot return. QUANTUM GRAVITY Although general relativity accurately describes the universe on a large scale, it has little to say about the subatomic world in which many scientists believe gravity must originate.This subatomic world is modeled by another great theory of physics, called quantum mechanics, telescope which itself has little to say about gravity.There is, it seems, little in common between the on Earth smooth, predictable interactions of space-time and matter predicted by general relativity and Sun the jumpy subatomic world modeled by quantum mechanics, in which changes in energy and matter MULTIDIMENSIONAL SPACE-TIME occur in quanta (discrete steps, see p.28). A key goal These figures, called Calabi–Yau of modern physics is to find a unifying theory—a spaces, are purported to hold six or more dimensions “curled up” within “quantum theory of gravity” or “theory of everything”— them. By incorporating one of these that unites relativity and quantum mechanics, and tiny objects at each point in space- harmonizes gravity with the other fundamental forces of time, string theorists envisage ten nature. One of the best hopes lies in string theory (see or more dimensions. p.31). Most early-21st-century theories of everything suppose that the universe has more dimensions than the easily observed three of space and one of time. GRAVITY BENDING LIGHT Calabi–Yau INTRODUCTION The effect of gravity on the path of space light is not obvious unless an observer looks deep into space at the universe’s most massive objects—clusters of galaxies. This image shows galaxies as white blobs. Their combined gravity bends light so much that the images of more distant galaxies appear as blue streaks, stretched and squashed by the galaxy cluster’s gravity.

44 WHAT IS THE UNIVERSE? EXPANDING SPACE the universe 6 billion years ago was much smaller 22–23 The scale of the universe A CRUCIAL PROPERTY of the universe is that it 34–37 Radiation is expanding. It must be growing, because distant galaxies galaxies are quickly receding from Earth and more close together The Big Bang 48–51 distant ones are receding even faster. Assuming that The fate of the universe 58–59 the universe has always been expanding, it must free gas and dust once have been smaller and denser—a fact that not yet absorbed Galaxy clusters 326–27 strongly supports the Big Bang theory of its origin. into galaxies MEASURING EXPANSION cones represent two 6EABRISLLAIOGNO possible histories of Y The rate of the universe’s expansion can be calculated by comparing the the expanding 3 distances to remote galaxies and the speeds at which they are receding. universe The galaxies’ velocities are measured by examining the red shifts in their light spectra (see p.35).Their distances are calculated by detecting 15 billion years ago, a class of stars called Cepheid variables in the galaxies and measuring size of universe is the stars’ cycles of magnitude variation (see pp.282, 313).The result zero—a possible is a number known as the Hubble constant—an expression of the Big Bang occurs universe’s expansion rate.The value of the constant has been debated by cosmologists, but is currently thought to be about 50,000 mph universe expanding (80,000 km/h) per million light-years.This means, for example, that at a constant rate in the past two galaxies situated 1 billion light-years apartrecessional velocity (measured by red shift) 12 billion years BILLION YEARS AGO are receding from each ago, size of other at 50 million mph universe is zero (80 million km/h). On a familiar time scale, this is universe actually a very gradual expanding at a expansion—an increase faster rate in the galaxies’ distance then slowing of 1 percent takes tens down of millions of years. AGE OF THE UNIVERSE Cosmologists can estimate present day the age of the universe by distance from Earth (measured by variable stars) extrapolating its expansion HUBBLE CONSTANT rate backward to the point at which the The recession velocity of remote galaxies rises with distance, and this relationship forms a size of the observable universe was zero. straight line on a graph. Estimates of the line’s slope yield values of the Hubble constant. Depending on how the expansion rate has some galaxies evolve into changed, estimates for the universe’s age spiral shapes range from 12 to 15 billion years. The PRESENT DAY current best estimate is 13.7 billion years. galaxies becoming less crowded THE NATURE OF EXPANSION galaxy cluster, Several notable features have been established about the bound by gravity, does not expand universe’s expansion. First, although all distant galaxies are I3NBTIHLLEIOFUNTYUERAERS moving away, neither Earth nor any other point in space is at the center of the universe. Rather, everything is receding from everything else, and there is no center. Second, at a local scale, gravity dominates over cosmological expansion and holds matter together.The scale at which this happens is surprisingly large—even entire clusters of galaxies resist expansion and hold together.Third, it is incorrect to think of INTRODUCTION galaxies and galaxy clusters moving away from each other “through” space. A more accurate picture is that of space itself expanding and carrying objects with it. Finally, the expansion rate almost certainly varies. Cosmologists are greatly interested in establishing how the expansion rate LOCAL GRAVITY may change in the future.The future rate The galaxies above are not moving apart. They will of expansion will decide the eventual continue to collide despite cosmological expansion. fate of the universe (see pp.58–59). Galaxy clusters are also held together by gravity.

EXPANDING SPACE 45 TIME AND EXPANDING SPACE EDWIN HUBBLE PEERING INTO DEEP SPACE The continued expansion of space, combined The American astronomer Edwin This Hubble “deep-field” with the constant speed of light, turns the Hubble (1889–1953) is famous for photograph shows a jumble of universe into a giant time machine.The light being the first to prove that the galaxies viewed at different from a remote galaxy has taken billions of years universe is expanding. He showed distances. Each appears as it to reach Earth, so astronomers see the galaxy as the direct relationship between the existed billions of years ago. it was billions of years ago. In effect, the deeper recession speeds of remote galaxies astronomers look into space, the farther they and their distances from Earth, diffuse, young young, peer into the universe’s history. In the remotest now known as Hubble’s Law. galaxy not yet blue galaxy regions, they see only incompletely formed Hubble is also noted for his earlier condensed into 4 billion light- galaxies as they looked soon after the Big Bang. proof that galaxies are external to a tight spiral years away, The most dim and distant of these galaxies is the Milky Way and for his system pictured as it receding from Earth at speeds approaching the of galaxy classification.The Hubble was 4 billion speed of light. Should astronomers observe such Space Telescope objects for millions of years, they would see and the Hubble years ago constant are both them evolving more slowly than if they named after him. elliptical galaxy, spiral galaxy, were closer and not being carried 6 billion light-years away 3 billion light- away so fast. At greater distances years distant yet, beyond the observable universe (see p.23), there may exist other objects that have moved away so fast that light from them has never reached Earth. LOOKBACK DISTANCE The expansion of space complicates the expression of distances to very remote objects, particularly those that we now observe as they existed more than 5 billion years ago.When astronomers describe the distance to such faraway objects, by convention they use the “lookback” or “light-travel-time” distance.This is the distance that light from the object has travelled through space to reach us today, and it tells us how long ago the light left the object. But because space has expanded in the interim, the distance of the galaxy when the light began its journey towards Earth is less than the lookback distance. Conversely, the true distance to the remote object (called the “comoving” distance) is greater than the lookback DIVERGING WORLDS distance.These distinctions need to be An object described as being remembered when, for example, a galaxy 11 billion light-years away is stated as being 10 billion light-years away. (lookback distance) has a greater true distance (comoving distance), due to the effects of photon leaves galaxy X the universe’s expansion. 1. Eleven 11 BILLION YEARS AGO Milky Way distant galaxy X receding billion years voids between galaxy ago, a photon of INTRODUCTION clusters progressively light departs distant enlarge and become galaxy X traveling almost empty of dust toward the Milky Way. and gas The two galaxies are separated by 4 billion ACCELERATING EXPANSION light-years of space. This is a conceptual interpretation of how a region of space may have changed over a 9-billion-year 2. Six billion years period. As space has expanded, so the galaxies later, the photon has within it have been carried apart, evolving as they not yet reached its go. This interpretation shows expansion speeding destination, because up—a scenario gaining support from cosmologists. space has expanded, carrying the galaxies much farther apart. 5 BILLION YEARS AGO photon travels toward Milky Way galaxy X still receding 3. The photon reaches PRESENT DAY photon arrives lookback distance true, comoving distance the Milky Way, where an observer sees X as it was 11 billion years ago, 11 billion light- years away (lookback distance). Meanwhile, X’s true (comoving) distance has increased to 18 billion light-years.

46 INTRODUCTION

“Some say the world will end in fire, Some say in ice.” Robert Frost THE STORY OF THE UNIVERSE can be traced back to its very first instants, according to the Big Bang theory of its origins. In the Big Bang model, the universe was once infinitely small, dense, and hot.The Big Bang began a process of expansion and cooling that continues today. It was not an explosion of matter into space, but an expansion of space itself, and in the beginning, it brought time and space into existence.The Big Bang model does not explain all features of the universe, however, and it continues to be refined. Nonetheless, scientists use it as a framework for mapping the continuing evolution of the universe, through events such as the decoupling of matter and radiation (when the first atoms were formed and the universe became transparent) and the condensation of the first galaxies and the first stars. Study of the Big Bang and the balance between the universe’s gravity and a force called dark energy can even help predict how the universe will end. CRADLE OF STAR BIRTH This pillar of gas and dust is the Cone Nebula, one of the most active cradles of star formation in the Milky Way. The clouds of material giving birth to these stars were once parts of stars themselves. The recycling of material in the life cycles of stars has been key to the universe’s enrichment and evolution. THE BEGINNING AND END OF THE UNIVERSE

48 THE BEGINNING AND END OF THE UNIVERSE THE BIG BANG THE FIRST MICROSECOND The timeline on this page and the next 28–31 Matter TIME, SPACE, ENERGY, AND MATTER are all thought to have come shows some events during the first 34–37 Radiation into existence 13.7 billion years ago, in the event called the Big microsecond (1 millionth of a second or 44–45 Expanding space Bang. In its first moments, the universe was infinitely dense, 10 6 seconds) after the Big Bang. Over unimaginably hot, and contained pure energy. But within a tiny this period, the universe’s temperature The fate of the universe 58–59 fraction of a second, vast numbers of fundamental dropped from about 1034°C (ten billion Mapping deep space 339 particles had appeared, created out of energy as trillion trillion degrees) to a mere 1013°C (ten trillion degrees). The timeline refers to the diameter of the observable universe: this is the approximate historical diameter of the part of the universe we can currently observe. the universe cooled.Within a few hundred thousand years, these particles had combined to form the first atoms. IN THE BEGINNING The Big Bang was not an explosion in space, but an expansion of space, which happened everywhere. Physicists do not know what happened in the first instant after the THE PLANCK ERA Big Bang, known as the Planck era, but at No current theory of the end of this period, they believe that physics can describe gravity split from the other forces of nature, what happened in followed by the strong nuclear force (see the universe during p.30). Many believe this event triggered this time. “inflation”—a short but rapid expansion. If DIAMETER 3x10 26 ft/10 26 m 33 ft/10 m 105 m (62 miles/100 km) inflation did occur, it helps to explain why 1022K (18 billion trillion °F/10 the universe seems so smooth and flat. TEMPERATURE 1027K (1,800 trillion trillion °F (1,000 trillion trillion °C) During inflation, a fantastic amount of THE INFLATION ERA THE QUARK ERA Part of the universe expanded from Sometimes called the electroweak era, this period saw mass-energy came into existence, in billions of times smaller than a vast numbers of quark and antiquark pairs forming from proton to something between the energy and then annihilating back to energy. Gluons and tandem with an equal but negative size of a marble and a football field. other more exotic particles also appeared. amount of gravitational energy. By the end of singularity TIME A hundred-billionth of a yoctosecond A hundred-millionth of a yoctosecond 1 yoctosecond inflation, matter had at the start 10 24 seconds 10 35 seconds 10 32 seconds begun to appear. of time A ten-trillionth of a yoctosecond quark antiquark 10 43 seconds quark quark quark– THE GRAND UNIFIED THEORY ERA antiquark During this era, matter and energy were completely interchangeable. Three of the pair fundamental forces of nature were still unified. X-boson strong nuclear force superforce Grand Unified electroweak weak nuclear force electromagnetic force Force force gravitational force 10–43 SECONDS 10–36 SECONDS 10–12 SECONDS gluon SEPARATION OF FORCES PARTICLE SOUP Physicists believe that at the exceedingly high temperatures present just after the Big Bang, the four fundamental forces About 10 32 seconds after the Big Bang, the were unified. Then, as the universe cooled, the forces universe is thought to have been a “soup” separated, or “froze out,” at the time intervals shown here. of fundamental particles and antiparticles. These were continually formed from energy INFLATION as particle–antiparticle pairs, which then met and were annihilated back to energy. Among INTRODUCTION In a Big Bang without inflation, what are now widely these particles were some that still exist today as constituents of matter or as force carrier spaced regions of the universe could never have particles. These include quarks and their antiparticles (antiquarks), and bosons such as become so similar in density and gluons (see pp.30–31). Other particles may have been present that no longer exist or are temperature. Inflation theory proposes hard to detect—perhaps some gravitons (hypothetical gravity-carrying particles) and that our observable universe is Higgs bosons, also hypothetical, which impart mass to other particles. derived from a tiny homogeneous patch of the original universe. The effect of inflation is like expanding a wrinkled sphere—after the expansion, its surface WRINKLED SMOOTHER VERY SMOOTH EXTREMELY SMOOTH AND FLAT appears smooth and flat.