["","BEYOND THE MILKY WAY 300","\u201cThe history of astronomy is a history of receding horizons.\u201d Edwin Hubble OUTSIDE THE BOUNDS of the Milky Way stretch vast gulfs of space, the realm of the galaxies. The closest are on our own galactic doorstep\u2014there is even a small galaxy currently in collision with the Milky Way.The farthest lie billions of light-years away, at the edge of the visible universe\u2014their light has been traveling toward Earth for most of time. Galaxies range from great wheeling disks of matter to giant, diffuse globes of billions of stars and from starless clouds of gas to brilliant furnaces lit up by star formation.They are also violent\u2014despite their stately motion over millions and billions of years, collisions are frequent and spectacular. Collisions disrupt galaxies, sending material spiraling into the supermassive black holes at their centers, fueling activity that may outshine ordinary galaxies many times over. Galaxies influence their surroundings and form constantly evolving clusters and superclusters. At the largest scale, it is these galaxy superclusters that define the structure of the universe itself. COSMIC RING A circlet of brilliant star-forming regions, 300 million light-years from Earth, surrounds the yellow hub of what was once a normal spiral galaxy. This ring galaxy, AM 0644-741, is probably the result of a cosmic collision with a smaller galaxy. BEYOND THE MILKY WAY","302 TYPES OF GALAXY TYPES OF GALAXY EDGE-ON SPIRALS NGC 4013 is a spiral galaxy that 24\u201327 Celestial objects THROUGHOUT THE UNIVERSE, galaxies exist happens to lie edge-on to Earth. 34\u201337 Radiation in enormous diversity.These vast wheels, Such edge-on views reveal the 38\u201339 Gravity, motion, and orbits globes, and clouds of material vary hugely in thinness and flatness of spiral galaxies. This Hubble image Galaxy evolution 306\u2013309 displays the dense dust within Galaxy clusters 326\u201327 the disk, and shows how few stars lie above or below the disk. size and mass\u2014the smallest contain just a few million stars, the largest around a million million. Some are just a few thousand light-years across, others can be a hundred times that size. Some contain only old red and yellow stars, while others are blazing star factories, full of young blue and white stars, gas, and dust.The features of galaxies are clues to their history and evolution, but astronomers have only recently begun to put the entire story together\u2014and there are still many gaps in their knowledge. THE VARIETY OF GALAXIES Galaxies can be classified by their shape, size, and color. At the most basic level, they are divided by NUMBERING ELLIPTICALS shape into spiral, elliptical, and irregular galaxies. Edwin Hubble (see p.45) devised a more precise The class of an elliptical classification, still used today, that subdivides these galaxy shapes. Hubble classed spiral galaxies as types galaxy is found by dividing Sa to Sd\u2014an Sa galaxy has tightly wound spiral arms, an Sd very loose arms. Spirals with a bar across the difference between its their center are classed as SBa to SBd. Hubble classed elliptical galaxies as E0 to E7 according to their long and short axes by the long-axis length and then shape in the sky\u2014circular galaxies are E0, and elongated ellipses E7. Elliptical galaxies appear as two- multiplying by ten, making dimensional ellipses, but in reality they are three-dimensional ellipsoids ranging from roughly ball-shaped this galaxy, M110, an E6. star clouds to cigar shapes. So Hubble\u2019s classification does not reflect their true geometry, since an E0 long axis = galaxy could be a cigar shape viewed end-on from 8.7 arc-minutes IRREGULAR GALAXY Earth. Hubble also recognized an intermediate type of short axis = Clouds of stars that lack clear galaxy\u2014the lenticular (type S0), with a spiral-like disk, 3.4 arc-minutes disk- or ellipse-like structure a hub of old yellow stars, but no spiral arms. Finally, are called irregular galaxies. irregular galaxies (type Irr) are usually small, rich in gas, Sb SPIRAL galaxy NGC 4622 The Small Magellanic Cloud dust, and young stars, but have few signs of structure. is one such irregular galaxy. ELLIPTICAL GALAXY E0 ELLIPTICAL galaxy M89 E6 ELLIPTICAL galaxy M110 Balls of stars, from perfect spheres, through egg shapes Sa SPIRAL Sc SPIRAL (such as M59, pictured here) galaxy NGC 7217 the whirlpool to cigar-shaped ellipsoids, are galaxy (M51) called elliptical galaxies. SPIRAL GALAXY E2 ELLIPTICAL galaxy M32 S0 LENTICULAR Vast, rotating disks of stars, galaxy NGC 2755 dust, and gas are classed as spiral galaxies. Spirals have HUBBLE\u2019S CLASSIFICATION a ball-shaped nucleus inside a disk with spiral arms. M33 Hubble arranged his galaxy types in a fork shape, with ellipticals is a nearby spiral galaxy. along the handle, and spirals and barred spirals as prongs. This SBa BARRED SBb BARRED SBc BARRED SPIRAL GALAXIES excludes irregular galaxies. He thought his scheme indicated the SPIRAL SPIRAL SPIRAL evolution of galaxies\u2014today astronomers know it is not so simple. galaxy galaxy NGC 7479 galaxy NGC 1300 NGC 660 Some 25\u201330 percent of galaxies in the nearby universe are spirals. BARRED SPIRAL Similar to our own In each one, a flattened disk of gas- and dust-rich material orbits a galaxy, M83 (right) is a typical barred spiral, spherical nucleus, or hub, of old red and yellow stars, which is often having a straight bar on either side of the distorted into a bar. Stars occur throughout the disk, but the brightest galactic nucleus. BEYOND THE MILKY WAY clusters of young blue and white stars are found only in the spiral chaotic orbit arms.The space between the arms often looks empty viewed from Earth, but it is also full of stars. Above and below the disk is a spherical \u201chalo\u201d region, where globular clusters (see p.289) and stray stars orbit. Spiral galaxies rotate slowly\u2014 typically once every few hundred million years\u2014but they do not behave like a solid object. Stars orbiting ORBITS IN SPIRALS farther away take longer Stars in the disk of a to complete an orbit spiral galaxy follow than those close to the elliptical, nearly elliptical orbit core.The resulting circular orbits in a \u201cdifferential rotation\u201d is single plane. Those in the key to understanding the hub have wildly the spiral arms. irregular orbits at a multitude of angles.","303 FLOCCULENT SPIRAL The spiral galaxy NGC 4414 is flocculent, with bright stars clumped throughout the disk. Its star formation seems to be caused by local collapses of material rather than a large-scale density wave. SPIRAL ARMS The continued presence of spiral arms in most disk-shaped galaxies was once a mystery. If the arms orbit more quickly near the nucleus, then, during a galaxy\u2019s multi-billion-year lifetime, they would become tightly wrapped around the core. It now seems that the arms are in fact rotating regions of star formation, not rotating chains of stars themselves.The arms arise from a \u201cdensity wave\u201d\u2014 a zone that rotates far more slowly than the galaxy itself.The density wave is like a traffic jam\u2014stars and other material slow down open as they move into it and accelerate as they move out, but the clusters jam itself advances only slowly.The increased density helps of longer- to trigger the collapse of gas clouds and the start of star lived stars formation.The strength of the density wave varies between move out spirals. If the wave is strong, the result is a neat, \u201cgrand from spiral arm design\u201d spiral with two clearly defined arms. If it is weak or nonexistent, disk stars will tend to form in localized regions, creating the more clumpy \u201cflocculent\u201d spirals. PERFECT GALAXY young \u201cOB\u201d star BEYOND THE MILKY WAY In this diagram of an ideal galaxy, clusters never objects follow neatly aligned move far from elliptical orbits around the spiral arm nucleus. They travel fastest before when close to the nucleus and dying slowest when farthest away. SPIRAL REALITY new stars ignite in HII molecular DETAIL OF A SPIRAL ARM In a real galaxy, the orbits do not region (star-forming nebula) cloud is As material orbiting in a galaxy\u2019s disk line up neatly. The variety of approaches the denser region marked alignments, coupled with the compressed by the spiral arm, it packs together. slower movement when farther Dark molecular clouds form, some of from the nucleus, creates spiral density wave causes material to build up which turn into star-forming nebulae zones in which objects are (see pp.238\u201339). New stars of all kinds moving more slowly and so sparse stars orbit faster than ignite here, but the brightest ones soon become bunched together. the spiral arm and move into die, so they always mark the spiral arms. arm from behind","304 TYPES OF GALAXY ELLIPTICAL GALAXIES Elliptical galaxies show little structure other than a simple ball shape. They span the range from the largest to the smallest galaxies. At one end, dwarf ellipticals are relatively tiny clusters of a few million stars, often very loosely distributed, appearing faint and diffuse. Such galaxies are scattered in the space between larger galaxies and must contain significant amounts of invisible material simply to hold them together. Some of this could be in a central black hole, but much of it seems to be mysterious \u201cdark matter\u201d (see p.27) scattered through the whole of the galaxy. At the other extreme lie the giant ellipticals\u2014galaxies only ORBITS IN ELLIPTICAL GALAXIES found near the centers of large galaxy clusters and often containing The orbits of stars in an elliptical many hundreds of billions of stars. Some giant ellipticals, called cD galaxy vary wildly, from circles to very galaxies, have large outer envelopes of stars and even multiple long ellipses, and are not confined to concentrations of stars at their centers, suggesting they may have formed any specific direction. from the merging of smaller ellipticals. Almost all the stars in elliptical galaxies are yellow and red, and there is rarely any sign of star-forming gas and dust.The dominance of old, long-lived stars implies that any star formation in these galaxies has long since ended. Each star orbits the galaxy\u2019s dense core in its own path.The chances of collision are very remote, because stars are so small relative to the distances between them.With no gas and dust clouds to interact with, there is nothing else to flatten the stars into a single plane of rotation. Ellipticals are described according to their degree of elongation\u2014how much they deviate from a perfect sphere (see p.302)\u2014but the largest galaxies are always very close to perfect spheres. GIANT ELLIPTICAL INTERMEDIATE GALAXY DWARF ELLIPTICAL M87 is the giant elliptical at the heart of the nearby Virgo M49 in the Virgo galaxy cluster is a The Leo I galaxy is a nearby dwarf cluster. It is a type E1 or E0, almost perfectly spherical and large elliptical of type E4. With a elliptical, and one of the few we can containing roughly a trillion stars. At lower right, three diameter of about 160,000 light-years, study closely. With so few stars, smaller galaxies can be seen. it is classed by some astronomers as there must be a large amount of a giant elliptical, although its mass is dark matter holding the galaxy much less than that of the true giants. together with its gravity. LENTICULAR GALAXIES DUSTY LENTICULAR At first glance, lenticular galaxies appear to be relatives of Lying 25 million light-years away, ellipticals\u2014they are dominated by a roughly spherical galaxy NGC 2787 is one of the nucleus of old red and yellow stars. However, around this closest lenticular galaxies. Dust nucleus, these galaxies also have a disk of stars and gas.This lanes can be seen silhouetted against the nucleus, marking links them to spiral galaxies, and they are similar in overall the plane of its disk. size and general shape, although the nucleus is often considerably bigger than it would be in a spiral of similar elliptical orbits in size.The overall shape is often described as that of a lens, the disk which is the root of the name \u201clenticular.\u201dThe key chaotic orbits BEYOND THE MILKY WAY difference between lenticulars and spirals in the hub is that lenticulars have no spiral arms and little sign of star-forming activity in their disks.Without the bright blue star clusters that illuminate the disks of spirals, lenticulars are sometimes hard to tell apart from ellipticals.Those that are face-on may be indistinguishable from ellipticals and misclassified. An edge-on spiral galaxy with a large nucleus can equally be misclassified as lenticular, because at oblique angles spiral structure ORBITS IN LENTICULAR GALAXIES is often invisible. Astronomers are Stars in the nucleus of a lenticular uncertain how lenticular galaxies form, galaxy follow orbits with no specific but they could be spiral galaxies that plane, similar to those in an elliptical have lost most of their dust and gas. galaxy or a spiral nucleus. Gas and dust in the disk orbits in a more orderly plane.","TYPES OF GALAXY 305 IRREGULAR DWARF EXPLORING SPACE The irregular dwarf galaxy NGC 4449 contains clusters of young bluish stars GALAXIES AT DIFFERENT WAVELENGTHS interspersed with dustier reddish regions of current star formation. Radiation of different wavelengths can reveal hidden structures within COMBINED IMAGE OF NGC 1512 galaxies.The hottest stars appear brightest in ultraviolet, while cool, diffuse gas may be visible only in infrared. By overlaying images from different spectral regions, astronomers build up a full picture of a galaxy. FROM ULTRAVIOLET TO INFRARED These images of galaxy NGC 1512 increase in wavelength from left to right. Each wavelength is represented by a false color. FAR NEAR GREEN YELLOW NEAR MID- FAR ULTRA\u2013 ULTRA\u2013 VISIBLE VISIBLE VIOLET VIOLET LIGHT LIGHT INFRARED INFRARED INFRARED IRREGULAR GALAXIES IRREGULAR STARBURST M82 is an irregular starburst Not all galaxies fit into the scheme of spirals, ellipticals, galaxy crossed with dark dust and lenticulars. Some of these misfit galaxies are colliding lanes. It is undergoing an with companions or being pulled out of shape by a intense period of star birth. neighbor\u2019s gravity.These are usually classed under the catch-all term \u201cpeculiar\u201d or \u201cPec.\u201d Many more are true irregulars (type Irr).These galaxies typically contain a lot of gas, dust, and hot blue stars. In fact, many irregulars are \u201cstarburst\u201d galaxies, with great waves of star formation sweeping through them. Irregulars frequently have vast, pink hydrogen-emission nebulae where star formation is taking place. Some irregulars show signs of structure\u2014central bars and sometimes the beginnings of spiral arms.The Milky Way\u2019s brightest companion galaxies, the Large and Small Magellanic Clouds (see pp.310\u201311), are typical irregular galaxies. EXPLORING SPACE PECULIAR GALAXY NGC 4650A is a rare example of a ASTRONOMY FROM polar-ring galaxy, perhaps created THE SOUTH POLE in a galactic collision. A blue-white star-forming ring, aligned with the Some of the best Earth-based poles, extends from the nucleus. observations of galaxies come from an automated observatory at the CENTRAL BLACK HOLES BEYOND THE MILKY WAY South Pole.The AASTO project takes advantage of the dryness on Many, if not all, galaxies have a dark region within the Antarctic Plateau\u2014the driest their nucleus that seems strange by contrast with the place on Earth.With no water outer parts.The fast orbits of stars near galactic nuclei vapor in the atmosphere, near- suggest an enormous concentration of mass in a tiny infrared light is volume at the center of most spiral and elliptical not absorbed, galaxies\u2014often billions of Suns\u2019 worth of material in so it reaches a space little larger than the solar system.The only the ground object that can reach such a density is a black hole unhindered. (see p.26). Despite the tremendous gravity of this \u201csupermassive\u201d black hole, in most nearby galaxies the material has long since settled into steady orbits around HIDDEN SUPERMASSIVE BLACK HOLE it.With no material to absorb, the black hole remains An X-ray image of galaxy M82 shows glowing dormant.When a gas cloud or other object comes hot gas and intense point sources of X-rays. too close, however, the black hole may awake, pulling These are probably stellar-mass black holes in the stray material and heating it, producing radiation.The black hole surrounding a central supermassive black hole. may generate any type of radiation from low-energy radio waves to high- energy X-rays. In extreme cases called \u201cactive galaxies\u201d (see pp.320\u201321), the radiation from the nucleus is the galaxy\u2019s dominant feature.","306 GALAXY EVOLUTION GALAXY EVOLUTION 22\u201323 The scale of the universe THE PROCESSES BY WHICH DIFFERENT 54\u201355 Out of the darkness 302\u2013305 Types of galaxy types of galaxies form have puzzled astronomers for almost a century, Active galaxies 320\u201321 but today a new generation of Galaxy clusters 326\u201327 telescopes, capable of studying Galaxy superclusters 336\u201339 galaxies billions of light-years away, is finally resolving some key questions. Light from these remote galaxies left on its long journey to Earth when the universe was very young, so it can reveal the secrets of the early stages of galactic evolution. THE DISTRIBUTION OF GALAXIES DUSTY LENTICULAR GALAXY This image of the lenticular galaxy Astronomers can only ever see a \u201csnapshot\u201d of a brief moment in a NGC 1316 in the constellation of Fornax galaxy\u2019s long life story, so they have to build up a picture of galactic was taken by the Hubble Space Telescope. evolution by studying many individual galaxies. Such studies have It reveals a complex series of dust lanes revealed certain patterns, such as the fact that large elliptical galaxies and dust patches in the galaxy, indicating are found only in substantial galaxy clusters. Changes in the type of that it was formed from the merger of galaxies seen at different distances\u2014and therefore at different stages two galaxies rich in dust and gas. in cosmic history\u2014can also reveal patterns in the way galaxies have developed. However, capturing the light of the most distant early EXPLORING SPACE galaxies is an enormous challenge, requiring techniques such as long- exposure deep-field photography and the use of gravitational lensing. HERSCHEL GRAVITATIONALLY Launched in 2009, the European LENSED GALAXIES Space Agency\u2019s Herschel Space This image of a small area in Observatory is designed to observe the constellation Hydra was the longest infrared wavelengths taken by the Herschel Space (the far infrared and submillimeter Observatory and reveals more wavelengths on the boundary with than 6,000 galaxies. The radio waves). Its primary mirror is white squares indicate 11.5 ft (3.5 m) in diameter and its distant galaxies that have instruments are cooled to -456\u00b0F been gravitationally lensed (-271\u00b0C), enabling it to map some by foreground galaxies. of the coolest and most distant These distant galaxies are objects in the universe. brighter at the submillimeter wavengths detected by secondary Herschel than at visible- mirror light wavelengths. sun shield apparent position and distorted shape of primary multiple galaxy images mirror BEYOND THE MILKY WAY GRAVITATIONAL LENSING path of light galaxy cluster THE HERSCHEL without acting as a SPACE OBSERVATORY The technique of gravitational gravitational gravitational lens lensing actual position and lensing relies on the fact that shape of galaxy the gravity of a massive object deflects light passing nearby\u2014in effect, the object acts as a lens. In this way, a massive object, such as a galaxy cluster, situated between Earth and a more light bent toward distant galaxy (or other observer by lens object) can focus light from the distant galaxy onto Earth. This often produces multiple distorted images of the distant galaxy but also intensifies its light. observer on Earth","","308 GALAXY EVOLUTION GALAXY COLLISIONS COLLIDING GALAXIES Situated about 450 light-years from Earth Relative to their size, galaxies are quite closely packed together\u2014 in the constellation Hercules, two spiral although they are separated by distances of hundreds of thousands galaxies (NGC 6050 and IC 1179, also of light-years, galaxies themselves are typically tens of thousands of collectively known as ARP 272 ) are colliding. light-years across. Furthermore, the enormous gravity exerted by large Tidal forces in both galaxies are triggering galaxies and their tendency to form within large-scale clusters allow enormous waves of star formation, them to influence and attract one another. As a result, collisions and manifested in the bright clusters around close encounters between galaxies are comparatively common. In 1966, their spiral arms. US astronomer Halton Arp compiled the first catalog of galaxies that did not fit neatly into the common categories of spiral, irregular, and elliptical.With the benefit of more recent observations, it now seems that most of Arp\u2019s unusual galaxies were the result of past collisions and interactions between galaxies. Even some apparently normal galaxies are now thought to have interacted with other galaxies in the past, and it is also clear that many large galaxies are \u201ccannibals,\u201d tearing apart and ultimately absorbing smaller galaxies that stray too close. However, during intergalactic collisions individual stars rarely collide, and it may take several billion years before the mutual gravity of colliding galaxies finally pulls together most of their material into a single combined cloud of stars. SEYFERT\u2019S SEXTET Despite its name, this group contains only five galaxies\u2014the bright patch on the right is an unwinding spiral arm. Only four of the galaxies are at the same distance from Earth, about 190 million light-years away; the face- on spiral galaxy is about five times that distance. The four nearest galaxies are being distorted by gravitational forces between them. BEYOND THE MILKY WAY THE SPLINTER GALAXY SPIRALS IN COLLISION Also known as the Knife Edge Galaxy or The galaxy NGC 520 (also known as Arp 157) in NGC 5907, this galaxy is an edge-on spiral the constellation Pisces is believed to be a pair of that lies about 40 million light-years from colliding spirals seen edge-on. The collision began Earth in the constellation Draco. It is about 300 million years ago, and the galaxies are now surrounded by extraordinary looping trails in the middle stages of merging: their disks have come of faint stars, nicknamed the \u201cGhost Stream,\u201d together but their nuclei have not yet merged. which are thought to be the remnants of a smaller galaxy that has now been consumed by NGC 5907. COLLISIONS AND EVOLUTION The process of collision is now thought to play a key role in transforming galaxies from one type to another. In the early stages of a collision, stars that may have had relatively orderly orbits are pushed into highly elongated and tilted paths, and powerful shock waves passing through interstellar gas and dust generate tremendous bursts of new star formation. In the longer term, the remaining gas may become energized to such a degree that it can escape the galaxy\u2019s gravity altogether, depriving it of the means to continue star formation. In this way, spiral and irregular galaxies can be transformed into ellipticals surrounded by clouds of hot gas, as seen in the central regions of many galaxy clusters. However, it has also been theorized that this process can be reversed, at least in the relatively short term. According to this theory, cold intergalactic gas is constantly drawn in by the galaxy\u2019s gravity and can ultimately form a flattened disk in which star formation can begin again and spiral arms reform. If this theory is correct, then the rare lenticular galaxies mark an intermediate phase between elliptical and spiral galaxies. However, over time, merging spiral galaxies will form larger and larger ellipticals, while dwindling reserves of cold intergalactic gas will slow the regeneration of spiral galaxies.","GALAXY EVOLUTION 309 TIDAL FORCES As two galaxies approach each other, their gravitational fields interact and can affect their shapes. For example, because the galaxies\u2019 gravitational fields pull more strongly on the near side of each of the galaxies than on their more distant sides, their near sides become stretched out toward each other. Such gravitational distortion is greater on the less massive of the two galaxies, because of the stronger gravity of the other, more massive galaxy. However, the disks of even large spiral galaxies can be warped by the gravity of relatively small neighbors.When spirals collide with one another, one or more of the spiral arms may unwind, transforming into a long trail of stars that stretches out on the opposite side from the collision. Among the THE TADPOLE GALAXY best-known examples of these This galaxy, in the constellation Draco, has \u201ctidal tails\u201d are the ones associated a tail that stretches for some 280,000 light- with the Tadpole Galaxy and the years and is thought to have formed when Antennae Galaxies. one of the spiral galaxy\u2019s arms unwound in a close encounter with a smaller galaxy. STARBURSTS BEYOND THE MILKY WAY Intergalactic collisions can send immense shock waves through the galaxies involved, compressing large areas of interstellar gas and triggering enormous waves of star formation known as starbursts. During these events, starbirth occurs much faster than normal, giving rise to huge \u201csuper star clusters\u201d that may (if they survive) evolve into globular clusters. Starbursts are commonly seen in direct collisions, such as that of the Antennae Galaxies, but can also occur in close encounters between galaxies, as seen in the Cigar Galaxy (see p.314) due to its close encounter with Bode\u2019s Galaxy. Radiation from the numerous massive stars being formed, coupled with shock waves from supernovae as the heaviest stars rapidly age and explode, may blow gas and dust out of the galaxy, and it is this dispersion that may ultimately bring the starbursts to an end. STARBIRTH IN THE ANTENNAE GALAXIES This image of the Antennae Galaxies (NGC 4038 and 4039), which lie about 45 million light-years from Earth, reveals stars being born in huge starburst regions. The newborn stars are a brilliant white-blue and are surrounded by glowing pink emission nebulae.","310 GALAXIES Astronomers are drawn naturally to the brightest, the most beautiful, and the most intriguing galaxies. However, of the 100 billion galaxies in the observable universe, only a minority are spectacular spirals and giant ellipticals. Astronomers are beginning to understand that most galaxies are relatively small and faint\u2014diffuse balls and irregular clouds of stars.The faintest and most common BIG AND BRIGHT galaxies are dwarf ellipticals, which are like oversized globular star Spirals such as Bode\u2019s Galaxy, M81, may be clusters of only a few million stars.These feeble galaxies are visible the most attractive type of galaxy, but they only if they lie nearby in intergalactic terms.The most brilliant are far from the most common. Making up are the giant ellipticals, which can be 20 times as luminous as less than 30 percent of all galaxies, they are the Milky Way. outnumbered by smaller, fainter galaxies. DWARF ELLIPTICAL GALAXY galactic neighbor. It was not found STAR DENSITY until 1994 and was supplanted only SagDEG\u2019s existence came SagDEG by the discovery of the even closer to light only when a Canis Major Dwarf in 2003. SagDEG survey of Sagittarius found SAGITTARIUS CATALOG NUMBER remained hidden for so long because, regions of increased star like all dwarf ellipticals, it is a very density\u2014the bright None faint scattering of stars. It is also well patches in this image. disguised by its position behind the DISTANCE great Sagittarius star clouds that mark SagDEG\u2019s existence so our galaxy\u2019s center. SagDEG is small close to our galaxy is 88,000 light-years and obscure, but it has at least four a puzzle. It orbits the orbiting globular clusters, which are Milky Way in less than DIAMETER brighter and more obvious. One of a billion years and so must have gone these, M54, was discovered by Charles through several close encounters that 10,000 light-years Messier more than 200 years before should have ripped it apart and the parent galaxy was found. scattered its stars through the galactic MAGNITUDE 7.6 for M54 star cluster in SagDEG The Sagittarius Dwarf Elliptical halo. It has survived only due to Galaxy, often called SagDEG, was a large amount of dark matter, until recently our closest known producing more gravity than SagDEG\u2019s visible stars. IRREGULAR GALAXY counterpart, the Small Magellanic SUPERNOVA BUBBLE Cloud, the LMC appears from This image shows a bubble of gas Large Magellanic Earth to be a distinctive, isolated around the site of a supernova that Cloud region of the Milky Way, some exploded about 400 years ago in the 10 degrees across, with its own LMC. The image is a composite from CATALOG NUMBER areas of nebulosity and star clusters. the Hubble Space Telescope and None The LMC is in fact an irregular Chandra X-ray Observatory. Green and galaxy, orbiting the Milky Way DISTANCE roughly once every 1.5 billion blue indicate hot, X-ray emitting years on a path that brought it to material, and pink shows the visible 179,000 light-years within 120,000 light-years of our gas shell shocked by the blast wave galaxy at its closest approach DIAMETER around 250 million years ago. from the supernova explosion. Although the LMC is irregular The bubble is about 23 light-years 20,000 light-years and is being distorted by the gravity of the Milky Way, it shows across and is expanding at over MAGNITUDE 0.1 some signs of basic structure. Many 11 million mph (18 million kph). of its stars are concentrated in a DORADO central barlike nucleus, curved at TARANTULA NEBULA one end. Some astronomers have Massive stars run through their entire life cycle in the The Large Magellanic Cloud (LMC) likened the LMC to a barred spiral Tarantula Nebula. This image shows a new open cluster, bears the name of 16th-century with just one arm. Hodge 301, whose biggest stars have already gone explorer Ferdinand Magellan (see supernova. As the shock waves spread, they ripple the panel, opposite). However, cultures Like all irregular galaxies, nearby gas clouds, triggering further star formation. native to the Southern Hemisphere the LMC is rich in gas, dust, have recognized its existence since and young stars, including prehistoric times. Like its smaller some of the largest known regions of star birth. One such BEYOND THE MILKY WAY RADIO MAP region is the magnificent This false-color radio image of the LMC is Tarantula Nebula, also known centered on the Tarantula Nebula. It shows as 30 Doradus. It is so brilliant intense radiation as red and black, indicating that, if transported to the ionized hydrogen and star formation. location of the Orion Nebula (see p.241)\u2014only 1,500 light- years away in the Milky Way\u2014it would be bright enough to cast shadows on Earth at night. In recent times, the LMC was host to the only bright supernova since the invention of the telescope. Supernova 1987A (see p.266) was observed by astronomers around the world both during and after its explosion, and it has taught astronomers a lot about the final stages of the stellar life cycle.","GALAXIES 311 IRREGULAR GALAXY EXPLORING SPACE Small Magellanic MAGELLAN\u2019S Cloud DISCOVERY CATALOG NUMBER NGC The southernmost sky was not 292 visible to Europeans until they visited the Southern Hemisphere. DISTANCE The Portuguese explorer Ferdinand Magellan was among the first to 210,000 light-years do so during his round-the-world voyage of 1519\u201321. He was the DIAMETER first European to record two isolated patches of the Milky Way, 10,000 light-years which were later named after him. MAGNITUDE 2.3 FERDINAND MAGELLAN TUCANA Like the Large Magellanic Cloud, the is far from CLOUD OF STARS Small Magellanic Cloud (SMC) is proven. It has The SMC forms a distinctive wedge-shaped an irregular galaxy in orbit around one known globular cloud in southern skies. The pinkish areas in the Milky Way. It was in the SMC cluster in orbit, but the this optical photograph show the galaxy\u2019s that Henrietta Leavitt discovered the SMC lies deceptively close in major star-forming regions. Cepheid variable stars that were to the sky to one of the Milky Way\u2019s unlock the secrets of the galactic largest globulars\u201447 Tucanae. previous encounters.This \u201cMagellanic distance scale (see pp.282, 356).Thanks Stream\u201d has allowed astronomers to to her discovery, astronomers know Both the Magellanic Clouds are trace and refine their models for the that the SMC is both more distant ultimately doomed to be torn to orbits of the clouds. and genuinely smaller than the LMC, shreds and absorbed into our own with around one-tenth of the larger galaxy.They have survived several cloud\u2019s mass. Like the LMC, the small close passes of the Milky Way, but cloud is also undergoing intense star now share their orbit with a trail of formation. Some astronomers argue gas, dust, and stars torn away during that the SMC also shows signs of a central barlike structure, but the case SC SPIRAL GALAXY FLOCCULENT SPIRAL M33 is an example of Triangulum Galaxy a flocculent spiral\u2014a galaxy with arms that CATALOG NUMBERS divide like split ends and separate into M33, NGC 598 patches. The clumpy star clouds are DISTANCE thought to form due to localized changes 3 million light-years in density. DIAMETER the larger and brighter Andromeda Galaxy (M31), and the two lie close to 50,000 light-years each other in the sky. M33 is affected by its larger neighbor\u2019s gravity, and it MAGNITUDE 5.7 may even be in a long, slow orbit around the giant Andromeda spiral. TRIANGULUM Seen from Earth, M33 is fainter After the Andromeda Galaxy and the and more diffuse than M31\u2014partly Milky Way, the Triangulum Galaxy because it is closer to face-on than (M33) is the third major member of the Local Group of galaxies. edge-on, and partly because it really It is slightly more is less spectacular. However, the distant than Triangulum Galaxy is more typical of spiral galaxies than its unusually bright BEYOND THE MILKY WAY companions. As with several Local Group galaxies, M33 is large and bright enough in the sky for its features to be cataloged, and several of them have NGC numbers. Most prominent is the star- forming region NGC 604, the largest emission nebula known. At 1,500 light-years across, it dwarfs anything in our own galaxy. CLOUD DETAILS NEBULA NGC 604 In this image, the LMC\u2019s central barlike This emission nebula\u2019s gas glows as it is nucleus appears as the bluish star cloud excited by ultraviolet light from a central at the upper left, with the pink star-birth star cluster. The stars are so massive and regions on the right. bright that they emit most of their light in ultraviolet, and so are not prominent in visible-light photographs such as this.","312 BEYOND THE MILKY WAY Sb SPIRAL GALAXY astronomers as typical a thought that M31 and spiral galaxy as it Andromeda Galaxy other \u201cspiral nebulae\u201d appears. For example, might be solar systems in the despite its huge size, it CATALOG NUMBERS process of formation, while others appears to be less massive guessed correctly that they were than the Milky Way, with a sparse M31, NGC 224 independent systems of many stars. halo of dark matter. Despite this, It was in the early 20th century that astrophysicists calculate that M31\u2019s DISTANCE Edwin Hubble (see p.45) revealed the central black hole has the mass of 30 true nature of M31, at a stroke hugely million Suns, almost ten times more 2.5 million light-years increasing estimates of the size of than the Milky Way\u2019s central black the universe (see panel, opposite). hole.The huge mass of M31\u2019s black DIAMETER Astronomers now know that M31, hole is surprising, because a galaxy\u2019s like the Milky Way, is a huge galaxy black hole is thought generally to 250,000 light-years attended by a cluster of smaller reflect the mass of its parent galaxy. MAGNITUDE 3.4 orbiting galaxies, which occasionally Furthermore, studies at different ANDROMEDA fall inward under M31\u2019s gravity wavelengths have revealed disruption and are torn apart. in the galaxy\u2019s disk, possibly caused by The Andromeda Galaxy (M31) is an encounter with one of its satellite the closest major galaxy to the Milky Despite being intensively studied, galaxies in the past few million years. Way and the largest member of the the Andromeda Galaxy still holds Local Group of galaxies. Its disk is many mysteries, and it may not be M31 and the Milky Way are twice as wide as our galaxy\u2019s. moving toward each other, and they should collide and begin to coalesce M31\u2019s brightness and size mean it in around 5 billion years. has been studied for longer than any other galaxy. First identified as a \u201clittle CENTRAL BLACK HOLE cloud\u201d by Persian astronomer Al-Sufi This X-ray image of a small area of M31\u2019s (see p.421) in around ad 964, it was core shows its central black hole as a blue for centuries assumed to be a nebula, dot\u2014it is cool and inactive compared to the at a similar distance to other objects galaxy\u2019s other X-ray sources (yellow dots). in the sky. Improved telescopes revealed that this \u201cnebula,\u201d like many others, had a spiral structure. Some GALAXY CORE This X-ray image of the central area of M31 shows numerous point X-ray sources and a diffuse cloud of gas (in orange), which is being heated by shock waves from supernova explosions.","313 GALACTIC NEIGHBORS Dark dust lanes are silhouetted against glowing gas and stars in this view of the Andromeda Galaxy and its two close companions, the dwarf elliptical galaxies M32 (upper left) and M110 (bottom). EXPLORING SPACE the same BEYOND THE MILKY WAY star at its INTERGALACTIC DISTANCE brightest The study of M31 played a key role in the discovery Cepheid variable that galaxies exist beyond our own. Although the V1 at its faintest spectra of galaxies suggested they shone with the light of countless stars, no one could measure their immense distance. In 1923, Edwin Hubble (see p.45) proved that M31 lay outside our galaxy. He found the true distance of M31 by calculating the luminosities of its Cepheid variable stars (see pp.282\u201383), and relating their true brightness to their apparent magnitude.","314 GALAXIES Sb SPIRAL GALAXY core Bode\u2019s Galaxy CATALOG NUMBERS X-RAY SOURCES A Chandra X-ray image shows a M81, NGC 3031 strong X-ray source at the galaxy\u2019s core, surrounded by smaller sources, DISTANCE probably X-ray binary stars. 10.5 million light-years DIAMETER 95,000 light-years MAGNITUDE 6.9 URSA MAJOR Bode\u2019s Galaxy, also known as M81, millions of years.The near PERFECT SPIRAL is one of the brightest spiral galaxies miss created tidal forces that M81 is a beautifully symmetrical spiral visible from the Northern Hemisphere. enhanced the density waves galaxy, tilted at an angle to our line of It is the dominant member of a galaxy (see p.303) in M81.The rate sight. This Hubble view shows star clusters, group lying near to the Local Group. of star birth around the dust, and gas clouds in its spiral arms. The galaxy is named after Johann density waves increased, Elert Bode, a German astronomer highlighting the spiral arms. who found it in 1774. A long, straight Bode\u2019s Galaxy has dust lane along had a close encounter one side of the with M82, the Cigar core could also Galaxy (see below), in have been created in the past few tens of the encounter. CLUSTERS REVEALED By measuring the This combined visible and Doppler shifts of light ultraviolet image shows from either side of the core, the hottest and brightest astronomers have found that the star clusters (blue and outer regions rotate more slowly than white blobs), lying in the in most galaxies.This suggests that M81 core and spiral arms. has little of the dark matter that creates higher rotation rates in other galaxies. IRREGULAR DISK GALAXY Sb SPIRAL GALAXY Cigar Galaxy Black Eye Galaxy CATALOG NUMBERS M82, CATALOG NUMBERS NGC 3034 M64, NGC 4826 DISTANCE DISTANCE 12 million light-years 19 million light-years DIAMETER DIAMETER 40,000 light-years 51,000 light-years MAGNITUDE 8.9 MAGNITUDE 8.5 URSA MAJOR COMA BERENICES BEYOND THE MILKY WAY The brightest and most spectacular This distinctive galaxy has a dark dust example of a \u201cstarburst galaxy,\u201d the lane, running in front of its core, from Cigar Galaxy (M82) is an irregularly which it gets its name.The dust lane shaped cloud of stars that looks like a is unusual because it arcs above the cigar from Earth. It is undergoing a galaxy\u2019s core in an orbit of its own. period of intense star birth as a result Because it has not yet settled into the of a close encounter with Bode\u2019s plane of the galaxy\u2019s rotation, it must Galaxy (M81).The near miss has have a recent origin and probably disrupted the galaxy\u2019s center, creating dates from the galaxy\u2019s absorption of a smaller galaxy that strayed too close. the dark dust lanes that obscure Another bizarre feature of the Black much of the core and triggering Eye Galaxy is that its outer regions the creation of many massive, are rotating in the opposite direction brilliant star clusters in an of the inner regions.This could be area a few thousand light- another effect of the collision. years across. At infrared GAS STREAMERS wavelengths, M82 is the M82\u2019s most spectacular brightest galaxy in the sky, features can be observed and it is also a strong radio only at the radio wavelength source.The infrared light emitted by ionized hydrogen, comes from disturbed gas here represented as and dust around the core. magenta. This wavelength reveals a huge envelope of X-RAY VIEW STARBURST GALAXY M64\u2019S CENTRAL REGION AND DUST LANE gas above and below the The intense activity in M82\u2019s core, blown out in long cluster of active core is luminous at optical and streamers by fierce radiation black holes X-ray wavelengths. The young from the central star clusters. stars illuminate the nebulae OPTICAL with visible light, while those IMAGE that have rapidly completed their life cycle form active black holes, emitting X-rays.","GALAXIES 315 Sc SPIRAL AND IRREGULAR GALAXIES LIGHT INTENSITY WILLIAM PARSONS Plotting the intensity of light Whirlpool Galaxy from different regions of M51 William Parsons (1800-67) was reveals the brightness of the an Irish nobleman who used his CATALOG NUMBERS M51, two galactic cores (the twin great wealth to build the largest NGC 5194, peaks on the graph). telescope of his time and made the NGC 5195 first detailed studies of nebulae. In 1845, he made detailed drawings DISTANCE NGC 5195 NGC 5194 and noticed the spiral structure of some \u201cnebulae,\u201d as galaxies were 31 million light-years thought to be at the time.This was an important step to discovering DIAMETER foreground star that galaxies were 100,000 light-years not nebulae but MAGNITUDE 8.4 separate star systems. CANES VENATICI PARSONS\u2019S Discovered by Charles Messier (see CONTRASTING PAIR large and luminous\u2014it is similar SKETCH p.73) in 1773, the Whirlpool Galaxy This infrared image, taken by the Spitzer in size to the Milky Way, but brighter OF M51 is now known to be a pair of galaxies Space Telescope, shows the Whirlpool overall because of the large young that is interacting\u2014the brightest Galaxy and its companion. The Whirlpool star clusters in its spiral arms. It is and clearest example of such a pair itself is rich in dust, which is colored red, thought to be the dominant member visible from Earth.The individual while the companion is largely dust-free of a small group of galaxies, called components are a spiral galaxy viewed and appears blue. simply the M51 group, which also face-on (NGC 5194) and a smaller includes the galaxy M63. irregular galaxy (NGC 5195). In undergoing a burst of star formation, visible light, the connection between which explains its unusual brightness, them cannot be seen, but images at while NGC 5194\u2019s core is also much other wavelengths reveal an envelope brighter than expected. It is even of gas connecting the two. One effect classified by some astronomers as an of the interaction is to enhance the active Seyfert galaxy (see p.320). density wave in the larger galaxy, triggering increased star formation The Whirlpool Galaxy is very and making its spiral arms stand out bright despite its distance, very clearly.The Whirlpool was in indicating that it is fact the first \u201cnebula\u201d in which spiral structure was recognized, by William Parsons (see panel, right). The interaction has also triggered increased activity in the cores of both of the galaxies\u2014NGC 5195 is LUMINOUS WHIRLPOOL BEYOND THE MILKY WAY This Hubble image combines data from different filters to reveal detail in M51, such as dark dust behind each spiral arm and bright pink regions of star birth.","316 GALAXIES SC SPIRAL GALAXY RELATIVE RED SHIFT This computer image shows Pinwheel Galaxy the red shift and blue shift of objects within M101, CATALOG NUMBERS revealing its rotation. Yellow and red regions are M101, NGC 5457 moving away, green and blue parts are approaching. DISTANCE lopsided, spiral-arm 27 million light-years system, giving the appearance that the core is offset DIAMETER from the galaxy\u2019s true center. M101 is one of the largest spirals known\u2014 170,000 light-years its visible diameter is more than twice that of our own galaxy. Its large MAGNITUDE 7.9 angular size in the sky (larger than a full moon) makes it one of the few URSA MAJOR galaxies whose individual regions can be isolated for study. Cataloged by Charles Messier (see p.73) as M101, the Pinwheel Galaxy ASYMMETRICAL DISK is a bright, nearby spiral galaxy, but M101\u2019s lopsided shape is thought to be one that reveals its nature only when caused by uneven distribution of mass studied with powerful telescopes or in the disk affecting the orbit of its stars. seen on long-exposure photographs. Because it lies face-on to Earth, most of the Pinwheel\u2019s light is spread out across its disk, and a casual glance reveals only the bright central core. Detailed photographs show that M101 has an extensive, though rather DUST LANE The thick dust lane around the Sombrero Galaxy is silhouetted against its bright disk in this Hubble Space Telescope image. SA SPIRAL GALAXY although its core is COMBINED VIEW of these was William Herschel, who unusually large and This composite image was the first to note the dark dust Sombrero Galaxy bright. Another shows the Sombrero at lanes that are M104\u2019s most distinctive odd feature is the X-ray (blue), optical feature. More recently, the Sombrero BEYOND THE MILKY WAY CATALOG NUMBERS dense swarm of (green), and infrared provided some of the first evidence globular star (orange) wavelengths. for objects lying far beyond our own M104, NGC 4594 clusters orbiting the galaxy (see panel, below). galaxy. More than DISTANCE 2,000 have been counted\u2014ten times more than orbit the Milky Way. 50 million light-years In the galaxy\u2019s core is a disk of DIAMETER bright material tilted relative to the VESTO SLIPHER galaxy\u2019s plane. It is probably the 50,000 light-years accretion disk of a central supermassive US astronomer Vesto Slipher black hole. X-ray emission from the (1875\u20131969) was one of the first to MAGNITUDE 8.0 region suggests some material is still suggest that the universe is bigger being absorbed by the hole. than our galaxy. In 1912, at Lowell VIRGO Observatory in Flagstaff, Arizona, he M104 was a late addition to identified red-shifted lines in M104\u2019s The dark dust lane and bulbous core Messier\u2019s catalog of celestial objects. spectrum.The lines told him the of the Sombrero Galaxy (M104) give He added it by hand to his copy of galaxy was receding at 2.25 million it a likeness to the traditional Mexican the catalog after discovering it in mph (3.6 million km\/h)\u2014too fast hat after which it is named. From 1781. Several other astronomers for it to reside within the Milky Way. Earth, we see the Sombrero Galaxy also found it independently. One from six degrees above its equatorial plane\u2014an ideal angle to provide a clear view of the core while also revealing the spiral arms. It is usually classified as an Sa or Sb spiral,","GALAXIES 317 S0 LENTICULAR GALAXY The Spindle (NGC 5866) is an E2 ELLIPTICAL GALAXY larger volume. It probably has a mass attractive galaxy orientated edge-on of several trillion suns, and is orbited Spindle Galaxy to observers on Earth. It is usually M60 by thousands of globular clusters. classified as a lenticular galaxy\u2014a disk Using the Hubble Space Telescope to CATALOG NUMBERS of stars, gas, and dust with a typical CATALOG NUMBERS measure the motions of M60\u2019s stars, bulging core, but with no sign of true astronomers have discovered that a M102 (not confirmed), spiral arms. However, spiral structure M60, NGC 4649 black hole of 2 billion solar masses NGC 5866 is hard to detect in an edge-on galaxy. lies at the galaxy\u2019s heart. DISTANCE DISTANCE The Spindle Galaxy is the major CLOSE NEIGHBORS member of the NGC 5866 Group, a 58 million light-years M60 lies very close to the spiral M59 (upper 40 million light-years small cluster of galaxies. Astronomers right), and the two galaxies are thought to have measured the way these galaxies DIAMETER be interacting. In a billion years, M60 DIAMETER move and have found that the Spindle may even swallow must contain an enormous mass of 120,000 light-years M59 entirely. 60,000 light-years material\u2014up to 1 billion solar masses, or 30 to 50 percent more than the MAGNITUDE 8.8 MAGNITUDE 9.9 Milky Way. DRACO VIRGO The Spindle Galaxy could be the mysterious entry number 102 M60 is one of several giant elliptical in Charles Messier\u2019s catalog of galaxies in the Virgo galaxy cluster astronomical features. Messier included (see p.329), the central cluster in our the object at first without a location, own Local Supercluster of galaxies. then later gave coordinates that did The galaxy and its neighbor, M59, not match any feasible object. Some were discovered in 1779 by German believe that Messier had listed the astronomer Johann K\u00f6hler, who Pinwheel Galaxy, M101, twice. More was observing a comet that likely, however, is that M102 was the passed close by. Charles Spindle, and he added 5 degrees to Messier (see p.73) found his measurements in error. them a few nights later, and added them to his MASSIVE SPINDLE catalog of objects that From Earth we see the Spindle Galaxy edge- might confuse comet on, giving it a cigar-shaped appearance with hunters. a fine silhouetted dust lane. M60 is similar in diameter to many spiral galaxies but, as an E2 elliptical, it is very nearly spherical, containing a much DISRUPTED SPIRAL GALAXIES The Antennae have been studied for what they can tell Antennae Galaxies us about galaxy collisions. Detailed images of the central CATALOG NUMBERS region show that it is lit by hundreds of bright, intense star NGC 4038, NGC 4039 clusters.These are thought to be forming as gas clouds in the DISTANCE galaxies become compressed by the collision, triggering 63 million light-years starbursts (see the Cigar Galaxy, p.314). Astronomers can use DIAMETER 360,000 the clusters\u2019 redness to estimate light-years (total) their age\u2014older clusters emit redder light because the brighter MAGNITUDE 10.5 blue stars are the most massive and therefore the first to die. CORVUS The Antennae Galaxies, NGC 4038 and 4039, are among the sky\u2019s most spectacular interacting galaxies. Seen from Earth, they appear as a central bright double-knot of material, with two long streamers of stars stretching in opposite directions, resembling an insect\u2019s antennae. However, powerful telescopes reveal that each streamer is in fact a spiral arm, uncurled from its parent galaxy by the tremendous gravitational forces of an intergalactic collision that began around 700 million years ago and continues today. THE BIGGER PICTURE CLOUDS AND CLUSTERS BEYOND THE MILKY WAY A wide-field view of the Antennae taken Turbulent dust clouds and brilliant star from Earth reveals both the bright, distorted clusters appear in a Hubble view of cores and the long, faint streamers formed the colliding Antennae Galaxies at by the disrupted spiral arms. right. The image above\u2014a composite of a Hubble visible-light view with microwave observations from the Atacama Large Millimeter Array in Chile\u2014reveals clouds of dense, cold gas (pink, red, and yellow areas) from which new stars are forming.","318 GALAXIES DISRUPTED SPIRAL GALAXY Despite being referred to only by a the collision is still going on, and the gas than in the stars, so it is usually number rather than a name (its long dust lane is the \u201cghost\u201d of a galaxy most obvious at radio wavelengths. ESO 510-G13 designation comes from the European that ESO 510-G13 has swallowed\u2014as Our near neighbor M31 (see pp.312\u2013 Southern Observatory\u2019s catalog), seen in the active galaxy Centaurus A 13) has such a distortion, and the CATALOG NUMBER ESO 510-G13 ESO 510-G13 is one of the most (see p.322). Alternatively, the disk Milky Way seems to have one, too, DISTANCE 150 million light-years intriguing galaxies in the sky. It is an might have been warped by the perhaps caused by interaction with its edge-on spiral with a clear dust lane gravity of a nearby galaxy.The galaxy own family of smaller neighbors. DIAMETER marking its central plane.The dust responsible might be a small neighbor lane has an obvious twist. or a more distant but larger member WARPED DISK 105,000 of the same group. As their techniques The bright core of ESO 510-G13 silhouettes light-years The most obvious explanation for and instruments improve, astronomers the galaxy\u2019s warped dust lane in this image. the kink is that ESO 510-G13 has are finding this kind of distortion is The blue glow on the right is a huge area of MAGNITUDE had a close encounter or collision common in spirals, although it often bright young stars\u2014evidence, perhaps, of a with another galaxy in its recent past. shows up more in the distribution of collision in the galaxy\u2019s recent history. 13.3 Some astronomers have suggested that HYDRA SB0 BARRED SPIRAL GALAXY DISRUPTED SPIRAL GALAXIES EXPLORING SPACE NGC 6782 The Mice SIMULATING GALAXY COLLISIONS CATALOG NUMBER CATALOG NUMBER The great challenge for astronomers studying NGC 6782 NGC 4676 colliding galaxies is that they can only ever see DISTANCE DISTANCE one stage in a story that unfolds over millions 183 million light-years 300 million light-years of years. Fortunately, today\u2019s supercomputers 0 MY DIAMETER DIAMETER can help to speed things up. By building \u201cmodel\u201d galaxies with simplified star 82,000 light-years 300,000 light-years clouds, gas, dust, and dark matter, then smashing them into each other in a MAGNITUDE 12.7 MAGNITUDE 14.7 computer, astronomers can measure how gravity affects the fate of the galaxies. PAVO COMA BERENICES 400 MY 650 MY SPIRAL COLLISION SIMULATION 1,000 MY The Hubble Space Telescope imaged The object classified as NGC 4676 This computer simulation shows two spiral galaxies BEYOND THE MILKY WAY the apparently normal barred spiral is in fact a pair of colliding galaxies\u2014 interacting and merging to form a large, irregular galaxy NGC 6782 in 2001. Using called the Mice because they appear galaxy. Time is measured in millions of years (My). ultraviolet detectors, it studied the to have white bodies and long, narrow pattern of the galaxy\u2019s hottest material. tails. As with the Antennae Galaxies DESTINED TO UNITE The image (see below) showed, in (see p.317), the long streamers are the Although currently moving apart from pale blue, two rings of stars so brilliant result of the spiral arms \u201cunwinding\u201d a close encounter, the Mice are and hot that they emit most of their during the collision\u2014though in this gravitationally locked together light as ultraviolet.The inner ring lies case one of the arms lies edge-on to and doomed eventually to in the galaxy\u2019s bar and could have us and so appears to be long and merge, perhaps resulting been ignited by tidal forces between straight, despite being strongly curved in the formation of the bar and the rest of the galaxy.The away from us. Knots of bright blue a new giant outer star ring is at the galaxy\u2019s edge. stars in the streamers and the main elliptical galaxy. bodies of the galaxies show where bursts of star formation are taking place. Computer simulations of the collision (see panel, right) suggest that the galaxies are now separating after a closest approach 160 million years ago. ULTRAVIOLET STAR RINGS HIDDEN EXTENT Image processing allows astronomers to amplify faint light from the outlying parts of the Mice, revealing their true shape and extent.","GALAXIES 319 DISRUPTED SPIRAL GALAXY through each other at high speed formation. An inward-traveling shock disrupted shape and vigorous star while orientated at right angles to wave is probably responsible for the formation, while a yellow galaxy Cartwheel Galaxy each other.The rotating density wave core\u2019s unusual \u201cbull\u2019s-eye\u201d appearance. could have been stripped of its that is normally responsible for the star-forming gas in the encounter. CATALOG NUMBER ESO spiral arms was disrupted in this case, For years, most astronomers However, recent radio observations 350-G40 resulting in the disappearance of the suspected that one of the Cartwheel\u2019s have shown a telltale stream of gas spiral structure. Meanwhile, a shock two immediate neighbors was leading from the Cartwheel toward DISTANCE wave spread to the outer edge of the responsible for the collision. Both another small galaxy, a quarter of a galaxy, creating a ring of vigorous star showed signs of being the culprit\u2014 million light-years away. 500 million light-years a nearby small, blue galaxy has a DIAMETER 150,000 light-years MAGNITUDE 19.3 SCULPTOR If the Cartwheel Galaxy looks unusual, it\u2019s because it is the victim of an intergalactic \u201chit-and-run.\u201dThe Cartwheel was once a normal spiral galaxy. As we see the galaxy, it is recovering from a head-on collision with a smaller runaway galaxy many millions of years earlier in its history. Such events are rare in the cosmos\u2014 galactic collisions usually involve grazing encounters or a slow dance toward an eventual merger.The Cartwheel shows what happens when two galaxies pass CLOUDS IN THE CORE SPIRAL REGENERATION So-called \u201ccomet The \u201cspokes\u201d of the Cartwheel Galaxy clouds,\u201d each a (on the left) are the ghostly outlines thousand light-years of returning spiral arms. long, are found in the Cartwheel\u2019s core. They are thought to arise as hot, fast-moving gas set in motion by the collision plows through denser, slower-moving matter. RING GALAXY LOW-SURFACE-BRIGHTNESS GALAXY Hoag\u2019s Object Malin 1 CATALOG NUMBER PGC CATALOG NUMBER 54559 None DISTANCE DISTANCE 500 million light-years 1 billion light-years DIAMETER DIAMETER 120,000 light-years 600,000 light-years MAGNITUDE 15.0 MAGNITUDE 25.7 SERPENS COMA BERENICES Hoag\u2019s Object is one of the most Despite its dull appearance, Malin 1 BEYOND THE MILKY WAY bizarre galaxies in the sky. Although is an extremely important galaxy. its ring structure suggests parallels Discovered by accident in 1987, it is to the Cartwheel Galaxy (a an enormous but faint spiral that is spiral disrupted by a head-on for some reason poor at forming collision, see above), there are no stars. It seems that such low-surface- nearby galaxies that could have brightness galaxies could account caused an impact. One of two for up to half the galaxies in the theories might account for the universe, though Malin 1 is one shape of Hoag\u2019s Object and of the largest of the type. that of similar ring galaxies. The galaxies may be members of an unusual class of spiral in which the two arms develop into a circle. Alternatively, they may be former elliptical galaxies that have each swallowed another galaxy, creating a surrounding ring of star-forming material. SEE-THROUGH GALAXY MALIN 1 IN A NEGATIVE IMAGE The gap between Hoag\u2019s Object\u2019s core and its ring is truly transparent\u2014a background galaxy can be seen through it near the top of this image. However, the gap could still contain large numbers of faint stars.","320 ACTIVE GALAXIES ACTIVE GALAXIES material blasted from the nucleus expands into a lobe as it is slowed by the intergalactic medium 28\u201331 Matter MANY GALAXIES ACROSS THE UNIVERSE show surprising 34\u201337 Radiation features that mark them as out of the ordinary. Although 40\u201343 Space and time there are several types of these strange galaxies, their 226\u201329 The Milky Way unusual behavior can always be traced back to powerful 302\u2013305 Types of galaxy activity in their nucleus\u2014it seems that there is an underlying 306\u2013309 Galaxy evolution similarity between them, and for this reason they are often studied together under the term \u201cactive galaxies.\u201d WHAT ARE ACTIVE GALAXIES? Astronomers think that the features of active galaxies are linked to their central giant black holes. Most, if not all, galaxies have black holes with the mass of many millions jet of particles of suns, known as supermassive black holes, at their nuclei (see p.305), but most such shooting from black hole\u2019s magnetic pole black holes are dormant\u2014all material in these galaxies is in a stable orbit around the star being ripped apart by black hole. In active galaxies, matter is still falling inward, and as it falls it is heated by intense gravity intense gravity, generating a brilliant blast of radiation. As the black hole \u201cengine\u201d pulls location of black hole matter in, the superheated material forms a spiraling accretion disk.The hot disk torus of dust, emits X-rays and other fierce, high-energy magnetic field line typically 10 light- radiation. Around the outer edge of the electron years across jet expands into disk, a dense torus (doughnut shape) photon of radio- lobe thousands of dust and gas forms.The intense wavelength of light-years long magnetic field surrounding the radiation black hole also catches some of the infalling material, firing it out as two narrow beams at the poles, at spinning right angles to the plane of the accretion disk accretion disk.These jets shine with of heated gas radio-wavelength radiation, due to the synchrotron mechanism (right). BLACK-HOLE ENGINE SYNCHROTRON RADIATION The black hole of an active galactic nucleus is As electrons from the black-hole jets move through the surrounded by a bright accretion disk and an black hole\u2019s magnetic field, they are forced into spiral outer dust cloud. Jets of material flow paths, releasing synchrotron radiation\u2014a type of EM outward from the black hole\u2019s poles. radiation that is most intense at long radio wavelengths. ACTIVE TYPES RADIO GALAXY QUASAR In a radio galaxy such In quasars, Earth- Astronomers distinguish between four major as NGC 383, the bound observers can types of active galaxies. Each displays its own set central region of the see over the dust of active features, and in each case these features nucleus is hidden by ring, and brilliant are evidence of the violent activity at the the edge-on dust ring, light from the nucleus nucleus. Radio galaxies are the most intense and observers on and disk drowns out natural sources of radio waves in the sky.The Earth see only the the light of the emissions typically come from two huge lobes radio jets and lobes. surrounding galaxy. on either side of an apparently innocuous parent galaxy (and often linked to it by narrow jets). radio jet SEYFERT GALAXY Seyfert galaxies are relatively normal spirals with dust ring In Seyfert galaxies a compact, luminous nucleus that may vary in such as M106, the BEYOND THE MILKY WAY brightness over just a few days. Quasars appear RADIO SOURCE 3C31 QUASAR PG 0052+251 nucleus and accretion as starlike points of light that show similar but (RADIO GALAXY NGC 383) disk are exposed to more extreme variability. Red-shifted lines in BLAZAR our view, as in a their spectra reveal that they are extremely distant BLAZAR 3C 279 Blazars are active quasar, but the galaxies\u2014powerful modern telescopes can galaxies aligned so that activity is weak. resolve them as galaxies with incredibly brilliant observers on Earth look cores.They are more powerful and more distant straight down the black- cousins of the Seyfert galaxies. Finally, blazars hole jet onto the nucleus. (also known as BL Lacertae objects) are starlike The galaxy is hidden variable points similar to quasars, but with no by the brilliant light, significant lines in their spectra.The standard but radio lobes can model of the black-hole engine (above) can sometimes be detected, explain the major features of each type\u2014how as in blazar 3C 279. the galaxy appears depends on the intensity of its activity, and the angle at which we see it. SEYFERT GALAXY M106","321 THE HISTORY OF NUDGED BACK INTO LIFE Optical images of Centaurus A ACTIVE GALAXIES clearly show the dark dust lane of a spiral colliding with The distribution of different types of this elliptical galaxy. The active galaxies in the universe provides overlaid radio map shows the burst of activity\u2014the jets and clues about how they evolve. Quasars and plumes\u2014triggered by this event. blazars are never seen close to Earth.They are always faint and distant, with red shifts indicating that they lie billions of light-years from Earth\u2014we are seeing them as they were in much earlier times. Radio and Seyfert galaxies, in contrast, are scattered throughout the nearby universe, and radio jets are linked to both spiral and elliptical galaxies. So what happened to the quasars and blazars? It seems likely that they represent a brief phase in a galaxy\u2019s evolution, false-color soon after its birth. At this time, material in the central regions would radio image have had chaotic orbits, and the central black hole engine would have of jet of particles been fueled by a continuous supply of infalling stars, gas, and dust. As the black hole swept up the available matter, objects with stable orbits at a safe distance remained. Starved of fuel, the engine would have petered out, and the quasar became dormant\u2014a normal galaxy such as the Milky Way.Today, such galaxies can become active again if they are involved in collisions dust lane false-color that send new material falling in toward the black (optical radio image of hole. Many nearby radio and Seyfert galaxies show image) galaxy\u2019s lobe evidence of recent collisions or close encounters, and some of these galaxies are close enough for optical view of infrared telescopes to image galaxy\u2019s elliptical arrangement of stars disk of the dust rings around their EXPLORING SPACE spiral galaxy cores directly (see p.323). However, levels of recent SUPERLUMINAL JETS jet of particles activity are restrained\u2014 emitting radio even the most spectacular Year Some quasars and blazars appear to defy the laws waves radio galaxies generate 1992 of physics. Image sequences, taken years apart, little energy compared to active nucleus of quasars, while Seyferts show jets of material blasting away from the galaxy, containing are the feeblest type nucleus, apparently traveling faster than the an active black hole of active galaxy. 1994 speed of light.This apparent motion is called surrounded by a \u201csuperluminal.\u201d In reality, it is an illusion, created bright accretion when jets traveling at very high speeds, of up to disk and a dust ring 1996 99 percent of the speed of light, happen to be pointing almost directly toward us. ACTIVE GALAXY This idealized active galaxy is a spiral with a bright nucleus, which hides an active black hole. From the 1998 TIME-LAPSE SEQUENCE black hole\u2019s poles blast two jets of particles, leaving at These images show jet emissions from blazar 3C 279, 20 40 60 80 taken at intervals of almost two years, and showing close to light speed, only slowing and billowing out Distance (light-years) motion apparently five times the speed of light. into lobes many thousands of light-years away, as the particles hit the intergalactic medium. IS THE MILKY WAY ACTIVE? ANTIMATTER FOUNTAIN This gamma-ray image traces positrons The Milky Way galaxy, like any galaxy with a central black hole, has the (antielectrons) around the Milky Way. potential to be active, and there is intriguing evidence that it might The horizontal feature is the plane of the Galaxy, with the fountain above it. have burst into activity in the recent past. In 1997, scientists discovered a huge cloud of gamma-ray emission above the galactic center.The BEYOND THE MILKY WAY radiation has a distinctive frequency, suggesting it is the result of electrons encountering positrons\u2014their antimatter equivalent (see p.31)\u2014and annihilating in a burst of energy.The positrons might have been generated by activity at the core\u2014 perhaps an infall of matter into the black hole\u2014 and are now meeting scattered electrons in the outer galaxy and mutually annihilating to produce the distinctive glow. Since the clouds lie just 3,000 light-years from the galactic center, the activity must have occurred recently. GALACTIC CENTER This near-infrared image, taken using the Very Large Telescope in Chile, shows the center of the Milky Way. By following the motions of its central stars over more than 16 years, astronomers were able to determine that the supermassive black hole at the core is about 4 million times as massive as the Sun.","322 ACTIVE GALAXIES There are no simple rules governing the appearance of active galaxies. Some have a disrupted structure, seen either in visible light or at other wavelengths, while others appear normal at first, but radiate unusually large amounts of energy at certain wavelengths. In fact, the majority of galaxies show activity of one kind or another. However, a smaller proportion of galaxies have particularly active nuclei, powered by matter spiraling into their central black hole.These JET FROM AN ACTIVE GALAXY include Seyfert galaxies, radio galaxies, quasars, and blazars.The vast Pictured in radio waves and false colors, majority of known active galaxies are distant quasars. Objects lying this jet of particles blasted from the core nearer to the Milky Way, although less spectacularly violent, are at of the galaxy M87 is a typical feature of least close enough for astronomers to study in detail. active galaxies with black-hole engines. TYPE-II SEYFERT GALAXY for so long partly because it lies just the galaxy, around 1,300 light- 4 degrees below the plane of the Milky years across, where great bursts Circinus Galaxy Way and is obscured by star clouds.The of star formation are occurring. full extent of the Circinus Galaxy\u2019s Finally, Hubble showed a cone- CIRCINUS CATALOG NUMBER extraordinary nature was revealed shaped cloud billowing above only when it was observed by the the plane of the galaxy.This is ESO 97-G13 Hubble Space Telescope in 1999.The matter ejected by the magnetic galaxy is a Seyfert (see p.320)\u2014a spiral fields of the black hole and glows SHAPE Sb spiral with an unusually bright, compact as it is heated by the ultraviolet region at its core, thought to result radiation from the nucleus. DISTANCE from material slowly drifting onto a massive central black hole. Hubble\u2019s CONE OF MATTER 13 million light-years infrared camera revealed how the The pinkish-white region near the core galaxy\u2019s gas is concentrated in a central of the Circinus Galaxy shows where DIAMETER ring, just 250 light-years in diameter, matter is being flung out, in a cone around the black hole. Also apparent shape, from the central black hole 37,000 light-years is a loose outer ring in the plane of into the gas cloud above the galaxy. MAGNITUDE 11.0 Although it is one of the nearest active galaxies to Earth, the spiral galaxy in Circinus went undiscovered until just a few decades ago. It remained hidden RADIO CONTINUUM COMPOSITE VIEW Centaurus A has been imaged at various wavelengths (left and below). The image at far left is a composite at optical, microwave, and X-ray wavelengths. RADIO (21-CM WAVELENGTH) jet OPTICAL WAVELENGTHS X-RAY WAVELENGTHS BEYOND THE MILKY WAY DUSTY DISK This Hubble Space Telescope close- up of Centaurus A (right) reveals dark interstellar dust, glowing orange gas clouds, and brilliant blue star clusters formed in the collision between two galaxies. RADIO GALAXY A ball of old yellow stars, NGC 5128 wavelengths. The shows some features typical of an Hubble Space Telescope Centaurus A elliptical galaxy, but its most striking looked through the dust aspect is the dark dust lane that cuts lanes with its infrared camera CENTAURUS CATALOG NUMBER across it, bisecting the uniform glow and found a huge accretion of stars with a ragged silhouette.What disk at the center\u2014a sure sign NGC 5128 is more, the galaxy is at the center of of an active black hole pulling a pair of vast radio lobes, 1 million in matter at Centaurus A\u2019s core. SHAPE Peculiar elliptical light-years across.The name of this It is now generally agreed radio source, Centaurus A, is now the that NGC 5128 is an elliptical DISTANCE most widely used name for the galaxy galaxy absorbing a spiral.The itself. Astronomers have studied ghost of the spiral is shown by 15 million light-years Centaurus A in detail at a range of the dust lane and by the bright star clusters that stud it\u2014perhaps DIAMETER generated by shock waves as the two galaxies merge. 80,000 light-years MAGNITUDE 7.0","ACTIVE GALAXIES 323 RADIO GALAXY class of galaxy often found at the cores TYPE-II SEYFERT GALAXY CARL SEYFERT old galaxy clusters.This huge ball of M87 stars seems to have a diameter roughly Fried Egg Galaxy US astronomer Carl Seyfert (1911\u2013 equivalent to that of the Milky Way, 60) was the son of a pharmacist VIRGO CATALOG NUMBERS but, because its stars are distributed CATALOG NUMBER NGC from Cleveland, Ohio. He studied across its spherical structure, it contains 7742 at Harvard and went on to work M87, NGC 4486 many more stars\u2014probably several at McDonald Observatory, then at trillion. Long-exposure photographs SHAPE Sb spiral Mount Wilson in California. It was SHAPE E1 giant have revealed that the galaxy also has here that he first identified the class elliptical an extensive halo of more loosely DISTANCE of galaxies with unusually bright scattered stars, extending well beyond nuclei that bear his name (see DISTANCE the central region in a more elongated 72 million light-years p.320). In 1951, he also shape.The galaxy also has an unrivaled discovered Seyfert\u2019s 60 million light-years collection of globular star clusters in DIAMETER Sextet, an interesting, orbit\u2014some astronomers estimate as compact cluster of DIAMETER many as 15,000 such groups. 36,000 light-years galaxies (see p.329). 120,000 light-years What is more, M87 is an active PEGASUS MAGNITUDE 11.6 galaxy\u2014its location coincides with MAGNITUDE 8.6 the Virgo A radio source, and with a The small spiral galaxy NGC 7742 strong source of X-rays.There is even resembles a fried egg because of the Lying at the heart of the Virgo galaxy a sign of this activity that is visible at intense yellow glow from its core.The cluster (see p.329), M87 is the closest optical wavelengths, in the form of a core is much brighter than is usual example of a giant elliptical galaxy\u2014a long, narrow jet of material being for a galaxy of this size, because this blasted from its interior. is a Seyfert galaxy, with a moderately GALACTIC ERUPTION active core. Seyferts emit radiation The black hole at M87\u2019s center is producing across a broad band of wavelengths\u2014 jets of energetic particles that are rising NGC 7742 is a Type-II\u2014a galaxy that through the surrounding cooler gas in a similar is brightest in way to gas erupting from a volcano on Earth. infrared light. energetic SEYFERT\u2019S OBSERVATORY radio-emitting At Nashville, Seyfert found time to give particles public lectures as well as raising support and supervising the construction of the cooler gas Arthur J. Dyer Observatory (above). supermassive black hole CELESTIAL EGG RADIO GALAXY The elliptical galaxy NGC 4261 lies material in NGC 4261 come from? TYPE-I SEYFERT GALAXY at the center of two great lobes of The most likely answer is that the NGC 4261 radio emission measuring 150,000 elliptical galaxy has merged with a NGC 5548 light-years from tip to tip. In many ways spiral in its relatively recent history. VIRGO CATALOG NUMBER a typical radio galaxy, it is also one of The spiral\u2019s individual stars have now CATALOG NUMBER the few active elliptical galaxies to become indistinguishable from the NGC 4261 have revealed its internal structure to stars that were originally part of the NGC 5548 astronomers. Infrared images from the elliptical galaxy, but the ghostly outline SHAPE E1 elliptical Hubble Space Telescope pierced the of the galaxy\u2019s gas and dust remains. SHAPE Sb spiral obscuring clouds of stars to reveal an DISTANCE unexpectedly dense disk of dusty DUST WHIRLPOOL DISTANCE material, apparently spiraling onto The Hubble Space Telescope\u2019s close-up image 100 million light-years the galaxy\u2019s central black hole. of the core reveals a dusty spiral of matter 220 million light-years within a ring of glowing outer clouds. A distinct DIAMETER Most elliptical galaxies are thought DIAMETER to be relatively dust-free, cone shows where matter is being 60,000 light-years so where did the flung off from the active 100,000 light-years galactic nucleus into MAGNITUDE 10.3 the radio lobes. BO\u00d6TES MAGNITUDE 10.5 NGC 5548 is a Type-I Seyfert galaxy; BEYOND THE MILKY WAY that is, a Seyfert that emits more ultraviolet and X-ray radiation than visible light. Like all Seyferts, it has a bright, compact core, but, unlike the Fried Egg Galaxy (see above), its core is an intense blue-white. Using the Chandra X-ray telescope, astronomers have detected an envelope of warm gas expanding around the core.The gas eventually forms two lobes of weak radio emission around the galaxy. RADIATING PLUMES HUBBLE IMAGE OF NGC 5548 Combining optical and radio images of NGC 4261 reveals its full extent. The visible part of the galaxy is the white blob in the center, while the orange plumes mark the radio-emitting regions.","324 RADIO GALAXY NGC 1275 CATALOG NUMBER NGC 1275 SHAPE Elliptical and distorted spiral DISTANCE 235 million light-years PERSEUS DIAMETER 70,000 light-years MAGNITUDE 11.6 ATYPICAL ELLIPTICAL GALAXY CLUSTERS IN THE NUCLEUS Despite being cataloged as a Seyfert NGC 1275 is unusual for an elliptical The core of NGC 1275 offers galaxy by Carl Seyfert himself (see galaxy in having a Seyfert-like core. clues to the origin of globular p.323), NGC 1275 has remained a The dark dust lanes are the remains clusters\u2014numerous globular-like mystery. Recent observations have of a now-disrupted separate spiral clusters are found here, but they shown that there are two objects\u2014 galaxy in front of NGC 1275. are composed of young blue, rather than old yellow stars. one in front of the other. A ghostly spiral galaxy, revealed by its bright blue star clusters, is responsible for the dust lanes that cross the bright central region, but this brighter region is in fact a separate galaxy. Despite its Seyfert-like core, it is an elliptical, not a spiral.This galactic giant lies at the heart of the Perseus galaxy cluster, and the foreground spiral is racing toward it at 6.7 million mph (10.8 million km\/h), its structure already disrupted by the elliptical\u2019s gravity. Adding to the complexity, the elliptical galaxy is also a radio source, and some astronomers have argued that it shows blazar-like activity (see BL Lacertae, opposite).Whatever the details, NGC 1275 displays many of the typical features of an active galactic nucleus. RADIO GALAXY Cygnus A CYGNUS CATALOG NUMBER 3C 405 SHAPE Pec (peculiar) DISTANCE 600 million light-years DIAMETER 120,000 light-years (excluding radio lobes) MAGNITUDE 15.0 BEYOND THE MILKY WAY The most spectacular and powerful (see p.322), which is thought to be at either end. Studies by the Chandra LOBES EMITTING RADIO WAVES radio galaxy in the nearby universe, an elliptical galaxy that has recently X-ray telescope have shown that This radio map of Cygnus A shows the Cygnus A was discovered as soon as swallowed a spiral. Recent detection Cygnus A lies at the center of a cloud galaxy\u2019s extremely narrow jets blasting from radio telescopes began operating in of a large cloud of red-shifted gas of hot but sparse gas.The jets have its core, the hot spots at the end its radio the 1950s. It features two huge lobes moving through the Cygnus A galaxy blown out a football-shaped cavity lobes, and the tendrils of hot gas falling of material emitting radio waves.The suggests that a collision may indeed in the gas so vast that it dwarfs the back toward the central galaxy. lobes are visibly linked to their origin be the root cause of the activity. central galaxy.Tendrils of gas, which at the heart of a faint, central, elliptical are emitting X-rays and radio waves, drawn by its gravitational pull.The galaxy by two long, narrow jets. From Astronomers have also argued are also falling back down through the hot spots are apparently created where lobe to lobe, the entire structure about the origin of the \u201chot spots,\u201d cavity onto the poles of the galaxy, the outward blast of the jets collides extends over half a million light-years. where the radio lobes glow brightest with the hot gas falling inward. Despite its prominence in the radio sky, mysteries still surround Cygnus A, largely because of its great remoteness. Early observations led astronomers to believe the central galaxy was in fact a pair of colliding galaxies. Hubble Space Telescope images suggested a resemblance to NGC 5128, the Centaurus A galaxy","ACTIVE GALAXIES 325 BLAZAR (BL LAC OBJECT) changed. For a variable star, it was QUASAR quasars, as well as other galaxies close very mysterious, showing rapid but to the quasars.The images showed BL Lacertae completely unpredictable variations. PKS 2349 that in many cases quasars do not just At the same time, it displayed a totally sit at the centers of their host galaxies, CATALOG NUMBER featureless spectrum\u2014it had neither CATALOG NUMBER but are involved in violent interactions the dark absorption lines seen in stars, with neighboring galaxies and other BL Lac nor the bright emission lines found in PKS 2349 quasars. PKS 2349 was referred to as galaxies (see p.35). It was not until a \u201csmoking gun\u201d because it showed SHAPE Elliptical 1969, when BL Lac was found to be SHAPE Disrupted these interactions so clearly.The a strong radio source, that astronomers quasar is surrounded by a ring of DISTANCE realized it might be a new type of DISTANCE faint material that may mark the active galaxy.Today it is seen as the outline of its host galaxy\u2014though, if 1 billion light-years founder member of a class of active 1.5 billion light-years so, the quasar itself is remarkably \u201coff- galaxies called blazars or BL Lac center.\u201d A small companion galaxy, DIAMETER objects. Blazars show many similarities DIAMETER about the size of the Large Magellanic to quasars but also some differences, Cloud (see p.310), also lies nearby Unknown most notably their featureless spectra. Unknown and seems doomed to collide with LACERTA MAGNITUDE 12.4\u201317.2 The mystery of BL Lac was solved PISCES MAGNITUDE 15.3 the quasar itself. in the 1970s, when two astronomers BL Lacertae (BL Lac for short) was blocked out or \u201cocculted\u201d BL Lac\u2019s The Hubble Space Telescope offered first cataloged as an irregular variable bright core to study its surroundings. astronomers an unprecedented star by German astronomer Cuno This revealed that it was embedded in chance to study quasars in detail Hoffmeister in the 1920s. Since then, a faint elliptical galaxy, whose light during the 1990s. One of their most astronomers\u2019 understanding of the was normally drowned out. Red- intriguing subjects was the otherwise object has shifted lines in the spectrum of this undistinguished quasar PKS 2349 galaxy confirmed BL Lac\u2019s great (referred to by its designation distance (see p.44).Today, blazars are in the catalog of the accepted as rare cases in which Earth\u2019s Australian Parkes radio position happens to align directly telescope). For the first with the jet of material blasting out time, astronomers of an active galactic nucleus, with no were able to see the obscuring material in the way. faint host galaxies surrounding MAP OF A BLAZAR This radio map of BL Lacertae shows the QUASAR CLOSE-UP intensity of radiation (contour lines) and In Hubble\u2019s image of also its polarization (color)\u2014an indication PKS 2349, the of magnetic field strength. The red object quasar is the bright at the top is the galaxy\u2019s nucleus, while central object, the the lower regions are parts of a radio jet. companion galaxy is the smaller bright region above it, and the supposed host galaxy is the fainter ring extending from the quasar. QUASAR QUASAR FIRST QUASAR At first, 3C 48 is 3C 273 3C 48 indistinguishable from foreground CATALOG NUMBERS CATALOG NUMBERS stars. It was only its unpredictable 3C 273, PKS 1226+02 3C 48, PKS 0134+029 variability and radio emission that SHAPE E4 elliptical SHAPE SB interacting marked it out as something special. DISTANCE DISTANCE could not have been emitted by any 2.1 billion light-years 2.8 billion light-years known element. Studies of similar lines in the optical counterpart of DIAMETER 160,000 light- HOST GALAXY DIAMETER 3C 273 (left) suggested that the lines years (excluding jet) By blocking the light from 3C 273\u2019s nucleus, of 3C 48 were hydrogen lines with a the Hubble Space Telescope was able to 100,000 light-years huge red shift, suggesting the object photograph detail (above) in the fainter was extremely distant and receding at VIRGO MAGNITUDE 12.8 surrounding galaxy, including traces of a TRIANGULUM MAGNITUDE 16.2 great speed. 3C 48 was therefore the spiral structure and a dust lane. first quasi-stellar object, or quasar, to The brightest quasar in the sky, 3C The radio source 3C 48 has a unique be discovered. 273 was the second to be discovered. the lines could have been formed by place in the history of the study of The existence of this radio source was hydrogen, oxygen, and magnesium if active galaxies. It was detected in the already known when, in 1963, the light was heavily red-shifted and 1950s, and in 1960 Allan Sandage Australian astronomer Cyril Hazard its source was racing away from us at (see panel, below) confirmed that it used an occultation by the Moon (see 16 percent of the speed of light, or coincided with a faint, blue, starlike p.69) to precisely establish its position, 107 million mph (173 million km\/h). object.The object\u2019s spectrum revealed linking the radio source to what We now know that the object is not strange emission lines (see p.35) that appeared to be an irregular variable a star, but a distant active galaxy. star.The star\u2019s spectrum had a forest of ALLAN SANDAGE BEYOND THE MILKY WAY unidentifiable dark emission lines (see p.35). Astronomers finally realized that Beginning his astronomical career as a student under Edwin Hubble RADIO JET (see p.45), Allan Sandage (1926\u2013 An enormous jet of particles, 100,000 light-years long, streams out 2010) has had a great influence on from the center of 3C 273. As the particles move away from the core our understanding of the universe\u2019s (the white square), their energy diminishes, as shown in this image by evolution. Sandage\u2019s studies have the transition from blue (indicating X-rays) to red (infrared radiation). focused on detecting Cepheid variable stars in distant galaxies, for use in measuring cosmological expansion. His many quasar discoveries were a natural offshoot from his studies of deep space.","326 GALAXY CLUSTERS GALAXY CLUSTERS DENSE CLUSTER The massive galaxy cluster Abell 1689 lies 22\u201323 The scale of the universe GALAXIES ARE NATURALLY GREGARIOUS. 2.2 billion light-years away. The yellow elliptical 24\u201327 Celestial objects Pulled together by their enormous gravity, galaxies are surrounded by arcs of light, which 38\u201339 Gravity, motion, and orbits they cluster tightly, sometimes orbiting one are images of more distant galaxies 40\u201343 Space and time another, often colliding. As galaxies slowly distorted by the cluster\u2019s 44\u201345 Expanding space move within a cluster, the cluster\u2019s gravitational lensing. 302\u2013305 Types of galaxy structure changes.The evolution of clusters can tell astronomers about dark matter, and clusters can even be used as cosmic \u201clenses\u201d to peer back into the early universe. TYPES OF CLUSTERS Some galaxy clusters are sparse, loose collections of galaxies. The smallest clusters are usually termed \u201cgroups.\u201dThe Local Group (see p.328), of which the Milky Way is a member, is one such cluster. Other clusters, such as the nearby Virgo Cluster (see p.329), are denser, containing many hundreds of galaxies in a chaotic distribution.Yet other clusters, such as the Coma Cluster (see p.332), are even more dense, with galaxies settled into a neat, spherical pattern around a center dominated by giant elliptical galaxies. Although clusters differ in density, the volume of space they occupy is generally the same\u2014a few million light-years across. Not all galaxies exist in clusters\u2014there are more isolated \u201cfield galaxies\u201d than there are cluster galaxies. Some galaxy types do not exist outside clusters, however. Giant ellipticals (see p.304) always lie near the center of large clusters, as do vast, diffuse cD galaxies (below right).The most numerous cluster components may be invisible, including faint, diffuse dwarf elliptical galaxies and proposed \u201cdark galaxies.\u201d A dark galaxy would consist of hydrogen gas and material too thin to condense and ignite stars.The Andromeda Galaxy (M31) first such galaxy may have been found, in the Virgo Cluster, in early 2005. SPARSE CLUSTER This sparse cluster, or group, of galaxies is in fact the Local Group, containing the Milky Way and its galactic neighbors. Most Milky of the galaxies are orbiting Way either the Milky Way or the Andromeda Galaxy (M31). BEYOND THE MILKY WAY dense core of cluster containing DWARF ELLIPTICAL cD GALAXY many large galaxies Most galaxies in the Local cD galaxies are similar Group, including the Sculptor to giant ellipticals but IDEAL DENSE CLUSTER Dwarf, are dwarf ellipticals. have extensive, sparse A dense cluster occupies the same They are invisible in distant outer haloes of stars. volume as a sparse cluster such as clusters, but must be present. They sometimes have the Local Group, but the galaxies hints of multiple cores, are mainly elliptical and have a suggesting the merger roughly spherical distribution of several smaller around the cluster\u2019s center. ellipticals. NGC 4889 (left) is a cD galaxy at the heart of the dense Coma Cluster.","GALAXY CLUSTERS 327 ABELL 2029 THE INTERGALACTIC MEDIUM This visible-light image of Abell 2029 shows Astronomers can estimate the overall mass of a galaxy cluster from the way in which its galaxies that it is an old, regular, spherical cluster full are moving, but also through the phenomenon of gravitational lensing\u2014an effect of general of elliptical galaxies. relativity (see pp.42\u201343).When a compact cluster lies in front of more distant galaxies, its mass bends the light passing close to it and deflects distorted images of the distant galaxies toward Earth. By measuring the strength of this effect, it is possible to measure the mass of the cluster and model how it is distributed. Galaxy clusters contain far more mass than the visible galaxies can account for, and most of it is in the matter that permeates the space between galaxies.This intergalactic medium is distributed around the cluster\u2019s center, rather than around the galaxies. X-ray satellites such as Chandra have revealed the nature of part INTERGALACTIC GAS of this material\u2014large galaxy clusters often contain huge clouds of An X-ray image of cluster Abell sparse, hot gas, glowing at X-ray wavelengths. Most is hydrogen, but 2029 shows the hot gas cloud heavier elements are present. It is thought to originate in the cluster around its center. If not for the galaxies, and to be stripped away during encounters and collisions. gravity of the cluster\u2019s dark Most of a cluster\u2019s mass is not gas, however, but dark matter. matter, this gas would escape. apparent position path of light without GRAVITATIONAL LENSING and distorted gravitational lensing Light leaves a distant galaxy in all shape of multiple directions. When it passes close to galaxy images light bent toward a massive cluster of galaxies, it is observer by lens deflected from its path, due to the way mass distorts space. Light paths arrive at Earth apparently from different directions, creating multiple, distorted images of the galaxy. actual position and shape of galaxy PERFECT ARC galaxy cluster This striking example of lensing is created acting as a by the cluster CL-2244-02. The lensed gravitational lens galaxy, unlike the cluster galaxies, is blue, so it must be a spiral or an irregular. CLUSTER EVOLUTION observer in the Milky Way Astronomers have built a picture of cluster development that complements their models of galaxy evolution (see VIOLENT MERGER pp.306\u2013309). According to their thinking, galaxy clusters Cluster Abell 400\u2019s core start as loose collections of gas-rich spirals, irregulars, and (left) shows two galaxies small ellipticals. Because of their proximity and huge gravity, merging to form a giant the spirals tend to merge, regenerating as spirals or forming elliptical. Radio images ellipticals. Each interaction drives off more of the galaxies\u2019 (below) reveal that both free gas into the intergalactic medium.The high temperature are active galaxies. Such a and speed of atoms in this medium prevents their recapture merger is typical of those by the cluster\u2019s galaxies. At this stage, the cluster is irregular, that shape galaxy clusters. or \u201cunrelaxed,\u201d and the pattern of galaxies and intergalactic IRREGULAR AND RELAXED CLUSTERS gas is irregular and chaotic. However, as galaxies swing BEYOND THE MILKY WAY The central regions of the Virgo Cluster around each other, their random motions are eliminated and they settle (above) and the Coma Cluster (below) show into a stable, spherical, \u201crelaxed\u201d distribution around the cluster\u2019s center. the difference between an irregular and a Eventually, even the largest elliptical galaxies begin to merge, more spherical (relaxed) pattern of galaxies. forming giant ellipticals and cD galaxies.The hot gas, freed from ties to individual galaxies, sinks into the center of the cluster, where it lies evenly around the cluster\u2019s major elliptical galaxies.What remains is an old, spherical, relaxed cluster full of ellipticals.","328 GALAXY CLUSTERS The shape and size of galaxy clusters are thought to be linked to their evolution. Clusters range from small groups comprising young, gas-rich irregular and spiral galaxies, to highly evolved clusters dominated by giant ellipticals, with a central cloud of gas so hot that it emits X-rays. Astronomers can study details in nearby clusters that are too faint to see in STEPHAN\u2019S QUINTET distant clusters. Earth\u2019s neighboring clusters do not offer This elegant group of five galaxies shows a spectacle to stargazers, however, because clusters are so that clusters are constantly changing\u2014two vast that their members are widely scattered across the sky. of its spiral galaxies are colliding, while a To appreciate clusters in a single picture, it is necessary to third is being distorted by their gravity and peer tens of millions of light-years into deep space. is doomed to collide with them one day. Andromeda Galaxy, M31 IRREGULAR CLUSTER the other of these large spirals.The spiral.The Local Group appears to be third large spiral in the group, M33 relatively young. Its major galaxies are Triangulum Galaxy, M33 Local Group (see p.311), may also be trapped in all spirals, and there is little matter in LOCAL GROUP MEMBERS a long orbit around M31. the space between galaxies\u2014most of DISTANCE the cluster\u2019s gas is still trapped in the Since Earth is in the midst of the Local Outnumbering these spirals is a spirals. It is in an early state of cluster Group, the galaxies are scattered around 0\u20135 million light-years host of dwarf elliptical and irregular evolution.The Milky Way is currently the sky. However, two large members, galaxies. Examples include SagDEG colliding with the Magellanic Clouds, M33 and M31, are near enough in the NUMBER OF GALAXIES 46 and the two Magellanic clouds (see and is heading inexorably toward an sky to appear in the same frame. pp.310\u2013311), as well as M110 and ultimate merger with M31. BRIGHTEST MEMBERS M32, both ellipticals orbiting the M31 Milky Way; M31 (magnitude 3.5) ANDROMEDA AND TRIANGULUM BARNARD\u2019S GALAXY This small, irregular galaxy (right), cataloged The Local Group is the small galaxy as NGC 6822, lies 1.7 million light-years away cluster of which the Milky Way is a within the Local Group. It is rich in gas and member. From Earth, its members dust, with many pinkish star-birth regions. appear dispersed throughout the sky, but some of its galaxies are grouped in the constellations of Andromeda and Triangulum. In space, the core of the group comprises about 30 members in a region just over 3 million light- years across. It is dominated by the Andromeda Galaxy (M31; see pp.312\u2013 313), and the Milky Way. Most of the smaller galaxies orbit close to one or FORNAX DWARF GALAXY This dwarf spheroidal galaxy (left) has no obvious nucleus. Such faint and diffuse galaxies are easily missed in more distant galaxy clusters, but they are probably the most numerous. BEYOND THE MILKY WAY THE MILKY WAY GALAXY A major member of the Local Group is the Milky Way galaxy. Earth is within the galaxy\u2019s disk, so our view is edge- on and stretched across the sky.","GALAXY CLUSTERS 329 IRREGULAR CLUSTER the Local Group. It is also a young IRREGULAR CLUSTER \u201cgroups\u201d is striking\u2014the Virgo Cluster cluster of irregular and spiral galaxies, contains around 160 major spiral and Sculptor Group with no major ellipticals. It is possible Virgo Cluster elliptical galaxies crammed into a that this group, the Local Group, and volume little larger than that of the ALTERNATIVE NAME another group called Maffei 1 were ALTERNATIVE NAME Local Group, along with more than once part of the same larger cluster. 2,000 smaller galaxies. At its heart lie South Polar Group Virgo I Cluster the giant ellipticals M87 (see p.323), The closest member to Earth is M84, and M86, which are thought to DISTANCE 9 million NGC 55, an irregular galaxy that, DISTANCE 52 million have formed from the collisions of light-years to center like the Large Magellanic Cloud (see light-years to center spirals over billions of years. Each p.310), shows enough structure for giant elliptical seems to be at the center NUMBER OF GALAXIES some astronomers to consider it a NUMBER OF GALAXIES of its own subgroup of galaxies\u2014 single-armed spiral.The dominant the cluster has not yet settled to 19 (6 major) galaxy, however, is NGC 253.This 2,000 (160 major) become uniform.The cluster\u2019s gravity large spiral is the same size as the influences a huge region, extending as SCULPTOR BRIGHTEST MEMBER Milky Way and more than twice the VIRGO BRIGHTEST MEMBER far as the Local Group and beyond\u2014 size of any other galaxy in the group. the Milky Way and its neighbors are NGC 253 (8.2) M49 (9.3) falling toward the Virgo Cluster at GALAXY NGC 253 900,000 mph (1.4 million km\/h). Lying just beyond the gravitational This large spiral dominates the Sculptor The Virgo Cluster is the nearest boundaries of the Local Group, the Group in this wide-field image. Most galaxy cluster worthy of the name; Sculptor Group is similar in size to of the other galaxies are it is a dense collection of galaxies at too faint to be seen the heart of the larger supercluster to without powerful which the Local Group also belongs. telescopes. The contrast with smaller galaxy STARBURST GALAXY CENTER OF THE CLUSTER NGC 253 is a spiral starburst galaxy\u2014 The Virgo Cluster\u2019s core has a high density a galaxy undergoing a surge of star of large galaxies. The two bright galaxies formation. The surge may have been on the right are the ellipticals M84 and M86. triggered by a series of supernovae. EXPLORING SPACE REGULAR CLUSTER mostly ellipticals, distributed evenly COMPACT GROUP around the giant elliptical NGC X-RAY IMAGING Fornax Cluster 1399. Dwarf galaxies lying between Seyfert\u2019s Sextet AND CLUSTER GAS the major ones are also mostly small FORNAX CATALOG NUMBER ellipticals, suggesting that the cluster SERPENS CATALOG NUMBERS Many galaxy clusters are strong formed long ago and that interactions sources of X-rays, and orbiting Abell S 373 between its galaxies have had time to NGC 6027 and X-ray telescopes can reveal features strip away most of their star-forming NGC 6027A\u2013C that remain hidden in visible-light DISTANCE 65 million gas (see p.327).This account of the images.While some X-ray sources light-years to center cluster\u2019s evolution has recently been DISTANCE are located at the centers of the confirmed by the orbiting Chandra cluster galaxies, the majority of NUMBER OF GALAXIES X-ray observatory (see panel, left). 190 million light-years radiation often comes from diffuse gas clouds, independent of the 54 major galaxies NUMBER OF GALAXIES 4 individual galaxies.The process that strips gas out of the cluster BRIGHTEST MEMBER BRIGHTEST MEMBER galaxies (see p.327) also heats it to generate the X-rays.The NGC 1316 (9.8) NGC 6027 (14.7) distribution of gas offers clues to a cluster\u2019s age and history. Fornax is home to a relatively nearby Seyfert\u2019s Sextet actually contains just four members\u2014each a misshapen galaxy cluster, centered at around the spiral galaxy locked to the others in a gravitational waltz within a region same distance as the Virgo Cluster. of space no larger than the Milky Way.The sextet, as seen from Earth, is However, the Fornax Cluster is at a completed by a small face-on spiral that happens to lie in the background, later stage of evolution and by a distorted star cloud (at lower right in the image below). than the younger GALAXY NGC 1365 Virgo group. Here, One of the Fornax spiral galaxies are Cluster\u2019s few spirals, rare\u2014the cluster\u2019s NGC 1365 has a dust BEYOND THE MILKY WAY major galaxies are bar through its core. FORNAX IN X-RAYS CLUSTER CORE This image of the Fornax cluster shows X-ray- In the Fornax Cluster\u2019s emitting gas in blue. Both central galaxies have central region lie NGC 1399 trailing plumes of gas, suggesting that the (upper left of center) and entire cluster is moving through sparser clouds. NGC 1365 (bottom right). As a rule, elliptical galaxies predominate. QUARTET PLUS TWO","THE VIRGO CLUSTER Over 2,000 galaxies reside in the Virgo Cluster (see p.329), the nearest large cluster to us, some 50 million light-years away. The brightest of them are visible through amateur telescopes. Just below center is the elliptical galaxy M87 (see p.320), also known as the radio source Virgo A. M87 has an estimated mass of 2.4 trillion Suns, making it the biggest galaxy in our region of the Universe.","","332 GALAXY CLUSTERS REGULAR CLUSTER COMPACT GROUP Hydra Cluster Stephan\u2019s Quintet CATALOG NUMBER Abell 1060 CATALOG NUMBER DISTANCE 160 million light-years Hickson 92 NUMBER OF DISTANCE GALAXIES 340 million light-years 1,000+ (NGC 7320: 41 million light-years) BRIGHTEST PEGASUS NUMBER OF GALAXIES 4\/5 HYDRA MEMBER BRIGHTEST MEMBER NGC 3311 (11.6) NGC 7320 (13.6) The Hydra Cluster is similar in size First observed by French astronomer QUINTET CLOSE-UP to the huge Virgo Cluster (see p.329). E. M. Stephan at the University of This detailed Hubble Space Telescope view of It is the closest example of a \u201crelaxed\u201d Marseilles in 1877, Stephan\u2019s Quintet Stephan\u2019s Quintet shows chains of stars linking cluster (see p.327) of mainly elliptical appears to be a remarkably compact several of its interacting galaxies. galaxies in a spherical distribution. Its cluster of five galaxies.The galaxies hot X-ray gas also forms a spherical are a mixture of spirals, barred spirals, cloud around the core.The cluster is and ellipticals and show clear signs centered on two giant elliptical galaxies of disruption from interactions.The and an edge-on spiral, each 150,000 largest galaxy as seen from Earth, light-years across.These galaxies are NGC 7320, is probably a foreground interacting\u2014the ellipticals\u2019 gravity object lying in front of a quartet of has warped the spiral, while both interacting galaxies.The spectral red ellipticals have distorted outer haloes. shift (see p.35) of NGC 7320 is much The cluster is the major member of smaller than those of the other four the Hydra Supercluster, which adjoins galaxies, and instead matches that of the Local Supercluster (see pp.336\u201339). several other galaxies close to it in the sky. Since it also appears physically HEART OF THE HYDRA CLUSTER different from the quartet, it seems In this image, the central giant ellipticals likely that NGC 7320 is much closer NGC 3309 and 3311 lie below the large, and the unusual red shift is a normal blue spiral NGC 3312. The two bright result of the expansion of space (see objects on either side are foreground stars. p.44). However, a few astronomers claim that trails of material link NGC SPIRAL SILHOUETTE 7320 to other Quintet galaxies. If this NGC 3314, an unusual is the case, then the red shift suggests case of one spiral that the galaxy is moving very fast galaxy silhouetted relative to its neighbors and toward against another, is Earth, therefore reducing its overall one of Hydra\u2019s most speed of recession and its red beautiful objects. shift. Or perhaps the red shift does not originate from its motion at all.These competing theories have turned Stephan\u2019s Quintet into a battleground for the small minority of astronomers who think that red shifts are not all caused by the expansion of space, and that Hubble\u2019s Law (see p.44) does not always apply. FOUR OR FIVE? The quintet consists of a quartet of yellow galaxies beside the white spiral NGC 7320. The contrasting appearance of NGC 7320 suggests it lies in front of the other galaxies. BEYOND THE MILKY WAY REGULAR CLUSTER in 1785, this is one of the nearest the cluster\u2019s center lie the giant elliptical highly evolved or \u201crelaxed\u201d galaxy NGC 4889 and the lenticular galaxy NGC Coma Cluster clusters (see p.327). It is very dense, 4874. Most of the spirals and irregulars are with over 3,000 galaxies, and is in the outer regions. X-ray images show two COMA BERENICES CATALOG NUMBER dominated by elliptical and lenticular distinct patches of cluster gas, suggesting that galaxies. Because it is near the north the cluster is absorbing a smaller Abell 1656 galactic pole (and therefore free of the cluster of galaxies. Like the dense star fields of the Milky Way), Virgo and Hydra clusters, DISTANCE it is well studied. Swiss-American Coma forms the core of its astronomer Fritz Zwicky used Coma own galaxy supercluster. 300 million light-years when he made the first measurements of galaxy movements within a cluster COMA ELLIPTICAL NUMBER OF GALAXIES in the 1930s. He found the cluster This image is dominated contained many times more mass than by the Coma Cluster 3,000+ its visible galaxies suggested\u2014an idea elliptical NGC 4881 that was not accepted until the 1970s. and a nearby spiral. BRIGHTEST MEMBER Overall, the cluster is moving away at The other galaxies 16 million mph (25 million km\/h). At are far more distant. NGC 4889 (13.2) Although it lies near the Virgo Cluster in the sky (see p.329), the Coma Cluster is much farther away. First recognized by William Herschel as a concentration of \u201cfine nebulae\u201d","GALAXY CLUSTERS 333 IRREGULAR CLUSTER stage of development. In keeping REGULAR CLUSTER GEORGE ABELL with the best models of such clusters\u2019 Hercules Cluster formation (see p.327), it shows little Abell 1689 George Abell (1927\u20131983) was a sign of structure.Within the cluster, career astronomer and popularizer CATALOG NUMBER several pairs or groups of galaxies CATALOG NUMBER of science who carried out the seem to be merging or interacting\u2014 first, and most influential, survey of Abell 2151 encounters that will transform them Abell 1689 galaxy clusters. After working on into different kinds of galaxies and the Palomar Sky Survey during HERCULES DISTANCE reduce their random movements until VIRGO DISTANCE the 1940s and 1950s, using the they become more evenly distributed. Palomar Schmidt telescope, he 500 million light-years The most prominent of these mergers 2.2 billion light-years turned his attention to analyzing is NGC 6050, a pair of interlocking the results, developing methods NUMBER OF GALAXIES spiral galaxies near the cluster\u2019s center NUMBER OF GALAXIES for distinguishing galaxy clusters that may eventually form the core of from isolated field galaxies, and 100+ a giant elliptical, such as those found 3,000+ classifying clusters into types. in more evolved clusters. BRIGHTEST MEMBER BRIGHTEST MEMBER NGC 6041A (14.4) Unnamed galaxy (17.0) The small Hercules Cluster is Abell 1689 is one of the densest galaxy dominated by spiral and irregular clusters known, with thousands of galaxies, suggesting that it is in an early galaxies packed into a volume of space only 2 million light-years across. Its ball shape makes it a fine gravitational lens, bending the images of distant galaxies into arcs. By noting the lensing power throughout the cluster, astronomers have worked out the distribution of the cluster\u2019s dark matter. HERCULES FIELD This wide-field view captures most of the bright galaxies in Hercules and shows their irregular, \u201cunrelaxed\u201d distribution. LENSING IN CLUSTER ABELL 1689 REGULAR CLUSTER IRREGULAR CLUSTER X-ray images reveal what optical ones them through the intergalactic cannot\u2014that the cluster is forming medium. A fainter cloud of almost Abell 2065 Abell 2125 from the merger of several smaller equal size, enveloping hundreds more clusters.The most intense cloud of galaxies, has remarkably few heavy CORONA BOREALIS CATALOG NUMBER URSA MINOR CATALOG NUMBER X-ray emitting gas shows \u201cclumpiness,\u201d elements, suggesting that the gas- which indicates it has recently come stripping process becomes more Abell 2065 Abell 2125 together. Spectra reveal that the cloud powerful and thorough over time, is enriched with heavy elements such and that the cloud is much younger DISTANCE DISTANCE as iron, and close-up images show gas than its fainter neighbor. actively being stripped away from 1 billion light-years 3 billion light-years galaxies such as C153.With it, the gas Since X-ray evidence shows so carries atoms of heavy metals created much activity within the cluster, NUMBER OF GALAXIES NUMBER OF GALAXIES in supernova explosions, distributing astronomers have also imaged it at other wavelengths. Infrared telescopes, 1,000+ 1,000+ for example, have revealed BRIGHTEST MEMBER BRIGHTEST MEMBER enormous bursts of star formation going on in galaxies PGC 54876 (16.0) Magnitude 17.0 far from the cluster center. One possible explanation is Abell 2065, also known as the Corona Abell 2125 has been the subject of that, even at distances of up to BEYOND THE MILKY WAY Borealis Cluster, contains 400 or more intense scrutiny from the orbiting 1 million light-years, the tidal large galaxies. A highly evolved cluster Chandra X-Ray Observatory.The forces from the center of a like the Coma Cluster (opposite), it cluster lies close enough to Earth to large cluster are enough to emits X-rays from a diffuse cloud of see detail, but so far away that images disrupt nearby galaxies and hot gas. However, X-ray observations reaching Earth show an early and still trigger starbursts. have found two distinct X-ray cores, active phase of its evolution, 3 billion suggesting that Abell 2065 may be years ago. Abell 2125 is therefore ideal two already ancient clusters merging for testing ideas on cluster formation. together.The cluster lies at the center of the Corona Borealis Supercluster. THE CORONA BOREALIS CLUSTER ZOOMING IN ON C153 This sequence of Chandra X-ray images zooms into the hot gas cloud at the core of Abell 2125, showing how gas is being stripped from galaxy C153 (right).","334 BEYOND THE MILKY WAY REGULAR CLUSTER The cluster has taught astronomers The galaxies beyond Abell 2218 lie much about galaxy clusters, and about much farther away, and therefore their Abell 2218 galaxies themselves.The cluster\u2019s images come from a much earlier density is so great that it affects the time. Most of the lensed galaxies are CATALOG NUMBER shape of the surrounding space, as blue-white, suggesting they are young predicted by Einstein\u2019s theory of irregulars and spirals very different Abell 2218 general relativity (see p.42). Many from Abell 2218\u2019s own aged ellipticals. more distant galaxies lie directly Some of the lensed galaxies align with DISTANCE behind the cluster, and as light rays X-ray sources, suggesting they are from these objects pass close to Abell active galaxies. Recent studies yielded 2 billion light-years 2218, their paths are deflected and images of a galaxy so far beyond Abell focused toward Earth, in the same 2218 that all its light has been red- NUMBER OF GALAXIES way that a magnifying lens focuses sunlight.This gravitational lensing HOLE IN THE COSMIC BACKGROUND 250 or more (see p.327) brightens the images of In this composite image of Abell 2218, yellow galaxies that would otherwise be too and red depict the X-ray-emitting gas around DRACO BRIGHTEST MEMBER far away to detect. It results in a series its core. The gas scatters the cosmic of distorted images of distant galaxies microwave background radiation, creating Unnamed galaxy (17.0) ringing the center of Abell 2218. a hole, outlined here by contours. Abell 2218 is a spectacular example of a highly evolved and extremely dense galaxy cluster. It contains more than 250 mostly elliptical galaxies in a volume of space roughly 1 million light-years across.","335 DISTORTED BY GRAVITY Most of the bright objects in this image are galaxies in the Abell 2218 cluster. The arcs are much more remote galaxies, their images distorted by Abell 2218\u2019s gravity. EXPLORING SPACE MAPPING THE MISSING MASS shifted into the infrared part of the Astronomers have now begun to use spikes The total mass of a cluster can be up to five BEYOND THE MILKY WAY spectrum. At the time, it was the most Abell 2218 to probe the origins of coincide times that of its visible galaxies, but the distant galaxy known, at 13 billion the universe. A phenomenon called with distribution of the other, dark matter was light-years from Earth. It must have the Sunyaev\u2013Zel\u2019dovich effect (see galaxies a mystery until recently. Gravitational formed shortly after the first stars, in caption, opposite) creates holes and lensing now allows astronomers to the aftermath of the Big Bang. ripples in the cosmic microwave measure the missing mass in clusters. background radiation shining through By analyzing images of lensed Gravitational lensing can also the cluster.This happens because gas galaxies, astronomers can reveal hidden properties of Abell 2218 around Abell 2218\u2019s core scatters pinpoint concentrations itself. Because the strength of lensing photons of microwave radiation, just of mass distorting depends on the cluster\u2019s density, it as Earth\u2019s atmosphere scatters light. the light as it offers a measure of the distribution of The strength of these ripples passes through all matter in the cluster\u2014including can be used to estimate the the cluster. the dark matter. Abell 2218 is one of true diameter of the cluster\u2019s core, the few galaxy clusters in which the and therefore its distance from Earth, cluster gas MAP OF CLUSTER CL0024+1654 pattern of visible matter (galaxies and independently of its red shift.The red and dark matter This mass map shows the difference X-ray-emitting gas) and the calculated shift and distance can then be used appear as a broad hump in distributions of visible and dark distribution of dark matter do not together to find the expansion rate around the cluster\u2019s core matter in a mature galaxy cluster. match, suggesting the cluster is not as of the universe (see p.44). uniform as it appears in visible light.","336 GALAXY SUPERCLUSTERS GALAXY SUPERCLUSTERS 22\u201323 The scale of the universe THE LARGEST-SCALE STRUCTURES in the universe are 24\u201327 Celestial objects galaxy superclusters\u2014collections of neighboring galaxy 28\u201331 Matter clusters that bunch together in chains and sheets stretching 34\u201337 Radiation across the cosmos.These structures are echoes of those 326\u201327 Galaxy clusters that formed in the Big Bang, and by studying the universe at these enormous scales astronomers can learn about the way it formed and our place within it. GALAXY SUPERCLUSTERS Just as galaxies are bound together by gravity into clusters, galaxy clusters PLOT OF GALAXIES themselves blur together at their edges to form even larger structures called This plot of a section of sky out to a distance superclusters.While individual clusters are typically about 10 million of 1 billion light-years shows how galaxies light-years in diameter (see p.326), superclusters are typically up to 200 cluster on the largest scale. million light-years across and merge with others at their edges.Where superclusters overlap, it is the gravitational behavior of individual clusters that determines to which supercluster they belong.The enormous size of superclusters and the great mass of galaxies in them allows them to modify the cosmological expansion of space (see pp.44\u201345), resulting in large- scale variations in the movement of galaxies.The best known example of this is a generalized flow of galaxies in our part of the universe, possibly toward a region known as the Great Attractor but more probably toward the more massive Shapley Supercluster directly behind it. Sculptor Group Local Group Virgo Cluster Virgo III groups Fornax Cluster Maffei group circle is 200 million Leo II groups light-years across MAP OF THE LOCAL SUPERCLUSTER This map of the Virgo Supercluster, THE GREAT ATTRACTOR centered on the Local Group, shows This view of the sky shows galaxies in the groups and clusters of galaxies linked direction of the Great Attractor. Recent into a chain. Each point denotes a studies suggest that galaxies are moving major galaxy\u2014there are thousands of toward the Shapley Supercluster behind smaller ones not pictured. the Attractor, rather than the Attractor itself. BEYOND THE MILKY WAY UKIDSS SURVEY THE LOCAL SUPERCLUSTER The core of the Virgo Cluster is seen here at A chain of galaxy clusters links our own small Local Group of galaxies, infrared wavelengths. containing the Milky Way, to the Virgo Cluster, some 52 million light- Infrared surveys give a years from Earth.This much larger cluster, containing up to 2,000 galaxies, more accurate measure marks the gravitational heart of the Local Supercluster (also known as the of the number of stars in Virgo Supercluster).The Local Supercluster contains at least 100 separate a galaxy than visible-light bright galaxy clusters scattered across 110 million light-years of space. surveys. They also reveal About two-thirds of these galaxy clusters are concentrated in a flattened, very distant galaxies disklike plane, while the remainder are scattered throughout a spherical whose light has been halo. However, in comparison with some other superclusters, the Local shifted into the infrared Supercluster appears to be relatively small and lightweight, with just a by cosmological single large cluster\u2014the Virgo Cluster\u2014at its heart surrounded by many expansion. The UKIRT smaller ones. Nevertheless, the Virgo Cluster\u2019s gravity is counteracting Infrared Deep Sky Survey cosmic expansion, pulling other clusters\u2014including our own\u2014toward it (UKIDSS) has mapped at speeds of up to 3.4 million mph (5.4 million kph). hundreds of millions of galaxies since 2005.","GALAXY SUPERCLUSTERS 337 LARGE-SCALE STRUCTURE A computer simulation of the large-scale structure of the universe shows the concentration of matter into filaments across a billion light-years of space. According to models of cosmic evolution, dark matter (yellow) formed clumps around which visible matter (lighter pinks) coalesced. Clusters of galaxies are concentrated at nodes where filaments meet; superclusters are strung out along the filaments. THE ORIGIN OF SUPERCLUSTERS The sheer size of galaxy superclusters makes their origins important in understanding the structure and properties of the universe as a whole.Theoretically, superclusters could either have formed from the gradual coalescence of matter pulled together by gravity alone, or they could mark large- scale concentrations of matter that were present from the earliest times and within which galaxies and galaxy clusters subsequently developed.The first detailed maps of the cosmic microwave background radiation (CMBR), produced by the COBE satellite in 1992, provided evidence that large-scale structures were present from the earliest times, confirming the second theory.The structures themselves are thought to have originated as microscopic variations in temperature and density in the early universe and then enlarged to enormous scale by cosmic expansion, eventually giving rise to the \u201cSwiss cheese\u201d distribution of matter in the universe today. RADIATION FROM THE SKY This elliptical map shows tiny variations in the temperature of the cosmic microwave background radiation (CMBR) over the whole sky, as measured by the Wilkinson Microwave Anisotropy Probe (WMAP). The variations\u2014anisotropies\u2014are color-coded according to the temperature scale shown below. TEMPERATURE VARIATION IN THE CMBR -0.0002\u00b0C -0.0001\u00b0C 0\u00b0C +0.0001\u00b0C +0.0002\u00b0C the red band running across the center of the map is caused by microwaves from the Milky Way radiation detected light from early galaxies BEYOND THE MILKY WAY by WMAP satellite observed by Hubble Space Telescope first stars ignite Big Bang Hubble Space THE GREAT COLD SPOT Telescope This large cold spot in the CMBR may be due to a huge void around 6\u201310 billion light-years away. afterglow radiation Wilkinson Microwave SEEING THE FIRST LIGHT Anisotropy Probe The cosmic background microwave radiation 377,000 YEARS 300 MILLION YEARS 1 BILLION YEARS (WMAP) detected by WMAP consists of photons that AFTER BIG BANG AFTER BIG BANG AFTER BIG BANG \u201cescaped\u201d from matter less than 380,000 PRESENT: 13.7 BILLION years after the Big Bang. This was after the DARK AGES YEARS AFTER BIG BANG first atoms formed but before the first stars ignited\u2014a period known as the Dark Ages. In contrast, Hubble can observe back only to about 400\u2013800 million years after the Big Bang, when early galaxies had formed.","","GALAXY SUPERCLUSTERS 339 MAPPING DEEP SPACE EXPLORING SPACE REGION DETAILED BY GALAXY MAP area depicted in While galaxy motions on a local scale are OBSERVING A The galaxy map below covers two survey galaxy map affected by gravitational influences such as the MILLION GALAXIES thin, wedge-shaped regions of presence of superclusters, on the scale of the space, still only representing Earth universe as a whole these effects should become Major galaxy red-shift surveys typically use a small fraction of the negligible in comparison to the overall cosmic multi-object spectrographs\u2014devices that can observable universe. edge of expansion resulting from the Big Bang (see simultaneously record the spectra of observable Universe pp.48\u201351). According to Hubble\u2019s Law, the speed hundreds of objects. Instruments such as the part of the Shapley Concentration, at which a far-off galaxy is moving away from us Gemini multi-object spectrographs, mounted or Shapley Supercluster; this is a is, on average, proportional to its distance, and as on two large telescopes in Hawaii and Chile, huge group of about 25 clusters a result the red shift in a distant galaxy\u2019s light can use special masks to separate light from the of galaxies be used as a measure of its distance.The first different objects before splitting it through a large-scale survey of galaxy red shifts, carried out diffraction grating to obtain the spectra. by the Harvard-Smithsonian Center for Astrophysics (CfA), started in 1977 and took five GEMINI OBSERVATORY years to measure 13,000 galaxies. Since then The Gemini observatory has two 26.9 ft (8.1 other surveys, such as the Sloan Digital Sky m) reflectors\u2014one in Chile (shown above), Survey and Two-degree-Field Galaxy Redshift the other in Hawaii\u2014each fitted with a Survey (2dFGRS), have mapped many more spectrograph for multi-object spectroscopy. galaxies.These surveys have confirmed that the large-scale pattern of galaxy distribution remains essentially identical out to distances of billions of light-years. The Sloan Great Wall, a giant filament, is the largest known structure in the universe, at 1 billion light-years across BEYOND THE MILKY WAY values on the red-shift GALAXY DISTRIBUTION PLOT FROM THE 2DFGRS 0.14scale are a measureCentered on Earth, this plot shows the positions of over 230,000 0.12of how fast galaxiesgalaxies. The dots are galaxies and the colors indicate density, 0.10are receding from with dense regions redder and less dense ones bluer. 0.08 Earth; they are also 0.06 an indication of 0.04 distance from Earth 0.02 Earth and the Milky IF EXPANSION HAS OCCURRED AT A STEADY RATE Way are at the central point of the map observer\u2019s view of a supernova ACCELERATING EXPANSION distances and apparent One of the most remarkable recent astronomical discoveries Earth type 1a supernova brightness change steadily has been the fact that cosmic expansion is accelerating. Studies of type 1a supernovae (see p.283) have revealed that APPARENT BRIGHTNESS 1 with increase in red shift they are unexpectedly faint in the most distant galaxies, which implies that they are further away than they should 1\/3 EVIDENCE FOR be if the rate of expansion of the universe was constant ACCELERATING EXPANSION or slowing down. Many cosmologists had expected the 1\/6 The brightness of type 1a expansion of the universe to slow down as the initial supernovae indicates their impetus from the Big Bang began to fade, so the discovery IF EXPANSION HAS ACCELERATED distance from Earth, while their that its expansion is getting faster implied that an important red shifts indicate how fast they factor was missing from cosmological theories. Furthermore, observer\u2019s view of a supernova are receding. If the universe is the acceleration seems to have begun only around 5 billion expanding at a steady rate, the years ago, with the universe slowing as predicted until Earth type 1a supernova brightness of a supernova should then. Since its discovery in 1998, the accelerating expansion be proportional to its red shift has been corroborated from other measurements, and it is APPARENT BRIGHTNESS 1 (top). However, studies have now generally believed to be due to dark energy (see p.58). found that distant supernovae According to recent measurements, dark energy may be the 1\/6 are fainter than their red shifts most abundant form of mass-energy in the universe, suggest (bottom), indicating accounting for almost 73 percent of the total. that expansion of the universe is accelerating. distance rises at an increasing rate with increase in red shift; highest red-shift supernovae are more distant and so fainter 1\/25","TWO-MICRON ALL-SKY SURVEY (2MASS) This panoramic view of the entire sky at near- infrared wavelengths illustrates the distribution of galaxies beyond the Milky Way. The plane of the Milky Way runs across the center of this projection. Galaxies are color-coded by their red shift, from blue (the nearest) via green (intermediate distances) to red (the farthest). The purple area at top center right is the Virgo Cluster of galaxies.","","","THE NIGHT SKY","THE NIGHT SKY 344","\u201cWhy did not somebody teach me the constellations, and make me at home in the starry heavens, which are always overhead, and which I don\u2019t half know to this day?\u201d Thomas Carlyle THE HUMAN EYE HAS ALWAYS seen patterns among the stars. Ancient peoples traced the figures of gods, heroes, and mythical animals onto the skies and used the relationship between these constellations to illustrate myths and legends. In most cases, stars within a constellation lie in the same region of sky merely by chance, however, and are not related. Despite the apparent permanence of the skies, these patterns are not fixed, because all the stars are moving relative to Earth. Over time, the shape of all the constellations will change, and hundreds of thousands of years from now, they will be unrecognizable. Future generations will need to invent constellations of their own. But for now, 88 constellations fill our sky, interlocking like pieces of an immense jigsaw puzzle. Some are large, others small, some richly stocked with objects of note, others faint and seemingly barren. All are featured in the following pages. PATTERNS IN THE SKY As darkness falls, a stargazer scans the sky with binoculars. The familiar shape of the Big Dipper looms overhead, part of the constellation Ursa Major, the Great Bear. The north pole star, Polaris, can be seen high up on the right. THE CONSTELLATIONS","346 THE CONSTELLATIONS THE HISTORY OF CONSTELLATIONS 62\u201363 The celestial sphere THE FIRST CONSTELLATIONS were patterns of stars that ancient 64\u201365 Celestial cycles peoples employed for navigation, timekeeping, and storytelling. 70\u201371 Star motion and patterns Recently, the pictorial aspect of constellations has become less 76\u201377 Naked-eye astronomy significant, and they have become simply delineated regions of the sky, although the attraction of the myths and legends remains. EARLY CONSTELLATION LORE ANTICANIS This page from a 9th-century edition of The constellation system used today stems from patterns recognized by the star myths of Hyginus shows the ancient Greek and Roman civilization.The earliest surviving account constellation Canis Minor, here termed of ancient Greek constellations comes from the poet Aratus of Soli Anticanis. Hyginus\u2019s words, in Latin, (c. 315\u2013c. 245 bc). His poem, the Phaenomena, written around 275 bc, form the shape of the dog\u2019s body. describes the sky in storybook fashion and identifies 47 constellations. It is based on a lost book of the same name by the Greek astronomer Eudoxus (c. 390\u2013c. 340 bc). Eudoxus reputedly introduced the constellations to the Greeks after learning them from priests in Egypt.These constellations had been adopted from Babylonian culture; they were originally created by the Sumerians around 2,000 bc. However, the Greeks attached their own myths to the constellations detailed by Eudoxus, and Aratus\u2019s storybook of the stars proved immensely popular. Sometime in the 2nd century ad, it was joined by a more elaborate work of constellation lore called Poetic Astronomy, written by the Roman author Hyginus. Many editions of both these works were produced and translated over the centuries. FILLING THE HEAVENLY SPHERE The oldest surviving star catalog dates from the 2nd century ad and is contained in a book called the Almagest, written by the Greek astronomer and geographer Ptolemy (see panel, opposite). It records the positions and brightnesses of one thousand stars, arranged into 48 constellations, based on an earlier catalog by Hipparchus of Nicaea (c. 190\u2013c. 120 bc). In the 10th century ad, an Arab astronomer, al-Sufi (see p.421), updated the Almagest in his Book of Fixed Stars, which included Arabic names for many stars.These Arabic names are still used today, although often in corrupted form. No more constellations were introduced until the end of the 16th century, when Dutch explorers sailed to the East Indies. From there, they could observe the southern sky that was below the European horizon. Two navigators, Pieter Dirkszoon Keyser and Frederick de Houtman (see p.416), cataloged nearly 200 new southern stars, from which they and their mentor, Petrus Plancius (see p.358), a leading Dutch cartographer, created 12 new constellations. Plancius also created other northern constellations, forming them THE NIGHT SKY between those listed by Ptolemy. Nearly a century later, Johannes Hevelius (see p.384), a Polish astronomer, filled the remaining GLOBAL COVERAGE This beautiful celestial globe gaps in the northern sky, and in was made around 1625 by Arnold van Langren, a celebrated POCKET GLOBE the mid-18th century, the French Dutch globe-maker. As with all This pocket globe from the National Maritime astronomer Nicolas Louis de celestial globes, the figures are Museum, England, positions the Earth within a Lacaille (see p.422) introduced shown reversed in comparison to shell that represents the surrounding celestial another 14 constellations in their appearance in the sky. sphere. On the inside of the open shell are the the southern sky. constellations, painted as mirror images.","THE HISTORY OF CONSTELLATIONS 347 STAR CHARTS AND ATLASES The first printed star chart was produced in 1515 by the great German artist Albrecht D\u00fcrer. Like a celestial globe, D\u00fcrer\u2019s chart depicted the constellations in reverse, showing the sky as it would be seen from an imaginary position outside the celestial sphere, but before long, charts were being made that could be compared directly with the sky. The finest early star atlas was Uranometria, produced in 1603 by the German astronomer Johann Bayer. This atlas remains SKETCHY FIGURES one of the most beautiful Leo, the Lion, an easily recognizable examples of the celestial constellation of the zodiac, is here depicted cartographer\u2019s art. Shortly on the Atlas Coelestis, by English astronomer after its publication, John Flamsteed, published in 1729. astronomy was revolutionized by the invention of the telescope. The first major star catalog and atlas of this new era was produced by England\u2019s first Astronomer Royal, John Flamsteed (1646-1719). Atlas Coelestis shows the Ptolemaic constellations visible from Greenwich, England, based on Flamsteed\u2019s own painstaking observations. The pinnacle of celestial mapping came in 1801, when Johann Bode, a German astronomer, published an atlas called Uranographia. Covering the entire sky, this atlas depicted over 100 constellations, some invented by Bode himself. Finally, in 1922, a list of 88 constellations was agreed upon by the International Astronomical Union, astronomy\u2019s governing body, which also defined the boundaries of each constellation. On modern star charts, the only sign of the traditional pictorial charts are the few lines that link the main stars, suggesting the overall shape of each constellation. PTOLEMY THE NIGHT SKY Ptolemy (c. ad 100\u2013170) lived and worked in the great metropolis of Alexandria, Egypt, which was then part of the Greek empire. He was one of the last\u2014and the greatest\u2014 of the ancient Greek astronomers. His Earth-centered model of the universe, outlined in the treatise Almagest, dominated astronomical theory for 1,400 years. Ptolemy also made a catalogue of 1,022 stars in 48 constellations, based on earlier work by Hipparchus. HEAVENLY PICTURE BOOK Ancient people imagined gods, heroes, and beasts among the stars, and these figures were depicted on star charts until the 19th century. These charts, from John Flamsteed\u2019s Atlas Coelestis (1729), show those 48 constellations known to the ancient Greeks depicted on the northern and southern halves of the sky.","348 THE CONSTELLATIONS VELA MAPPING THE SKY celestial \u02da-50 coordinates CENTAURUS The following pages divide the celestial sphere into six parts\u2014 constellation \u03bc \u03b3 CRUX two polar regions and four equatorial regions\u2014which show the border \u03bb\u03b2 \u03b4 location of the 88 constellations. Each constellation is then profiled in the following section. Each entry places the constellation and its deep-sky \u02da-60 NGC \u03b9 \u03b5 \u03b81,2 main features into the context of the rest of the sky. object Hadar 4755 \u03b1 Coal- linking lines join constellation sack \u03b6 CARINA figure Acrux \u03b7 VISIBILITY MAPS CONSTELLATION CHARTS MUSCA \u02da-70 80\u00b0N Each of the 88 constellation entries has its own chart, 14h 13h 12h centered around the constellation area. These charts 11h Not visible 60\u00b0N show all stars brighter than magnitude 6.5. Within the DEEP-SKY OBJECTS Partially visible 40\u00b0N constellation borders, every star brighter than magnitude 5 Galaxy is labeled. Deep-sky objects are represented by an icon. Visible 20\u00b0N 0\u00b0 Globular cluster Open cluster 20\u00b0S KEY TO STAR MAGNITUDES 40\u00b0S 60\u00b0S -1.5\u20130 0\u20130.9 1.0\u20131.9 2.0\u20132.9 3.0\u20133.9 4.0\u20134.9 5.0\u20135.9 6.0\u20136.9 Diffuse nebula The entry for each constellation contains a map showing 18h Planetary nebula or the parts of the world from which it can be seen. The entire 17h 19h supernova remnant constellation can be seen from the area shaded black, part is visible from the area shaded gray, and it cannot be seen from the Black hole or area shaded white. Exact latitudes for full visibility are given X-ray binary in the accompanying data set. CYGNUS THE NORTH DRACO 21h POLAR SKY 22h BOOTES CEPHEUS LACERTA 23h URSA MINOR URSA MAJOR 90\u02da 80\u02da 70\u02da 60\u02da \u02da50 0h Almost in the center of this chart is the star Polaris, in Ursa Minor, which lies less than 1\u00b0 from CASSIOPEIA the north celestial 1h pole. For observers in the Northern Hemisphere, the stars around the pole never set\u2014they are circumpolar. PERSEUS 2h The viewer\u2019s latitude will THE NIGHT SKY determine how much of the sky is circumpolar: the farther CAMELOPARDALIS north, the larger the circumpolar area.This chart shows the sky from 3h 4h declinations 90\u00b0 to 50\u00b0. STAR MAGNITUDES LYNX -1 0 1 2 3 4 5 Variable star 7h AURIGA Star magnitudes shown here are for the equatorial and polar sky charts 6h 5h"]
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