Big Ideas Simply Explained - The Astronomy Book

URANUS TO NEPTUNE 99 The evening of the third, Herschel qualified his suggestion The asteroid Vesta was visited by my suspicion was converted by reserving for himself “the liberty the Dawn spacecraft from 2011–12. into certainty, being assured of changing that name, if another, Its orbit lies within that of Ceres, more expressive of their nature, and it is the brightest asteroid as it was not a fixed star. should occur.” seen from Earth. I waited till the evening of the fourth, when I had the Nothing more expressive did that come particularly close satisfaction to see it had occur, and after the Celestial Police to Earth—the Near Earth Asteroids moved at the same rate as was disbanded in 1815, a steady (NEAs)—are monitored in the trickle of asteroid discoveries hope of predicting and preventing on the preceding days. continued. By 1868, their number devastating future impacts. Giuseppe Piazzi stood at 100; by 1985, it was 3,000. The advent of digital photography Trojans The Celestial Police kept up the and image analysis has now There are also asteroids known search and, in March 1802, Olbers boosted the number of recorded as trojans, which travel in the discovered a second body like asteroids to more than 50,000, same orbits as planets, gathering Ceres located at the same distance spread around the 28-Bode-unit gap. far from their host in gravitationally from the sun, calling it Pallas. In Olbers and Herschel had discussed stable “libration points.” Most of 1804, Karl Harding found a third, the possibility that the asteroids these are in the Jupiter system, named Juno, while it was Olbers were the remains of a planet that where they form two clusters: the again who spotted the fourth, once orbited in the gap before “Trojan Camp” and “Greek Camp.” Vesta, in 1807. All these bodies being smashed by an astronomical Mars and Neptune have trojans, were later shown to be smaller cataclysm. Today, it is thought and the first Earth trojan was than Ceres—Vesta and Pallas that the gravitational disruption discovered in 2011. were slightly more than 300 miles of nearby Jupiter prevented the (500 km) wide and Juno was asteroids from accreting into a In 2006, the International half that size. planet in the first place, as similar Astronomical Union gave Ceres disks had done elsewhere in the the status of dwarf planet, the only Asteroid belt primordial solar system. one in the asteroid belt. At the The Celestial Police called their same time, Pluto was reclassified discoveries minor planets, but Under constant influence from as a dwarf planet. The orbits of William Herschel chose another the cumulative gravity of other neither Neptune nor Pluto match name—asteroid, which means asteroids, about 80 percent of the predictions of Bode’s law. starlike. Herschel reasoned that, known asteroids have unstable Despite the fact that it was unlike true planets, these small orbits. The 13,000 or so bodies instrumental in the discovery of objects had no discernible features, Ceres, Bode’s law is now viewed or at least none that could be made They resemble small stars as a mathematical coincidence, out with the telescopes of the day, so much as hardly to be and not a key to unlocking the so would be indistinguishable from distinguished from them. formation of the solar system. ■ starlight were it not for the fact that From this, their asteroidal they moved. Perhaps still smarting appearance, if I take my name, from his failure to name the planet and call them asteroids. he had found 20 years earlier, William Herschel

100 OWAFSHTUOHRLEVEEHSYEUAORVFFEATNCHSEE THE SOUTHERN HEMISPHERE IN CONTEXT B etween 1786 and 1802, sky, his son’s observations would William Herschel published have to be made from somewhere KEY ASTRONOMER catalogs listing more in the southern hemisphere. John Herschel (1792–1871) than 1,000 new objects in the night sky. Following his death in Herschel settled on South BEFORE 1822, William’s son John continued Africa, then a part of the British 1784 Charles Messier his work, but expanded its scope Empire. He moved there in 1833, publishes a list of 80 and ambition to carry out a taking with him his wife and known nebulae. complete survey of the night sky. young family, an assistant, and William’s observations had all been his father’s 20-ft (6-m) focal length AFTER made from southern England, and telescope. This was the same 1887 The Cartes du Ciel, an so were limited to objects down instrument that had been used ambitious project to survey to around 33° below the celestial to survey the northern skies, the entire sky photographically, equator. To survey the rest of the and Herschel chose it to ensure is initiated by the director that the new information gathered of the Paris Observatory, from the southern hemisphere Amédée Mouchez. was comparable to that already produced. The family set 1918 The Henry Draper themselves up in a house near Catalog, covering most the base of Table Mountain, far of the sky, is published by enough away to avoid the clouds Harvard College Observatory. that often gathered on its summit, and Herschel spent the next four 1948–58 The Palomar years completing his survey. Observatory in California completes its major sky The southern skies survey, which includes The Magellanic clouds are two data from nearly 2,000 dwarf galaxies close to the Milky photographic plates. Way, and are only visible from the southern hemisphere. They can be 1989–93 The Hipparcos satellite gathers data that The Milky Way’s core is clearest allow more than 2.5 million from the southern hemisphere. The stars to be cataloged. dark regions are where starlight is blocked by interstellar dust.

URANUS TO NEPTUNE 101 See also: Messier objects 87 ■ The Milky Way 88–89 ■ Examining nebulae 104–05 From each hemisphere, part of the celestial sphere is always hidden. A survey taken from Britain misses everything 33° below the celestial equator. Adding observations from South Africa John Herschel would make a complete survey. John Herschel left Cambridge Combining observations from University in 1816, already both hemispheres produces a survey a renowned mathematician. of the whole surface of the heavens. He worked with his father, William, and continued his seen by the naked eye, but Herschel’s Due to the orientation of the work after William’s death telescopic surveys provided the first solar system within the Milky in 1822. Herschel became detailed observations available to Way, the brightest section of it, one of the founders of the astronomers. He compiled a list of which is now known to be the core Royal Astronomical Society more than 1,000 stars, star clusters, of the galaxy, is only visible low and served as president for and nebulae within these galaxies. on the horizon from the northern three separate terms. He hemisphere during the summer married in 1826 and fathered Herschel also made careful when nights are short. From the 12 children. Herschel had observations of the distributions southern hemisphere, the brighter numerous interests in addition of stars within the Milky Way. core is visible higher in the sky to astronomy. While in South and during the darker months of Africa, Herschel and his The stars are the the year, allowing easier and more wife produced a portfolio landmarks of the universe. detailed observations. of botanical illustrations. He also made important John Herschel The end result of Herschel’s contributions to photography, labors, The General Catalog experimenting with color of Nebulae and Clusters of Stars, reproduction, and published listed more than 5,000 objects papers on meteorology, in total. These included all the telescopy, and other subjects. objects observed by John and his father, and also many discovered Key works by others such as Charles Messier, since it was intended to be a 1831 A Preliminary complete catalog of the stars. ■ Discourse on the Study of Natural Philosophy 1847 Results of Astronomical Observations Made at the Cape of Good Hope 1864 General Catalog of Nebulae and Clusters of Stars 1874 General Catalog of 10,300 Multiple and Double Stars

102    MTAHNOEVAESPMPTAAERRNESTNOTF STELLAR PARALLAX IN CONTEXT P arallax is the apparent ab movement of a nearby KEY ASTRONOMER object against distant Earth’s Earth’s Friedrich Bessel (1784–1846) objects due to the changing position position in position of the observer. According in June December BEFORE to this phenomenon, nearby stars 220 bce Aristarchus suggests should appear to change position Sun that the stars are very far away against the background of more since no parallax can be seen. distant stars as Earth moves Due to the effects of parallax, a nearby around its orbit. The idea that it star’s apparent position against distant 1600 Tycho Brahe rejects might be possible to measure the background stars moves from b in June the Copernican sun-centered distance to nearby stars using to a in December. system partly because he parallax dates back to ancient cannot detect stellar parallax. Greece. However, it was not 1838, Bessel measured parallax with achieved until the 19th century, an angle of 0.314 arc seconds for the AFTER due to the distances involved being star 61 Cygni, which indicated that 1912 Henrietta Swan Leavitt far greater than anyone supposed. it was 10.3 light-years away. The discovers a link between the current estimate is 11.4 light-years, period of a type of variable Much of German astronomer giving Bessel’s measurement an star and its brightness, Friedrich Bessel’s career had error of just under 10 percent. ■ allowing these stars to be been dedicated to the accurate used as “standard candles” determination of the positions of for figuring out distances. stars and finding their proper motion (changes in position due to the 1929 Edwin Hubble discovers motion of the star, rather than the link between the redshift changes in apparent position due of a galaxy’s light and its to the time of night or the season). distance from Earth. By the 1830s, with improvements in the power of telescopes, there was 1938 Friedrich Georg a race to carry out the first accurate Wilhelm Struve measures the measurement of stellar parallax. In parallax of Vega, and Thomas Henderson measures the See also: The Tychonic model 44–47 ■ Measuring the universe 130–37 ■ parallax of Alpha Centauri. Beyond the Milky Way 172–77

URANUS TO NEPTUNE 103 SCAPYUCPNELSAEPSROTINS   THE SURFACE OF THE SUN IN CONTEXT S unspots are cooler areas the sun. He did not find Vulcan, but on the sun’s surface caused he did discover that the number of KEY ASTRONOMER by changes in its magnetic sunspots varied over 11-year cycles. Samuel Heinrich Schwabe field. The first written observations (1789–1875) of sunspots date from about 800 bce, Swiss astronomer Rudolf Wolf in China, but it was not until 1801 studied Schwabe’s and other BEFORE that the British astronomer William observations, including some 800 bce Chinese and Korean Herschel made the connection from as far back as Galileo, and astrologers record sunspots between sunspots and changes numbered the cycles starting at to help foretell events. in Earth’s climate. 1 for the 1755–66 cycle. Eventually, he saw that there are long periods 1128 English chronicler John Samuel Schwabe, a German in each cycle when the number of of Worcester draws sunspots. astronomer, started observing sunspots is low. Herschel had not sunspots in 1826. He was looking noticed the pattern because he was 1801 William Herschel links for a new planet thought to orbit observing during what is now called sunspot numbers and the price closer to the sun than Mercury, the Dalton Minimum, when overall of wheat, due to the effect of provisionally named Vulcan. It numbers of sunspots were low. ■ sunspots on Earth’s climate. would have been very difficult to observe such a planet directly, but Sunspots can last from a few AFTER Schwabe thought he might see it days to several months. The largest 1845 French physicists as a dark spot moving in front of can be the size of Jupiter. Hippolyte Fizeau and Léon Foucault photograph sunspots. See also: Observing Uranus 84–85 ■ The properties of sunspots 129 ■ Carrington (Directory) 336 1852 Irish astronomer Edward Sabine demonstrates that the number of magnetic storms on Earth correlates with the number of sunspots. 1908 US astronomer George Ellery Hale discovers that sunspots are caused by magnetic fields.

104 OWAFSAAPSRIRDREAATLNEFGCOETRMEMDENT EXAMINING NEBULAE IN CONTEXT To the naked eye, nebulae are fuzzy patches of light that could comprise gas or stars. KEY ASTRONOMER Lord Rosse (1800–1867) Larger telescopes Telescopes show reveal a spiral form some nebulae to be BEFORE clusters of stars. 1784 Charles Messier of arrangement. publishes a catalog of the visible nebulae. I n the 1840s, a British aristocrat Despite this difficulty, by 1845 named William Parsons, Lord Rosse had succeeded in casting 1785 William Herschel Rosse, decided to commit a mirror that was 72 in (1.8 m) publishes catalogs of some of his considerable wealth in diameter. He mounted it in nebulae and speculates that to building the world’s largest his telescope at Birr Castle, near many are similar in shape reflecting telescope. Rosse was Parsonstown in Ireland, where it and size to the Milky Way. curious to reexamine some of the became known as the Leviathan nebulae listed by John Herschel in of Parsonstown. This telescope 1833 John Herschel expands the early 19th century, in particular remained the world’s largest his father’s catalogs by those nebulae that did not appear reflecting type until the 100-in surveying objects from the to be clusters of stars. (2.5-m) reflector was built at Mount southern hemisphere. Wilson in California in 1917. To re-observe these nebulae, 1864 William Huggins Rosse needed to build a larger and Central Ireland proved a far discovers that some nebulae better telescope than that used by from ideal place to build a telescope, are clouds of luminous gas, Herschel. He experimented for many as overcast or windy conditions not aggregations of stars. years with methods for casting a often prevented viewing. The 36-inch (0.9-m) mirror. Mirrors at telescope itself had limited mobility, AFTER the time were made from a metal meaning that only a small area 1917 Vesto Slipher concludes called speculum, an alloy of copper of the sky could be examined. that spiral galaxies are “island and tin—a brittle material that was Nonetheless, when the weather universes” and that the Milky prone to cracking as it cooled. was clear, Rosse was able to use Way is one such galaxy seen by us from within.

See also: Messier objects 87 ■ The Milky Way 88–89 ■ The southern URANUS TO NEPTUNE 105 hemisphere 100–01 ■ Properties of nebulae 114–15 ■ Spiral galaxies 156–61 Lord Rosse the huge instrument to observe The light by which we and record the spiral nature of recognize the nebulae now William Parsons was born some nebulae—now called spiral must be merely that which in Yorkshire in 1800 and galaxies—for the first time. The became Third Earl of Rosse first of these spirals that Rosse left their surfaces a vast on the death of his father in identified was M51, later known as number of years ago … 1841. He was educated at the Whirlpool galaxy. Today, about phantoms of processes Trinity College, Dublin, and three-quarters of all the galaxies completed long in the Past. at Oxford University, where that have been observed are spiral he was awarded a first-class galaxies. However, these are Edgar Allen Poe degree in mathematics. He thought ultimately to transform married in 1836, but only four into elliptical galaxies. Formed of of his 13 children survived older stars, elliptical galaxies are to adulthood. Lord Rosse’s dimmer and much harder to spot, estates were in Ireland, but astronomers believe that they and this is where he built are probably the most common his telescopes. galaxy type in the universe. In 1845, after he made The nebular hypothesis stars in the Orion nebula, and public his findings on nebulae, In the mid-19th century, so for a time, the idea of gaseous Rosse was criticized by John astronomers debated whether nebulae was rejected. However, Herschel, who was convinced nebulae consisted of gas or stars. although the stars were real, that nebulae were gaseous In 1846, Rosse found numerous their presence did not mean that in nature. Both men accused there was no gas. The gaseous each other of using flawed The Leviathan Telescope at nature of some nebulae was not instruments. Ultimately, Parsonstown held a mirror weighing demonstrated until spectroscopic however, neither succeeded 3.3 tons (3 metric tons), inside a 54 ft- analysis was used by William in demonstrating sufficient (16.5 m-) long tube. The whole structure Huggins in 1864. ■ scientific evidence to resolve weighed about 13 tons (12 metric tons). conclusively the question of whether nebulae were composed of gas or stars. Key works 1844 On the construction of large reflecting telescopes 1844 Observations on some of the Nebulae 1850 Observations on the Nebulae

106 YTWOHHUEOPHSLAEAVPNEOEPSTOITINIOTNED OUT ACTUALLY EXISTS THE DISCOVERY OF NEPTUNE IN CONTEXT I n the months following William an undiscovered planet and using Herschel’s discovery of Uranus Newton’s law of gravity to work out KEY ASTRONOMER in 1781, astronomers found what its effect might be on Uranus. Urbain Le Verrier (1811–1877) irregularities, or perturbations, This prediction was compared to in its orbit. Most perturbations in observations of Uranus, and the BEFORE orbits are caused by the gravitational position was revised according to March 1781 William Herschel effects of other large bodies, but with the planet’s movements. After many discovers Uranus. Uranus there were no known planets repetitions of this process, Le Verrier that could cause the observed established the likely position of August 1781 Finnish−Swedish motion. This led some astronomers an unknown planet. He presented astronomer Anders Lexell finds to suggest that there must be a his ideas before the Académie des irregularities in Uranus’s orbit planet orbiting beyond Uranus. Sciences in 1846, and he also sent and suggests that they are due his predictions to Johann Galle to other, undiscovered planets. Searching for the invisible (1812–1910) at the Berlin Observatory. Frenchman Urbain Le Verrier tackled 1799–1825 Pierre-Simon the problem of the perturbations of Galle received Le Verrier’s letter Laplace explains perturbations Uranus by assuming the location of on the morning of September 23, mathematically. 1846, and obtained permission to 1821 French astronomer Alexis Unknown body Saturn Bouvard publishes predictions of future positions of Uranus. Calculations of Uranus’s Uranus Sun Subsequent observations predicted orbit took into Jupiter deviate from his predictions. account the gravitational Gravitational effects of the sun, Jupiter, pull AFTER and Saturn. However, the 1846 Briton William Lassell observed orbit deviated discovers Triton, Neptune’s from the calculations in a largest moon, only 17 days after way that suggested the pull the discovery of the planet. of another massive body farther out from the sun. 1915 Albert Einstein explains perturbations in the orbit of Mercury using relativity.

URANUS TO NEPTUNE 107 See also: The Milky Way 88–89 ■ Gravitational disturbances 92–93 ■ The theory of relativity 146–53 Perturbations in the orbits of planets can be explained by the gravitational effects of other bodies in the solar system. The orbit of Uranus has There may be another Urbain Le Verrier perturbations that cannot planet beyond Uranus. Urbain Le Verrier studied be explained by any at the École Polytechnique, known bodies. near Paris. After graduating, his initial interests were in Neptune is discovered Newton’s laws show chemistry, before he switched very near the place where to look for to astronomy. His astronomical predicted by the this planet. work was focused on celestial mathematics. mechanics—the description of the movements of the look for the planet. Working with planet. There was some controversy bodies in the solar system his assistant Heinrich D’Arrest, over who should have the credit for using mathematics. Le Verrier he located an unknown object the discovery, but Adams always obtained a position at the within 1° of the predicted position acknowledged that Le Verrier had Paris Observatory and spent that same night. Observations on the better claim. most of his life there, acting as subsequent nights showed that director from 1854. However, the object was moving against the Galle was not the first person his management style was background of stars and was, indeed, to observe Neptune. Once the orbit not popular and he was a planet—one that would later be of Neptune had been worked out, replaced in 1870. He took named Neptune at Le Verrier’s it was possible to go through old up the position again in 1873 suggestion. Galle later gave the records and find that others had after his successor drowned, credit for the discovery to Le Verrier. already observed it without realizing and held it until his own it was a planet, including both death in 1877. Independent discovery Galileo and John Herschel. Later, At the same time as Le Verrier Le Verrier used a similar technique Le Verrier spent his early was calculating the position to analyze the orbit of Mercury career building on Pierre- of the unknown planet, British and found that perturbations in Simon Laplace’s work on the astronomer John Couch Adams its orbit could not be explained stability of the solar system. (1819–92) was also looking at by Newtonian mechanics. He He later went on to study the cause of the perturbations in suggested that this might be due periodic comets before turning the orbit of Uranus. He arrived at to the influence of another planet his attention to the puzzle of a similar conclusion to Le Verrier, even closer to the sun, provisionally Uranus’s orbit. completely independently, but his named Vulcan. This speculation results were not published until ended when Einstein explained Key work after Galle had observed the new the perturbations using his general theory of relativity. ■ 1846 Recherches sur les Mouvements de la Planète Herschel (Research on the Movements of the Planet Herschel)

TASHETRROISP 1850–1915

EHYOSFICS

110 INTRODUCTION Germans Gustav Italian priest Angelo Pioneering American Kirchhoff and Robert Secchi starts a project to astrophotographer Henry classify stars according Bunsen investigate Draper takes the first the physics behind to their spectra. photograph of the Orion nebula. spectral lines. 1854 1863 1880 1862 1868 1888 Scottish physicist James British astronomer Using long-exposure Clerk Maxwell produces Joseph Norman photography, British amateur Lockyer discovers a astronomer Isaac Roberts a set of equations that new element in the sun, describe the wavelike which he calls helium. reveals the structure of the Andromeda nebula. behavior of light. I n the early 19th century, from the longest wavelength (red) Such lines in the sun’s spectrum astronomy was mainly to the shortest (violet). When a had been noted as early as 1802, concerned with cataloging spectrum is examined in close but the first physicists to examine the positions of stars and planets, detail, a multitude of fine variations the physics behind particular kinds and understanding and predicting can emerge. A typical star of spectra were Gustav Kirchhoff the movements of the planets. New spectrum appears crossed by and Robert Bunsen. Importantly, in comets continued to be discovered, numerous dark lines, some fine about 1860, Kirchhoff showed that and there was a growing awareness and faint, some broad and black. different patterns of dark lines are of assorted distant phenomena, such the spectral fingerprints of different as variable stars, binary stars, and Light is to us the sole chemical elements. Here was a nebulous objects. However, there evidence of the existence way to investigate the composition seemed little scope for learning of these distant worlds. of the sun and stars. It even led to more about the nature of these James Clerk Maxwell the discovery of the previously remote objects—their chemical unknown element, helium. composition or temperature, for example. The key that unlocked This new branch of astronomy these mysteries was the analysis was enthusiastically taken up by of light using spectroscopy. the British astronomer William Huggins and his wife, Margaret, Decoding starlight who also pioneered photography A glowing object gives out light as a way of recording observations. over a range of wavelengths, which They did not restrict themselves we perceive as a rainbow of colors to stars, but studied the spectra of nebulae as well.

THE RISE OF ASTROPHYSICS 111 The Harvard College While investigating X-rays, Harvard computer Henrietta Observatory French physicist Henri Swan Leavitt shows how stars Becquerel demonstrates called Cepheid variables can produces the first be used to measure distances Draper Catalog the effects of the radioactive of Star Spectra. decay of uranium. in the universe. 1890 1896 1907 1895 1900 1912 In experiments with Max Planck lays the Austrian physicist cathode ray tubes, foundation for quantum Victor Hess shows German physicist mechanics by suggesting that powerful rays, now Wilhelm Röntgen that energy can only exist called cosmic rays, discovers X-rays. in distinct sizes of “quanta.” come from space. By the end of the 19th century, spectrum. This information quickly would strongly influence the it seemed that, in order to fully paid dividends as astronomers future direction of astronomy. understand the nature of stars, it analyzed the new data. Cannon’s Significant developments in basic was necessary to systematically colleague at Harvard, Antonia physics impacted on astronomy. record their spectra and classify Maury, realized that the simple For example, Briton James Clerk them into different types. temperature sequence did not take Maxwell published his theory account of subtle variations within of electromagnetism in 1873, Star classification each star type. Ejnar Hertzsprung describing electromagnetic radiation This immense task was undertaken and Henry Norris Russell such as light in terms of its wavelike at Harvard College Observatory, independently followed this up, properties. X-rays were discovered where the director Edward leading to the discovery that stars in 1895 and radioactivity in 1896. In Pickering employed a large team of the same color could be giants or 1900, German physicist Max Planck of women to carry out the exacting dwarfs, and the identification of the prepared the ground for quantum work. Here, Annie Jump Cannon first known white dwarf star. physics with a leap of inspiration, devised the stellar classification postulating that electromagnetic system still used today, based The physics of the stars energy comes in “packets” of a on a temperature sequence. In an interval of some 50 years, particular size, called “quanta.” Cannon personally classified cutting-edge astronomy had These discoveries would lead some 500,000 stellar spectra. changed its focus. By the early to new ways of looking at the The star catalog included not only 20th century, physics—the study skies, and shed new light on the their position but also precise of matter, forces, and energy, and processes taking place within stars. information about their magnitude how they are related—could be Physics and astronomy would be (apparent brightness) and applied to the sun and stars, and inseparable from this point on. ■

112    BSS  EOODLFAIOURUMNATIDSMITNOOSTPHHEERE THE SUN’S SPECTRUM IN CONTEXT I n 1814, a German maker of hot, dense gas, such as the sun, optical instruments named will emit light at all wavelengths KEY ASTRONOMER Joseph von Fraunhofer and thus produce a continuous Gustav Kirchhoff invented the spectroscope (see spectrum. However, if the light (1824–1887) diagram on p.113). This allowed the passes through a cooler, lower- spectrum of the sun, or any other density gas, such as the sun’s BEFORE star, to be displayed and measured atmosphere, some of that light 1802 After creating an image with high precision. Fraunhofer might be absorbed by an element of the sun’s spectrum by noticed that there were more than (sodium, for example), at the same passing sunlight through 500 dark lines crossing the sun’s wavelengths at which the element a narrow slit and prism, spectrum, each located at a precise emits light when heated. The English chemist William wavelength (color). These came absorption of the light causes Hyde Wollaston notices seven to be known as Fraunhofer lines. gaps in the spectrum, which are dark lines in the spectrum. now known as absorption lines. ■ By the 1850s, German scientists 1814 Joseph von Fraunhofer, Gustav Kirchhoff and Robert Bunsen The path is opened for the German inventor of the had discovered that, if different the determination of the spectroscope, discovers 574 chemical elements are heated chemical composition of of the same dark lines in the in a flame, they emit light at one the sun and the fixed stars. sun’s spectrum. He maps or more wavelengths that are these in detail. characteristic for that element, Robert Bunsen acting like a fingerprint to indicate AFTER its presence. Kirchhoff noticed 1912 Danish physicist that the wavelengths of light given Niels Bohr introduces a off by some elements corresponded model of the atom in which to the wavelengths of some movements of electrons Fraunhofer lines. In particular, switching between different sodium’s emissions at wavelengths energy levels cause radiation of 589.0 and 589.6 nanometers to be emitted or absorbed exactly matched two Fraunhofer at particular wavelengths. lines. Kirchhoff suggested that a See also: Analyzing starlight 113 ■ The characteristics of stars 122–27 ■ Refining star classification 138–39 ■ Stellar composition 162–63

THE RISE OF ASTROPHYSICS 113 STGHTRAEORIURSPSECPDAENBCYTBREA  ANALYZING STARLIGHT IN CONTEXT A ngelo Secchi was Star one of the pioneers of KEY ASTRONOMER astrophysics, an arm Light Angelo Secchi (1818–1878) of science that focuses on the properties of a star, rather than Prism BEFORE merely its position in the sky. 1802 William Hyde Wollaston He was the first to group stars Spectroscopy uses Spectrum notices that there are dark according to their spectra, or the a prism to refract gaps in the sun’s spectrum. particular colors of light they emit. the light from a star, splitting the light to 1814 German lensmaker A Jesuit priest as well as a allow its constituent Joseph von Fraunhofer noted physicist, Secchi founded wavelengths to be measures the wavelengths a new observatory at the order’s measured with a high of these dark lines. Collegio Romano in Rome. There he degree of accuracy. became a pioneer of the technique 1860 Gustav Kirchhoff and of spectroscopy, a new way to were orange, with a complex array Robert Bunsen use a gas measure and analyze starlight. of elements present. In 1868, Secchi burner to make systematic added Class IV for redder stars with recordings of the wavelengths Gustav Kirchhoff had shown carbon present, and finally in 1877 produced by burning elements. that gaps in a stellar spectrum came Class V for stars that showed were caused by the presence of emission lines (not absorption lines, AFTER specific elements (see opposite). as in the other four). 1868 English scientist Armed with this knowledge, Secchi Norman Lockyer identifies began to class stars according to Secchi’s stellar classes were a new element, helium, their spectra. At first he used three later amended by other scientists, from emission lines in the classes: Class I were white or blue and in 1880 became the foundation sun’s light. stars that showed large amounts of of the Harvard System, which is hydrogen in their spectra; Class II used to classify stars to this day. ■ 1901 The Harvard System were yellow stars, with metallic for the classification of spectral lines (for astronomers, stellar spectra, devised “metallic” refers to any element by Williamina Fleming heavier than helium); and Class III and Annie Jump Cannon, supersedes Secchi’s system. See also: The sun’s spectrum 112 ■ The sun’s emissions 116 ■ The star catalog 120–21 ■ The characteristics of stars 122–27

114 ELMNUAOMSRISNMEOSOUUOSSFGAS PROPERTIES OF NEBULAE IN CONTEXT I n the 1860s, a pioneering wavelengths. Huggins, encouraged British astronomer named by his astronomer wife, Margaret, KEY ASTRONOMER William Huggins made turned his attention deeper into William Huggins (1824–1910) key discoveries by studying the space, toward nebulae, the fuzzy composition of stars and nebulae patches of light that had long BEFORE using a spectroscope. This mystified astronomers. He used 1786 William Herschel instrument, a glass prism attached spectroscopy to divide these publishes a list of nebulae. to a telescope, splits white light into patches into two distinct types. its constituent light wavelengths, 1850s Gustav Kirchhoff and producing a spectrum of color. The spectra of nebulae Robert Bunsen realize that hot Gustav Kirchhoff and Robert Bunsen Huggins observed that nebulae, gases produce bright emission had already noted the chemical such as the Andromeda nebula, had lines in their spectra of light, composition of the sun by studying a spectrum of light similar to that whereas cool gases absorb the the dark absorption lines that occur of the sun and other stars—a broad same wavelength, producing in its spectrum. These lines are band of color with dark absorption dark lines in spectra. caused by the atoms of different lines. The reason for this (not chemical elements absorbing discovered until the 1920s, after AFTER radiation at certain precise Huggins’s death) was that such 1892 Margaret Huggins is made an honorary Fellow of the Spectroscopes allow Some nebulae are found Royal Astronomical Society. astronomers to measure to have spectra similar a nebula’s spectrum 1913 Dane Niels Bohr depicts to those of stars. atoms as containing a central of light. nucleus surrounded by electrons. Spectral lines are These nebulae are Others have spectra that produced when the electron enormous masses emit energy at a single moves between energy levels. of luminous gas. wavelength. 1927 American Ira Bowen realizes that the two green lines caused by “nebulium” are produced by oxygen atoms that have lost two electrons.

THE RISE OF ASTROPHYSICS 115 See also: Observing Uranus 84–85 ■ Messier objects 87 ■ The sun’s spectrum 112 nebulae are indeed composed of Huggins was the first to analyze William Huggins stars, and are galaxies in their own the spectrum of a planetary nebula (the right. The second type of nebula he Cat’s Eye nebula), confirming that it After selling the family observed was entirely different. Its was gaseous and not composed of stars. drapery business when he light spectrum was made of single- was 30, William Huggins wavelength emission lines—energy energy in two strong green lines, ran a private observatory in was being emitted as one color; which did not correspond to any Tulse Hill in South London. there were no absorption lines. known chemical element. Some He used his new wealth astronomers suggested that they to buy a powerful 8-in Huggins realized that these were produced by a new element, (20-cm) refracting telescope. second kind of nebula were huge dubbed nebulium. clouds of hot, low-density gas. In 1875, age 51, Huggins Some of this gas could be in the Huggins concluded from his married 27-year-old Irish process of forming new stars; spectroscopic observations that astronomy enthusiast other gas clouds, like the planetary all of the heavenly bodies he had Margaret Lindsey, who nebulae, could have been ejected studied were made of exactly the encouraged him to adopt from evolving stars. same elements as Earth. However, photography to record his the mystery of nebulium was spectra and was an active Huggins’ 1864 observations not solved until after his death. partner in his later research, of the Cat’s Eye planetary nebula in In 1927, it was found to be simply co-authoring many papers. the constellation Draco revealed a doubly ionized oxygen—oxygen Huggins was a pioneer in the spectrum with a single absorption atoms that have lost electrons and use of photography to record line, produced by hot hydrogen gas. have a double positive charge. ■ astronomical objects. He However, the nebula also emitted also developed a technique to study the radial velocity of stars using the Doppler shift of their spectral lines. As a pioneer astronomical spectroscopist, Huggins was elected to serve as president of the Royal Society from 1900 to 1905. He died at his home in Tulse Hill in 1910, age 86. Key works 1870 Spectrum analysis in its application to the heavenly bodies 1909 Scientific Papers

116   DTPT  HREIFROEFRMSEERIUNSSNET’NFRSRCIYAOEELMLFLALONAWYME THE SUN’S EMISSIONS IN CONTEXT I n August 1868, the French A total eclipse of the sun reveals astronomer Pierre Jules César the chromosphere. This image of an KEY ASTRONOMERS Janssen traveled to India to eclipse was captured in 1919 by British Jules Janssen (1824–1907) observe a solar eclipse. The eclipse astronomer Arthur Eddington. Joseph Norman Lockyer covered the sun’s bright disk, (1836–1920) leaving only a narrow ring of light. he changed his mind—the light was This was the chromosphere, the not from sodium but from a hitherto BEFORE middle of three layers in the sun’s unknown element, which he named 1863 Gustav Kirchhoff develops atmosphere, which was normally helium, after helios, the Greek word spectroscopy, showing how hidden by the glare. Janssen for the sun. For some years, it was light can be used to identify found that the spectrum of the thought that helium only existed hot substances. chromosphere’s light contained on the sun, but in 1895, Scottish numerous bright emission lines. chemist William Ramsay succeeded 1864 William and Margaret Using discoveries made by Gustav in isolating a sample from a Huggins find that the spectra Kirchhoff, Janssen was able to radioactive uranium mineral. ■ of nebulae contain different confirm that the chromosphere emission lines, showing that was a layer of gas. He also noticed a they are largely clouds of gas. previously unseen yellow emission line in the sun’s spectrum. He AFTER assumed this unknown light was 1920 Arthur Eddington states produced by sodium, helping to that stars are fueled by the give the sun its yellow hue. fusion of hydrogen into helium. In October that year, English 1925 Cecilia Payne-Gaposchin astronomer Joseph Norman Lockyer shows that stars are largely developed a spectroscope for made from the elements observing the chromosphere directly. hydrogen and helium. He also detected its curious light and also assumed it was produced 1946 US cosmologist Ralph by sodium, but after consulting Alpher calculates that most the chemist Edward Frankland, of the universe’s helium was formed in the first few minutes See also: The sun’s spectrum 112 ■ Nuclear fusion within stars 166–67 ■ after the Big Bang. The primeval atom 196–97

THE RISE OF ASTROPHYSICS 117 BMOYFARACSHDAEISNNNSTEERLANSVEETRWSOEDRK   MAPPING MARS’S SURFACE IN CONTEXT B y the mid-19th century, Schiaparelli described various dark scientists were increasingly areas as “seas” and lighter areas KEY ASTRONOMER speculating about the as “continents.” He also portrayed Giovanni Schiaparelli possibility of life on Mars, which what seemed to him to be a (1835–1910) had been found to have certain network of long, dark, straight lines, similarities to Earth, including ice or streaks, crisscrossing Mars’s BEFORE caps, a similar length of day, and an equatorial regions. In his book Life 1858 Angelo Secchi first uses axial tilt that meant it experienced on Mars, Schiaparelli suggested the word canali (channels) in seasons. However, it had also been that, in the absence of rain, these connection with Mars. found that it did not rain on Mars. channels might be the mechanism by which water was transported AFTER Between 1877 and 1890, Italian across the dry surface of the 1897 Italian astronomer astronomer Giovanni Schiaparelli planet to allow life to exist there. Vincenzo Cerulli theorizes that carried out a series of detailed the Martian canals are just an observations of Mars to produce In the following years, many optical illusion. a map of the planet’s surface. eminent scientists, including American astronomer Percival 1906 A book by American Lowell, speculated that these astronomer Percival Lowell, dark lines were irrigation canals Mars and Its Canals, promotes constructed by intelligent beings the idea that there may be on Mars. However, others could artificial canals on Mars, not see the channels at all when built by intelligent beings. they looked for them—and by 1909, observations with telescopes of 1909 Photographs of Mars higher resolution had confirmed that taken at the new Baillaud the Martian canals did not exist. ■ dome at the Pic du Midi observatory in France discredit Schiaparelli’s 1888 atlas of Mars the Martian canals theory. shows land, seas, and a network of straight channels. Here, the south 1960s NASA’s Mariner flyby pole is shown at the top. missions fail to capture any See also: Observing Saturn’s rings 65 ■ Analyzing starlight 113 ■ images of the canals or find Life on other planets 228–35 any evidence of them.

118 PTHHOETSOTGARRASPHING ASTROPHOTOGRAPHY IN CONTEXT Photographs of the Photographing stars can be used to the stars requires KEY ASTRONOMER make very accurate long exposures. David Gill (1843–1914) star maps. BEFORE 1840 The first clear Accurate maps However, Earth’s photograph of the moon reveal that stars are rotation makes images is taken by American moving at different John Draper, using a blurry. A precise 20-minute exposure. speeds and in tracking mechanism different directions. 1880 John Draper’s son is needed to move Henry takes a 51-minute the camera. exposure of the Orion nebula. He also takes the I saac Newton’s theory Gill was a pioneer in the field of first wide-angle photograph of gravitation, like many astrophotography. In the mid-1860s, of the tail of a comet. advances of the Scientific then still an amateur astronomer Revolution (pp.42–43), was based working in his father’s backyard, AFTER on a belief that the universe he built a tracking mount for his 1930 US astronomer worked like clockwork. In the 12-in (30-cm) reflecting telescope, Clyde Tombaugh discovers 1880s, David Gill, a master and used it to photograph the Pluto by spotting a moving clockmaker from Aberdeen, moon with a clarity that had never object on photographic plates. Scotland, applied his precision been seen before. The photographs clockmaking machinery to earned Gill a fellowship in the 1970s Charge-coupled astronomical telescopes—and, Royal Astronomical Society, devices replace photographic ironically enough, offered a way and, by 1872, his first job as a plates and film with digital of showing that the stars were professional astronomer at the photographs. not all moving in clocklike unison. Dunecht Observatory in Aberdeen. 1998 The Sloan Digital Sky Survey begins to make a 3-D map of galaxies.

THE RISE OF ASTROPHYSICS 119 See also: The Tychonic model 44–47 ■ Mapping southern stars 79 ■ Messier objects 87 ■ Space telescopes 188–95 ■ A digital view of the skies 296 ■ Roberts (Directory) 336 ■ Kapteyn (Directory) 337 ■ Barnard (Directory) 337 Gill applied clockwork tracking Frank McClean, an astronomer motion of nearby stars relative to mechanisms to telescope mounts friend of David Gill, donated the more distant ones. This information so that the telescope could move McClean Telescope to the Cape was invaluable for measuring in near-perfect harmony with the Observatory in 1897. David Gill stellar distances on a large rotation of Earth. This allowed the used it extensively. scale, and it began to reveal to instrument to remain fixed and astronomers the true scale of the focused on a single patch of sky. photographic record of the southern galaxy and the universe beyond. ■ Gill was not the first to attempt sky. The result was the Cape to photograph the heavens with Photographic Durchmusterung telescopes, but imaging faint (catalog), showing the position celestial light required exposures and magnitude of nearly half a of several minutes at least, and million stars. Gill also became poor tracking meant that early a key figure in the Carte du Ciel star photographs were mostly (“Map of the Sky”) project, a global incomprehensible blurs. collaboration of observatories begun in 1887 with the goal of Southern sky making a definitive photographic In 1879, Gill became the chief map of the stars. This ambitious, astronomer at the Cape Observatory expensive, and decades-long in South Africa. By now, he was project involved teams of human using the latest dry-plate system computers who would measure (a photographic plate pre-coated the plates by hand. It was, however, in light-sensitive chemicals), which superseded by new methods and he employedto capture the “Great technologies before it was finished. Comet” that appeared over the southern hemisphere in 1882. The accurate maps produced by Gill’s photographic techniques Working in partnership with may seem fairly unremarkable the Dutch astronomer Jacobus today, but at the turn of the 20th Kapteyn, Gill spent the best part century they were the first reliable of the next two decades creating a means for showing the proper David Gill The eldest son of a successful measuring stellar parallax clockmaker, David Gill was (p.102). His measurements, destined to take over the family used in conjunction with his star business. However, while at the maps, did much to reveal the University of Aberdeen, he distances between stars. By the became a student of the great time he left the Cape Observatory physicist James Clerk Maxwell, in 1906, Gill was a renowned whose lectures gave Gill a passion astronomer. In one of his final for astronomy. When offered a job jobs, the government consulted as a professional astronomer in him on the implementation of 1872, Gill sold the family business daylight saving hours. and began work at the Dunecht Observatory, Aberdeen. Key work In addition to his pioneering 1896–1900 Cape Photographic work in astrophotography, Durchmusterung (with Gill developed the use of Jacobus Kapteyn) the heliometer, a device for

120 MOAFEPARTSHEUCERISSETEMAERNST THE STAR CATALOG IN CONTEXT E dward C. Pickering, in his help of the Harvard “computers”— role as director of the Harvard a team of mathematically KEY ASTRONOMER College Observatory from minded women upon whom Edward C. Pickering 1877 to 1906, laid the foundations Pickering relied to process the (1846–1919) for precise stellar astronomy. His huge amounts of data required team carried out star surveys that to create the catalog. BEFORE broke new ground in understanding 1863 Angelo Secchi develops the scale of the universe. Pickering More than 80 computers, spectral classification for stars. combined the latest techniques in known in those less enlightened astrophotography with spectroscopy days as “Pickering’s Harem,” 1872 American amateur (splitting light into its constituent worked at the Harvard Observatory. astronomer Henry Draper wavelengths) and photometry The first of them was Williamina photographs the spectral (measuring the brightness of stars) Fleming, who had been Pickering’s lines of Vega. to create a catalog that listed maid. Upon taking over the a star’s location, magnitude, and observatory, Pickering fired his 1882 David Gill begins to spectral type. He did this with the male assistant, deeming him survey the southern sky “inefficient,” and hired Fleming using photography. in his place. Other notable names among the computers included AFTER Antonia Maury, Henrietta Swan 1901 Annie Jump Cannon, Leavitt, and Annie Jump Cannon. along with Pickering, creates the Harvard Classification A woman had no chance Color and brightness Scheme, which forms the at anything in astronomy Pickering’s individual contributions basis of stellar classification. except at Harvard in the to the star catalog were twofold. In 1880s and 1890s. And even 1882, he developed a method of 1912 Henrietta Swan Leavitt there, things were rough. photographing multiple star spectra links the period of Cepheid William Wilson Morgan simultaneously by transmitting the variables to their distance. stars’ light through a large prism US astronomer and onto photographic plates. 1929 Edwin Hubble In 1886, he designed a wedge measures the distance photometer, a device for measuring to nearby galaxies using the apparent magnitude of a star. Cepheid variables. Magnitudes had previously been recorded psychometrically—using

THE RISE OF ASTROPHYSICS 121 See also: The sun’s spectrum 112 ■ The characteristics of stars 122–27 ■ Classifying star spectra 128 ■ Measuring the universe 130–37 the naked eye as a means of I do not know if God Edward C. Pickering comparing one star’s brightness is a mathematician, with that of another. The wedge but mathematics is the Edward C. Pickering was the photometer was much more loom on which God dominant figure of American objective; the observer viewed weaves the universe. astronomy at the turn of the a target star alongside one of Edward C. Pickering 20th century. Many of the several stars with an accepted first steps in the development brightness and then edged a Peru, to survey the southern of today’s astrophysics and wedge of calcite in front of the sky and produce the first all-sky cosmology were made by known source, diminishing its photographic map. people he employed at the magnitude in increments until Harvard College Observatory. the two sources looked to have In combination with the Known as a progressive for the same brightness. work of the Harvard computers, his attitudes to the education Pickering’s data was the basis of women and their role in In 1886, Mary Draper, the for the Henry Draper Catalogue research, Pickering nonetheless widow of spectral photography published in 1918, which contained asserted a rigid authority pioneer Henry Draper, agreed spectral classifications for 225,300 over his team. On more than to fund Pickering’s work in her stars across the entire sky. ■ one occasion, he forced out husband’s name. In 1890, the first researchers with whom he Draper Catalogue of Stellar Spectra did not agree, only for them was published. Pickering then later to be proved right; one opened an observatory in Arequipa, example of this is Antonia Maury, whose work on stellar Many of the Harvard computers spectra Pickering dismissed. were trained in astronomy, but as women, they were excluded from Pickering spent his whole academic positions. Their wages were career in academia, but was similar to those of unskilled workers. also an avid outdoorsman, and was a founding member of the Appalachian Mountain Club. The club became a leading voice in the movement to preserve wilderness areas. Key works 1886 An Investigation in Stellar Photography 1890 Draper Catalogue of Stellar Spectra 1918 Henry Draper Catalogue

CTLHAESSSITFAYIRNSG ACCORDING TO THEIR SAPGECETRAARNEVDEALSS TIHZEEIR THE CHARACTERISTICS OF STARS



124 THE CHARACTERISTICS OF STARS IN CONTEXT A merican astronomer Each substance sends out its Annie Jump Cannon was own vibrations of particular KEY ASTRONOMER the early 20th-century’s wavelengths, which may be Annie Jump Cannon leading authority on the spectra likened to singing its own song. (1863–1941) of stars. When she died in 1941, Cannon was described as “the Annie Jump Cannon BEFORE world’s most notable woman 1860 Gustav Kirchhoff shows astronomer.” Her great contribution stars according to their spectra. that spectroscopy can be used was to create the basis of the Pickering’s team modified this to identify elements in starlight. system for classifying the spectra system. By 1924, the catalog of stars that is still in use today. contained 225,000 stars. 1863 Angelo Secchi classifies stars using their spectra. Cannon worked at the Harvard Early approaches College Observatory, as part of the Williamina Fleming, the first of 1868 Jules Janssen and Joseph team of “Harvard Computers,” a Pickering’s female computers, Norman Lockyer discover group of women employed by the made the earliest attempt at a more helium in the solar spectrum. director Edward C. Pickering to help detailed classification system, by compile a new stellar catalog. The subdividing Secchi’s classes into 1886 Edward Pickering begins college’s catalog, begun in the 13 groups, which she labeled with compiling the Henry Draper 1880s with funding from the widow the letters A to N (excluding I), then Catalogue using a photometer. of astrophotographer Henry Draper, adding O, P, and Q. In the next used new techniques to collect phase of the work, fellow computer AFTER data on every star in the sky Antonia Maury, working with better 1910 The Hertzsprung−Russell brighter than a certain magnitude, data received from observatories diagram reveals the different including obtaining the spectra of sizes of stars. as many stars as possible. In the 1860s, Angelo Secchi had set out 1914 US astronomer Walter a provisional system for classifying Adams records a white dwarf. 1925 Cecilia Payne- The seven main classes of star, Gaposchkin finds that stars categorized according to spectra and are composed almost entirely temperature, are, from left to right: of hydrogen and helium. O, B, A, F, G, K, and M, with O the hottest and M the coolest.

THE RISE OF ASTROPHYSICS 125 See also: The sun’s spectrum 112 ■ Analyzing starlight 113 ■ The sun’s emissions 116 ■ The star catalog 120–21 ■ Analyzing absorption lines 128 ■ Refining star classification 138–39 ■ Stellar composition 162–63 around the the world, noticed more The spectra of stars cover a wide variety in the detail. She devised a range of star types. more complex system of 22 groups designated by Roman numerals, A star’s spectrum can Classifying the each divided into three subgroups. reveal its temperature, stars according Pickering was concerned that to their spectra applying such a detailed system luminosity, and would delay the task of compiling composition. reveals their the catalog. However, Maury’s age and size. approach to stellar classification proved a crucial step toward the appearance of its spectrum and Harvard system creation of the Hertzsprung–Russell made her classes a temperature Cannon’s 1901 system laid the diagram in 1910, and consequent sequence from hotter to cooler. foundations for the Harvard discoveries about stellar evolution. On this, Cannon followed Maury’s Spectral Classification system. lead. Some of Fleming’s letters By 1912, she had extended it to Cannon joined the Harvard were dropped because they were introduce a range of more precise College Observatory staff in 1896 unnecessary, so the final sequence subclasses, adding 0 to 9 after and began working on the next became O, B, A, F, G, K, M, based the letter, with 0 the hottest in the part of the catalog, which was on the presence and strength of class and 9 the coolest. A few new published in 1901. With Pickering’s certain spectral lines, especially classes have been added since. approval, to make classification those due to hydrogen and helium. clearer and easier, she reverted Students of astronomy still learn it The Harvard system essentially to Fleming’s spectral classes by remembering the mnemonic, classifies stars by temperature and designated by letters, but she “Oh Be A Fine Girl, Kiss Me,” takes no account of the luminosity changed the order. attributed to Henry Norris Russell. or size of the star. In 1943, however, luminosity was added as an ❯❯ Maury had realized that stars of similar colors have the same characteristic absorption lines in the spectra. She had also deduced that a star’s temperature is the main factor affecting the

126 THE CHARACTERISTICS OF STARS additional dimension, creating The strengths of the absorption lines of different elements the Yerkes classification system, vary according to the surface temperature of the star. Lines of otherwise called the MKK system heavier elements are more prominent in the spectra of cooler stars. after William Morgan, Philip Keenan, and Edith Kellman, the Neutral Ionized Hydrogen Ionized Neutral Molecules astronomers based at the Yerkes helium helium metals metals Observatory in Wisconsin who formulated it. This system denotes RELATIVE STRENGTH luminosity with Roman numerals, although a few letters are also used. OBA FGKM DECREASING TEMPERATURE The advantage of the MKK system is that it gives a star a size as are this hot. O-type stars burn their twice as large as the sun, main well as a temperature, so that stars fuel very quickly and release huge sequence A-type stars have a can be described in colloquial terms amounts of energy. As a result, they surface temperature of between such as white dwarf, red giant, or have a short life expectancy, which 7,500 and 10,000 K. They have blue supergiant. The main sequence is measured in tens of millions strong hydrogen lines in their stars, including the sun, are all small of years, compared to billions for spectra and emit a wide spectrum enough to be called dwarfs. The sun cooler stars. Members of this class of visible light, which makes them is a G2V star, which indicates that it have weak lines of hydrogen in look white (with a blueish tinge). is a yellow dwarf with a surface their spectra, and strong evidence As a result, they are some of the temperature of about 5,800 K. of ionized helium, which is present most easily seen stars in the night because of the high temperature. sky, and include Vega (in Lyra), Classes and characteristics Gamma Ursae Majoris (in the Big The hottest class of star, O-types With a surface temperature of Dipper), and Deneb (in Cygnus). have a surface temperature in excess between 10,000 and 30,000 K, B-type However, only 0.625 percent of main of 30,000 K. Most of the radiation stars are brighter in visible light than sequence stars are A-type stars. these stars emit is in the ultraviolet O-types, despite being cooler. This part of the spectrum and appear is because more of the radiation is Cooling stars blue when viewed in visible light. emitted as visible light, making them As dwarf stars cool, the hydrogen O stars are mainly giants, typically “blue-white.” Again, B-type dwarfs in their spectra becomes less 20 times as massive as the sun and are rare, making up less than 0.1 intense. They also exhibit more 10 times as wide. Only 0.00003 percent of main sequence stars. absorption lines due to metals. (To percent of main sequence stars When they do occur, they are an astronomer, everything heavier perhaps 15 times more massive than helium is a metal.) This is The prism has revealed to us than the sun. B-type stars have not because their composition is something of the nature of non-ionized helium in their spectra different from that of hotter stars and more evidence of hydrogen. but because the gas near the surface the heavenly bodies, and the Because they live for only a short is cooler. In hotter stars, the atoms photographic plate has made time, B-type stars are found in are too ionized to create absorption molecular clouds or star-forming lines. F-type stars have a surface a permanent record of the regions, since they have not had temperature of between 6,000 and condition of the sky. time to move far from the location 7,500 K. Called yellow-white dwarfs, in which they formed. About Williamina Fleming

THE RISE OF ASTROPHYSICS 127 they make up 3 percent of the main main sequence stars, making up Annie Jump Cannon sequence and are a little larger than 76 percent of the total, although the sun. The spectra of these stars no red dwarf is visible to the naked Born in Delaware, Annie Jump contain medium-intensity hydrogen eye. They are just 2,400–3,700 K Cannon was the daughter lines and strengthening lines for on the surface and their spectra of a state senator, and was iron and calcium. contain absorption bands for oxide introduced to astronomy compounds. The majority of the by her mother. She studied The sun’s class yellow, orange, and red dwarfs are physics and astronomy at Type-G yellow dwarfs, of which believed to have planetary systems. Wellesley College, an all- the sun is one, make up 8 percent women’s college. Graduating of the main sequence. They are Extended classification in 1884, Cannon returned to between 5,200 and 6,000 K on the Stellar spectral classes now cover her family home for the next surface and have weak hydrogen even more types of stars. Class W 10 years. On the death of her lines in their spectra, with more are thought to be dying supergiant mother, in 1894, she began to prominent metal lines. Type-K stars. Class C, or carbon stars, are teach at Wellesley and joined dwarfs are orange and make up declining red giants. Classes L, Y, Edward C. Pickering’s Harvard 12 percent of the main sequence. and T are a diminishing scale of Computers two years later. They are between 3,700 and colder objects, from the coolest red 5,200 K on the surface and have dwarfs to the brown dwarfs, which Cannon suffered from very weak hydrogen absorption are not quite large or hot enough to deafness, and the ensuing lines but strong metallic ones, be classed as stars. Finally, white difficulties in socializing led including manganese, iron, and dwarfs are class D. These are the hot her to immerse herself in silicon. Type-M are red dwarfs. cores of red giant stars that no longer scientific work. She remained These are by far the most common burn with fusion and are gradually at Harvard for her entire cooling. Eventually they should fade career, and is said to have A white dwarf sits at the heart of the to black dwarfs, but it is estimated classified 350,000 stars over Helix planetary nebula. When its fuel it will take a quadrillion years for 44 years. Subject to many ends, the sun will become a white dwarf. that to happen. ■ restrictions over her career due to her gender, she was finally appointed a member of the Harvard faculty in 1938. In 1925, she became the first woman to be awarded an honorary degree by Oxford University. Key work 1918–24 The Henry Draper Catalogue

128   OTTHWFEROREEKDIANSRDTESAR ANALYZING ABSORPTION LINES IN CONTEXT I n the late 19th and early 20th Red giant centuries, Edward Pickering KEY ASTRONOMER and his assistants carried Sun Ejnar Hertzsprung out extensive work classifying (1873–1967) star spectra. They cataloged the Red dwarf range of light wavelengths coming BEFORE from a star, which, among other A typical red giant has a diameter 1866 Angelo Secchi creates information, contains dark absorption about 50 times that of the sun, and the first classification of stars lines. These lines indicate the 150 times that of a typical red dwarf. according to their spectral presence of particular elements However, a red giant has only about characteristics. in the star’s atmosphere that are 8–10 times the mass of a red dwarf. absorbing those wavelengths. 1880s At the Harvard College category, or red-colored stars, Observatory, Edward Pickering One of Pickering’s assistants, he noticed that the “c-types” and Williamina Fleming Antonia Maury, developed her own were highly luminous, high-mass, establish a more detailed classification system, taking into comparatively rare stars—today, classification system. account differences in the width of these are called red giants or red absorption lines in star spectra. She supergiants, depending on their 1890s Antonia Maury develops noticed that some spectra, which she size. The remaining majority of her own system for classifying denoted as “c,” had sharp, narrow non-“c-type” M-class stars were star spectra, taking into lines. Using Maury’s system, Danish low-mass, faint stars that are now account differences in width astronomer Ejnar Hertzsprung saw known as red dwarfs. A similar and sharpness of spectral lines. that stars with “c-type” spectra were distinction of two main kinds also far more luminous than other stars. applied to K-class (orange) stars. ■ AFTER 1913 Henry Norris Russell Bright and dim red stars creates a diagram, similar Hertzsprung figured out that what to one made by Hertzsprung, Maury had identified as “c-type” that plots the absolute stars were radically different from magnitude (intrinsic brightness) other types in the same category. of stars against spectral class. For example, within the M-class This later becomes known as a Hertzsprung−Russell diagram. See also: The sun’s spectrum 112 ■ Analyzing starlight 113 ■ The star catalog 120–21 ■ The characteristics of stars 122–27 ■ Refining star classification 138–39

THE RISE OF ASTROPHYSICS 129 SARUENSMPAOGTNSETIC   THE PROPERTIES OF SUNSPOTS IN CONTEXT A merican George Hale was including the 60-inch (150-cm) just 14 when his wealthy Hale Telescope, built at California’s KEY ASTRONOMER father bought him his first Mount Wilson Observatory in 1908, George Ellery Hale telescope, and 20 when his father paid for by a bequest from his (1868–1938) built him an observatory on the father. Working at Mount Wilson family property. Two years later, that same year, Hale was able BEFORE while at MIT, Hale developed a new to take clear images of sunspots 800 bce The appearance of dark design for a spectroheliograph— in a deep red wavelength emitted spots on the sun is recorded in a device for viewing the surface by hydrogen. The speckled images the Chinese Book of Changes. of the sun one wavelength of light reminded Hale of the way iron at a time. He used this device to filings mapped the force field 1600 English physicist William study the spectral lines of sunspots. around a magnet. This led him to Gilbert discovers that Earth look for signs of the Zeeman effect has a magnetic field. Some years later, Hale organized in light coming from the sunspots. the building of some of the largest 1613 Galileo demonstrates telescopes in the world at the time, The Zeeman effect is a split that sunspots are features in spectral lines caused by the on the surface of the sun. presence of a magnetic field, first observed by the Dutch physicist 1838 Samuel Heinrich Pieter Zeeman in 1896. The Schwabe notes a cycle in spectral lines in light coming from the numbers of sunspots sunspots had indeed been split, seen each year. which suggested to Hale that sunspots were swirling magnetic 1904 British astronomers storms on the surface of the sun. ■ Edward and Annie Maunder publish evidence of an 11-year The variations in the strength sunspot cycle. of the sun’s magnetic field are shown in this magnetogram, produced AFTER using the Zeeman effect. The marks 1960 US physicist Robert correspond to the locations of sunspots. Leighton introduces the field See also: Galileo’s telescope 56–63 ■ The surface of the sun 103 ■ of helioseismology, a study of The sun’s vibrations 213 ■ Maunder (Directory) 337 the motion of the solar surface.

THE KEY SCALETO A DISTANCE OF THE UNIVERSE MEASURING THE UNIVERSE



132 MEASURING THE UNIVERSE IN CONTEXT S ome of the most important, A remarkable relation but often most challenging, between the brightness KEY ASTRONOMER measurements for astronomers of these (Cepheid) variables Henrietta Swan Leavitt to make have been the distances to and the length of their (1868–1921) extremely remote objects—which periods will be noticed. includes most celestial objects Henrietta Swan Leavitt BEFORE aside from the moon, sun, and other 1609 German pastor David planets of the inner solar system. by the year 1900, the distances Fabricius discovers the Nothing in the light coming from to only about 60 stars had been periodically variable star Mira. distant stars and galaxies gives measured. Furthermore, the parallax any direct indication of how far method could be applied only to 1638 Dutch astronomer that light has traveled through nearby stars. The difference in Johannes Holwarda observes space to reach Earth. perspective for more distant stars Mira’s variation in brightness over the course of a year was too over a regular 11-month cycle. For several hundred years, small to be accurately determined. scientists realized that it should be New methods were therefore needed 1784 John Goodricke discovers possible to measure the distances to measure large distances in space. a periodic variation in the star to relatively nearby stars by a Delta Cephei: the prototypic method called parallax. This is Measuring brightness example of a Cepheid variable. based on comparing the position In the 1890s and early 1900s, the of a nearby star against the Harvard College Observatory in 1838 Friedrich Bessel measures background of more distant stars Massachusetts was one of the the distance to the star 61 Cygni from two perspectives—usually world’s leading astronomical research using the parallax method. Earth’s different positions in space six months apart in its orbit around AFTER the sun. Although many others 1916 Arthur Eddington had tried (and failed) before him, studies why Cepheids pulsate. the first astronomer to measure a star’s distance accurately using 1924 Edwin Hubble uses this method was Friedrich Bessel, observations of a Cepheid in 1838. However, even with in the Andromeda nebula increasingly powerful telescopes, to calculate its distance. measuring star distances by parallax proved difficult and, Henrietta Swan Leavitt Henrietta Swan Leavitt developed Due to the prejudices of the an interest in astronomy while day, Leavitt did not have the studying at Radcliffe College, chance to use her intellect Cambridge, Massachusetts. After to the fullest, but she was graduation, she suffered a serious described by a colleague as illness that caused her to become “possessing the best mind increasingly deaf for the rest of at the Observatory.” She was her life. From 1894 to 1896 and remembered as hardworking then again from 1902, she worked and serious-minded, “little given at Harvard College Observatory. to frivolous pursuits.” Leavitt Leavitt discovered more than worked at the Observatory until 2,400 variable stars and four her death from cancer in 1921. novae. In addition to her work on Cepheid variables, Leavitt Key work also developed a standard of photographic measurements, 1908 1777 Variables in the now called the Harvard Standard. Magellanic Clouds

THE RISE OF ASTROPHYSICS 133 See also: A new kind of star 48–49 ■ Stellar parallax 102 ■ The star catalog 120–21 ■ Analyzing absorption lines 128 ■ Nuclear fusion within stars 166–67 ■ Beyond the Milky Way 172–77 ■ Space telescopes 188–95 Milky Way At the time, the SMC and LMC were thought to be very large star Earth clusters within the Milky Way, which itself was assumed to Large The Cepheid variable stars that comprise the entire universe. Magellanic Leavitt studied are in the Magellanic Today, they are known to be Cloud Clouds, known today to be galaxies relatively small, separate galaxies outside the Milky Way. The Large that lie outside the Milky Way. Magellanic Cloud is about 160,000 The Magellanic Clouds are visible light-years away; the Small Magellanic to the naked eye in the night sky Cloud is about 200,000 light-years away. of the southern hemisphere, but are Both are part of the Local Group galaxy never visible from Massachusetts, cluster that includes the Milky Way. where Leavitt lived and worked. Small Magellanic Cloud Therefore, although she examined numerous photographic plates institutions. Under the supervision eventually became the head of of the LMC and SMC obtained of its director, Edward C. Pickering, the photographic photometry by astronomers at an observatory the Observatory employed many department. This mainly involved in Peru, it is highly unlikely that men to build equipment and take measuring the brightness of stars, she ever physically observed photographs of the night sky, but a specific aspect of Leavitt’s them in the sky. and several women to examine work was to identify stars that photographic plates taken from fluctuate in brightness—known After several years’ work, telescopes throughout the world, as variable stars. To do this, Leavitt had found 1,777 variables in measure their brightness, and she would do a comparison of the SMC and LMC. One particular perform computations based on photographic plates of the same part kind that caught Leavitt’s attention, their assessment of the plates. of the sky, made on different dates. representing a small fraction of all These women had little chance Occasionally she would find a star the variables she had found (47 to do theoretical work at the that was brighter on different dates, out of 1,777), was of a type called Observatory, but several of them, indicating that it was a variable. a Cepheid variable. Leavitt called including Williamina Fleming, them “cluster variables”—the term Henrietta Swan Leavitt, Antonia Cluster variables Cepheid variable was introduced ❯❯ Maury, and Annie Jump Cannon, A specific task that Leavitt took nevertheless left a lasting legacy. on was to examine some of the One of the most striking photographic plates of stars in the accomplishments of Miss Henrietta Swan Leavitt, who had Small Magellanic Cloud (SMC) and Leavitt was the discovery originally joined the Observatory the Large Magellanic Cloud (LMC). of 1,777 variable stars in as an unpaid volunteer in 1894, the Magellanic Clouds. Solon I. Bailey Colleague of Leavitt

134 MEASURING THE UNIVERSE The period of the fluctuation in brightness of a galaxies. In examining her records Cepheid variable is closely related to its intrinsic brightness. of Cepheid variables in either the LMC or SMC, Leavitt noticed Measuring its period Comparing its intrinsic something that seemed significant. gives a value for its brightness to its Cepheids with longer periods seemed to be brighter on average intrinsic brightness. apparent brightness than those with shorter periods. from Earth gives a value In other words, there was a for its distance from Earth. relationship between the rate at which Cepheids “blinked” and their Cepheid variables can be used as “standard candles” brightness. Furthermore, Leavitt to measure distances in the universe. correctly inferred that, since the Cepheids she was comparing were later. These are stars that regularly in brightness followed by a slower all in the same distant nebula vary in brightness with a period tailing off. Today, they are known to (either the LMC or the SMC), they (cycle length) that could be anything be giant yellow stars that “pulsate”— were all at much the same distance from one to more than 120 days. varying in diameter as well as from Earth. It followed that any Cepheid variables are reasonably brightness over their cycles—and difference in their brightness as easy to recognize because they are are very rare. As a class of stars, viewed from Earth (their apparent among the brightest variable stars, they also have an exceptionally magnitude) was directly related and they have a characteristic light high average brightness, which to differences in their true or curve, showing fairly rapid increases means they stand out even in other intrinsic brightness (their absolute magnitude). This meant there was a definite relationship between the periods of Cepheid variables and their average intrinsic brightness or their optical luminosity (the rate at which they emit light energy). Leavitt published her initial findings in a paper that first appeared in the Annals of the Hottest state Coolest state A straight line can readily LUMINOSITY Period of one pulsation Light be drawn among each of curve the two series of points corresponding to maxima and TIME minima, thus showing that A Cepheid variable belongs to a class of star called a pulsating there is a simple relation variable. These stars expand and contract over a regular cycle, at the between the brightness of the same time regularly varying in brightness. They are hottest and brightest variables and their periods. shortly after reaching their most contracted phase. The graph of the Henrietta Swan Leavitt star’s luminosity (light output) against time is called its light curve.

THE RISE OF ASTROPHYSICS 135 Brightness and magnitudes of stars information about the distance to the SMC, nor indeed any accurate data about the intrinsic brightness of any Cepheid variable. Apparent magnitude Absolute visual Optical luminosity is Calibrating the variables is the brightness magnitude is the the rate at which a star To turn Leavitt’s finding into of a star as viewed brightness of a star emits light energy from a system that could be used to from Earth. as viewed from a set its surface and is closely determine absolute distances, not distance and indicates related to absolute just relative distances, it needed the true or intrinsic visual magnitude. calibrating in some way. In order brightness of a star. to do this, it would be necessary to measure accurately the distances Astronomical Observatory of One of the first people to appreciate to and intrinsic brightness of a few Harvard College in 1908. Then, the significance of Leavitt’s Cepheid variables. Hertzsprung in 1912, after further study, which discovery was Danish astronomer therefore set about determining the included plotting graphs of the Ejnar Hertzsprung. Due to the distances to a handful of Cepheids periods of Cepheid variables period−luminosity relationship in the Milky Way galaxy, using an in the SMC against values for discovered by Leavitt, Hertzsprung alternative complex method called their minimum and maximum realized that by measuring the statistical parallax, which involves brightness, she confirmed her period of any Cepheid variable calculating the average movement discovery in more detail. It became it should be possible to determine of a set of stars assumed to be at known as the “period−luminosity” its luminosity and intrinsic a similar distance from the sun. relationship. Formally, it stated brightness. Then by comparing that the logarithm of the period its intrinsic brightness to its Having obtained the stars’ of a Cepheid variable is linearly apparent magnitude (measured distances, it was a straightforward (i.e., directly) related to the star’s average brightness from Earth), it step to figure out the intrinsic average measured brightness. should be possible to calculate the brightness of each of the nearby distance to the Cepheid variable. Cepheids. Hertzsprung used Building on Leavitt’s work In this way, it should also be these values to calibrate a scale, Although it is possible that possible to determine the distance which allowed him to calculate Leavitt did not realize the full to any object that contained one the distance to the SMC and the implications right away, she had or more Cepheid variable star. intrinsic brightness of each of discovered an extremely valuable Leavitt’s Cepheids in the SMC. ❯❯ tool for measuring distances in the However, there was still a universe, far beyond the limitations problem to be solved: although I should be willing to pay of parallax measurements. Cepheid Leavitt had established the thirty cents an hour in view variables were to become the first important period−luminosity of the quality of your work, “standard candles”—a class of relationship, initially all this celestial objects that have a known promised was a system for although our usual price, luminosity, allowing them to be measuring the distance to remote in such cases, is twenty used as tools to measure vast objects relative to the distance distances in space. to the SMC. The reason for this five cents an hour. is that Leavitt had no accurate Edward C. Pickering

136 MEASURING THE UNIVERSE Leavitt left behind a legacy of a great astronomical discovery. Solon I. Bailey Following these calibrations, Hertzsprung was able to establish a system for determining the distance to any Cepheid variable from just two items of data—its period and its apparent magnitude. Further applications to the first realistic estimate of The star RS Puppis is one of It was not long before Leavitt’s the true size of the Milky Way, the brightest Cepheid variables findings, tuned by the work of was an important milestone in in the Milky Way. It is about 6,500 Hertzsprung, led to further galactic astronomy. light-years from Earth and has a cycle important results in terms of of variability lasting 41.4 days. helping to understand the scale Right up to the 1920s, many of the universe. From 1914 to 1918, scientists (including Harlow Andromeda nebula, allowing its the American astronomer Harlow Shapley) maintained that the distance to be measured. This Shapley (who was also the first Milky Way galaxy was the whole led directly to the confirmation person to show that Cepheid universe. Although there were that the Andromeda nebula is a variables are pulsating stars) those that believed otherwise, separate large galaxy (and is now was one of the first to use the neither side could conclusively called the Andromeda galaxy) newly developed concept that the prove their argument one way outside the Milky Way. Later, distances of variable stars could be or another. In 1923, however, the Cepheids were similarly used to found from knowing their periods American astronomer Edwin show that the Milky Way is just one and apparent brightness. Shapley Hubble, using the latest in of a vast number of galaxies in the found that objects called globular telescopic technology, found universe. The study of Cepheids star clusters—all part of the Milky a Cepheid variable in the Way—were distributed roughly in a sphere whose center lay in the direction of the constellation of Sagittarius. He was able to conclude from this that the center of the Milky Way galaxy is at a considerable distance (tens of thousands of light-years) in the direction of Sagittarius and that the sun is not, as had previously been supposed, at the center of the galaxy. Shapley’s work, which led

THE RISE OF ASTROPHYSICS 137 was also employed by Hubble in classical and Type II Cepheids, The measurement of distances his discovery of the relationship and started to be used for different to Cepheid variables for more between the distance and purposes in distance measuring. accurate calibration of period– recessional velocity of galaxies, luminosity relationships is still leading to confirmation that the Today, classical Cepheids are considered extremely important, universe is expanding. used to measure the distance of and it was one of the primary galaxies out to about 100 million missions of the Hubble Space Revising the scale light-years—well beyond the Telescope project when it was In the 1940s, the German local group of galaxies. Classical launched in 1990. A better astronomer Walter Baade was Cepheids have also been used to calibration is crucial, among working at the Mount Wilson clarify many characteristics of other things, to calculate the age Observatory in California. Baade the Milky Way galaxy, such as its of the universe. Leavitt’s findings made observations of the stars local spiral structure and the sun’s from over a century ago are still at the center of the Andromeda distance from the plane of the having significant repercussions galaxy during the enhanced galaxy. Type II Cepheids have been in terms of truly understanding viewing conditions afforded used to measure distances to the the scale of the cosmos. ■ by the wartime blackout. He galactic center and globular clusters. distinguished two separate populations, or groups, of Cepheid A simplified version Gravity Pressure variables that have different of the mechanisms that forces period–luminosity relationships. cause Cepheid variables A This led to a dramatic revision in to fluctuate in size is B the extragalactic distance scale— shown here. The pressure Pressure forces exceed for example, the Andromeda forces inside a star include gravity. The star begins Pressure and gravity are now galaxy was found to be double gas pressure, maintained to expand. in balance but inertia causes the distance from the Milky by heat output from the the star to expand further. Way that Hubble had calculated. star’s core, and radiation Baade announced his findings pressure. Another at the International Astronomical mechanism that may Union in 1952. The two groups be involved is a cyclical of Cepheids became known as change in the opacity (resistance to the transmission of radiation) in gas within the star’s outer layers. Hubble’s underwhelming C D E acknowledgment of Leavitt is an example of the ongoing With continued expansion, Pressure and gravity As the star contracts, the denial and lack of professional the pressure forces decrease, are in balance again pressure forces increase until and public recognition that as does the gravity, though but inertia causes the they exceed the inward- she suffers from, despite her to a lesser extent. Eventually, star to shrink further. pulling gravity. The star gravity exceeds the pressure stops shrinking and begins landmark discovery. from the pressure forces, and to expand again, starting Pangratios Papacosta the star stops expanding a new pulsation cycle. and begins to shrink. Science historian

138 DSGWITAAANRRTSFSSAORRE REFINING STAR CLASSIFICATION IN CONTEXT Among most stars, A round 1912, American Henry blue stars are brighter Russell began comparing KEY ASTRONOMER than yellow stars, which are stars’ absolute magnitude Henry Norris Russell brighter than orange/red (or true brightness) and their color, (1877–1957) or spectral class. Before the early stars. These are 20th century, no one had figured out BEFORE dwarf stars. how different star types might be 1901 Annie Jump Cannon, However, a few related in some overall scheme, but working at the Harvard College exceptionally bright stars it had long been recognized that Observatory, introduces the do not follow this rule. they differ in certain properties, star spectral classes O, B, A, F, These are giant stars. such as color. While some stars G, K, and M (based on surface Stars fall into two shine with a pure white light, others temperature of stars). distinct groups when have distinct colors: many have plotted on a diagram reddish or bluish hues, while the 1905 Based on analyses of star showing luminosity sun is yellow. In 1900, German spectra, Ejnar Hertzsprung and temperature. physicist Max Planck worked out states that there are two the precise mathematics to describe fundamentally different kinds Stars are either how the mix of wavelengths of light of star within some spectral giants or dwarfs. given off by hot objects, and hence classes, one of which is far their color, varies according to their more luminous. temperature. Thus, star colors are related to surface temperature—red AFTER stars have the coolest surfaces, and 1914 Walter Adams discovers blue stars the hottest. By around white dwarf stars—white-hot 1910, stars were considered to fit but relatively faint. into spectral classes related to their colors and surface temperatures. 1933 Danish astronomer Bengt Strömgren introduces The other obvious way in which the term “Hertzsprung–Russell stars differ is in their brightness. diagram” to denote a plot Since ancient times, stars have of the absolute magnitudes been classified into brightness of stars against spectral class. classes. This developed into the apparent magnitude scale, which rated stars according to how bright

THE RISE OF ASTROPHYSICS 139 See also: Analyzing starlight 113 ■ The characteristics of stars 122–27 ■ Analyzing absorption lines 128 ■ Measuring the universe 130–37 ■ Discovering white dwarfs 141 ■ Stellar composition 162–63 they look from Earth. However, it ABSOLUTE MAGNITUDE -10 Supergiants Giants The Hertzsprung− was realized that, in order to know Russell diagram a star’s absolute brightness, it -5 shows the distribution would be necessary to correct for of stars by absolute its distance from Earth: the farther 0 magnitude and away a star is, the dimmer it will spectral class. The appear. From the mid-19th century, +5 Main sequence diagram formed the reasonably precise distances to (Dwarfs) basis for developing some stars began to be calculated, theories about how and the absolute brightness of +10 stars evolve. (In the these stars could be established. +15 White dwarfs absolute magnitude scale, the lower the Russell’s discovery number, the higher Among the majority of stars, the magnitude.) Russell found a definite relationship—hot blue-white stars 20,000 10,000 5,000 2,500 (spectral classes B and A) tend to have higher absolute magnitudes TEMPERATURE (°C) than cooler white and yellow stars (classes F and G), while white and 1913. However, unknown to him, Russell called these ordinary stars yellow stars have higher absolute Danish chemist and astronomer “dwarfs”; Hertzsprung referred to magnitudes than orange and red Ejnar Hertzsprung had performed them as “main sequence.” The stars (classes K and M). However, a similar exercise a couple of years newly discovered hot but faint some exceptionally bright red, orange, earlier, and the diagram is now white dwarfs were later added to and yellow stars departed from this known as the Hertzsprung–Russell the diagram, forming a third group. rule. These were the “giant” stars. diagram. The diagram shows stars Today, it is known that most stars divided into a group of bright giant spend most of their lives on the Russell plotted the absolute stars and a much larger group of main sequence, some later evolving magnitudes of stars against ordinary stars running diagonally. into giants or supergiants. ■ their spectral classes on a scatter diagram, which he published in Henry Norris Russell Henry Norris Russell was born appointed as an instructor in Oyster Bay, Long Island, in in astronomy at Princeton 1877. At age 5, his parents University, and in 1911, he encouraged him to observe became professor of astronomy a transit of Venus across the there. He was also director sun’s disc, which inspired an of Princeton University interest in astronomy. He was Observatory from 1912 to 1947. awarded a doctoral degree by the astronomy department Key works of Princeton University for an analysis of the way that Mars 1927 Astronomy: A Revision of perturbs the orbit of the asteroid Young’s Manual of Astronomy; Eros. From 1903 to 1905, he Volume 1: The Solar System; worked at the Cambridge Volume 2: Astrophysics and Observatory, England, on star Stellar Astronomy photography, binary stars, and 1929 On the Composition of the stellar parallax. In 1905, he was sun’s Atmosphere

140     IPRFSREANOCDEMOIATMTSRIIPANOATGNCINEG COSMIC RAYS IN CONTEXT A ustrian-born physicist by substances in the ground, Victor Hess made a series meaning that air ionization should KEY ASTRONOMER of dangerous high-altitude decrease with altitude. However, Victor Hess (1883–1964) ascents over eastern Germany in a measurements made at the top hydrogen balloon in the years 1911 of the Eiffel Tower in Paris in BEFORE and 1912. His goal was to measure air 1909 indicated a higher level 1896 French physicist Henri ionization at a height of 3 miles (5 km). of ionization than expected. Becquerel detects radioactivity. Ionization is the process by Hess’s results showed that 1909 German scientist which electrons are stripped from ionization decreased up to an Theodor Wulf measures air atoms. In the early years of the altitude of about half a mile ionization near the top of 20th century, scientists were (1 km), and then increased above the Eiffel Tower. Levels are puzzled by the levels of ionization that point. He concluded that higher than expected. in Earth’s atmosphere. After the powerful radiation from space discovery of radioactivity in 1896, was penetrating and ionizing the AFTER it was suggested that ionization atmosphere. This radiation later 1920s American physicist was caused by radiation emitted became known as cosmic rays. Robert Millikan coins the term “cosmic ray.” In 1950, scientists found that cosmic rays consisted of charged 1932 American physicist particles, some possessing very Carl Anderson discovers the high energies. They smash into positron (antiparticle of the atoms in the atmosphere, creating electron) in cosmic rays. new subatomic particles that may themselves create collisions, which 1934 Walter Baade and Fritz in turn cause a cascade of collisions Zwicky propose the idea that called a cosmic ray shower. ■ cosmic rays come from supernova explosions. In 1951, the Crab nebula was found to be a major source of cosmic rays. 2013 Data from the Fermi Since then, supernovae and quasars Space Telescope suggest that have also been identified as sources. some cosmic rays come from supernova explosions. See also: Supernovae 180–81

THE RISE OF ASTROPHYSICS 141 SAISTWATRHOIOTTHEFAAHTIONTT DISCOVERING WHITE DWARFS IN CONTEXT I n the first decade of the 20th The answer could only be that, century, American astronomer although it was small (about the KEY ASTRONOMER Walter Adams developed a size of Earth), its density must be Walter Adams (1876–1956) method for calculating the absolute immense—about 25,000 times magnitude of stars from the relative that of the sun. 40 Eridani B was BEFORE intensities of particular wavelengths the first white dwarf star to be 1783 William Herschel in their spectra. One of the original discovered. White dwarfs were discovers 40 Eridani B and C. team members at the Mount Wilson later shown to be the hot stellar Observatory in California, Adams cores left behind when main 1910 Williamina Fleming used his method to investigate the sequence stars run out of fuel answers an inquiry from triple-star system 40 Eridani, which for nuclear fusion. ■ Henry Norris Russell about contained a mysterious star that the spectrum of 40 Eridani B, seemed very dim but also very hot. confirming that it is a Type A star. White dwarf Composed of material 3,000 The brightest of the three stars, times denser than anything AFTER 40 Eridani A, was being orbited you have ever come across, a 1926 British astronomer by a much dimmer binary pair, ton of [this] material would be Ralph Fowler applies new 40 Eridani B and C. Stars as a little nugget that you could ideas in quantum physics faint as 40 Eridani B and C were to explain the nature of the expected to be of spectral class M, put in a matchbox. extremely dense material meaning that their starlight is red, Arthur Eddington in white dwarfs. indicating a relative coolness. 40 Eridani C fitted this profile, but 40 describing white dwarfs 1931 Subrahmanyan Eridani B was one of the whitest Chandrasekhar calculates and hottest types of star. When that white dwarfs cannot be Adams published the data in 1914, more massive than 1.4 times astronomers were presented with the mass of the sun. a puzzle: a star that hot had to be getting its energy from somewhere. 1934 Walter Baade and Fritz Zwicky suggest that stars See also: Observing Uranus 84–85 ■ Refining star classification 138–39 ■ too massive to become white The life cycles of stars 178 ■ Energy generation 182–83 dwarfs form neutron stars.

AATNODMGSAL 1915–1950

LSATXAIRESS

144 INTRODUCTION Albert Einstein Observing a solar eclipse, Edwin Hubble finds a publishes his general Arthur Eddington shows relation between the redshift theory of relativity, which explains gravity that light from stars is and distance of nebulae, bent by the sun’s gravity, showing that spiral as a warping of just as relativity predicts. spacetime. nebulae are galaxies. 1916 1919 1924 1917 1920 1926 Vesto Slipher shows that At the Smithsonian Austrian physicist Erwin many nebulae show large museum, a “Great Schrödinger formalizes redshift, meaning that Debate” takes place over the equation describing whether or not spiral quantum mechanics, which they are moving away nebulae are galaxies. describes strange behavior from us rapidly. at the quantum level. D espite the vast difference understanding how the hierarchy In 1919, the New Zealand physicist in scale, atoms, stars, of matter in the universe is Ernest Rutherford was able to and galaxies share a organized. Underpinning these transmute atoms of nitrogen property in common: each in its developments was Einstein’s into oxygen by firing particles at own size domain is a fundamental general theory of relativity, in them from a radioactive element. construction unit of the universe. which the concepts of mass There was now ample evidence Galaxies define the distribution and energy are inseparable in that nuclear processes could of matter in the universe on the a unified fabric of space and time. produce new elements and grandest scale; stars are a defining release unimaginable quantities constituent of those galaxies Looking inside a star of energy. For any remaining (although galaxies may harbor Between 1916 and 1925, Briton doubters, Eddington reflected quantities of gas, dust, and Arthur Eddington worked on the on the experiments conducted at mysterious dark matter as well); physical nature of ordinary stars Cambridge University by pointing atoms are the units of matter that such as the sun. He pieced together out that “what is possible in the make up the hot gas of stars (with a detailed physical description Cavendish Laboratory may not some simple molecules in cooler of a sphere of hot gas, in which be too difficult in the sun.” stars). If we think of galaxies as energy makes its way from a central cities, stars are like individual source to the surface, from where it When British astronomer Cecilia buildings within the city, and then radiates into space. Eddington Payne-Gaposchkin, working in atoms are the bricks. also did much to convince the US, concluded in 1925 that astronomers that stars are fueled stars are overwhelmingly made In a mere 30-year period in by subatomic processes—what of hydrogen atoms, astronomers the first half of the 20th century, we would now call nuclear energy. at last had a real grasp on the astronomy took huge leaps in true nature of “ordinary” stars.

ATOMS, STARS, AND GALAXIES 145 At the Lowell Observatory in Georges Lemaître American astrophysicist Arizona, Clyde Tombaugh publishes a paper in Lyman Spitzer Jr. which he proposes that proposes putting discovers Pluto, which is the universe began telescopes in space. initially classified as from a tiny “atom.” the ninth planet. 1930 1931 1946 1930 1933 1946 Subrahmanyan Using an antenna he had British astronomer Chandrasekhar calculates built himself, American Fred Hoyle shows the conditions under which radio engineer Karl Jansky how elements are discovers radio waves a star can collapse into a made in stars. neutron star or black hole. coming from space. However, not all stars are quite holes was born, although many tiny “primeval atom” like a firework. so ordinary. White dwarfs, for astronomers found it hard to believe In just a handful of years, astronomers example, are clearly extraordinarily they could really exist. In any event, had learned that the universe was dense. In the 1930s, the tools of the it was four decades before the first far larger and more complex than new quantum physics were used to neutron stars and candidate black they had ever imagined. ■ explain how a star could become holes were identified. so compacted and predicted even We used to think that if more exotic types of collapsed The universe of galaxies we knew one, we knew two, star. It turned out that 1.46 solar Meanwhile, the whole concept of the masses would be the upper limit nature of the universe was changing because one and one are to make a white dwarf, but there rapidly. In 1917, American Vesto two. We are finding that was nothing to stop more massive Slipher recognized that many we must learn a great deal stars from collapsing into a much so-called “nebulae” were galaxies, denser neutron star or a black hole. akin to our own Milky Way, and in more about “and.” rapid motion. Some 10 years later, Arthur Eddington Black holes may be real Belgian priest Georges Lemaître Walter Baade and Fritz Zwicky realized that an expanding universe speculated that the central remnant was consistent with Einstein’s theory of a supernova explosion would of relativity. American Edwin Hubble be a neutron star, and with the discovered that the more distant a work of Indian Subrahmanyan galaxy, the faster it is receding from Chandrasekhar and others, the us, and Lemaître suggested that the theoretical concept of black universe began by exploding from a

GTIMREAAVNDITSPAATCEIOANND EHAXVEISNOTSEEPNARCATEE FROM MATTER THE THEORY OF RELATIVITY



148 THE THEORY OF RELATIVITY IN CONTEXT The speed of light A person undergoing is always constant acceleration cannot tell KEY ASTRONOMER even when observers if this is due to gravity or Albert Einstein (1879–1955) another force. Their body are moving. BEFORE This must mean that could be thought of as 1676 Ole Rømer shows that moving through space moving, or the universe light speed is not infinite. around it could be thought makes the flow of 1687 Isaac Newton publishes time slower. of as changing. his laws of motion and Mass exists not universal law of gravitation. just in space but in spacetime. Mass itself 1865 James Clerk Maxwell distorts spacetime. shows that light is a wave moving though an The slowing of time Gravity is best described electromagnetic field at makes an object’s as the result of spacetime a constant speed. mass increase. being warped by mass. AFTER Time and space and gravitation have 1916 Karl Schwarzschild uses no separate existence from matter. Einstein’s equations to show how much matter warps space. 1919 Arthur Eddington provides evidence for the warping of spacetime. 1927 Georges Lemaître shows that a relativistic universe can be dynamic and constantly changing, and proposes the Big Bang theory. A lbert Einstein’s general The theory of relativity arose from Measuring the speed of light is theory of relativity has a contradiction between the laws of not an easy thing to do. Danish been called the greatest motion described by Isaac Newton astronomer Ole Rømer tried in 1676 act of thought about nature ever and the laws of electromagnetism by measuring the time delay in the to take place in a person’s head. defined by Scottish physicist light arriving from Jupiter’s moons. It explains gravity, motion, matter, James Clerk Maxwell. Newton His answer was 25 percent too energy, space and time, the described nature in terms of matter slow, but he did show that light’s formation of black holes, the Big in motion governed by forces that speed was finite. By the 1850s, Bang, and possibly dark energy. act between objects. Maxwell’s more accurate measurements Einstein developed the theory over theories concerned the behavior had been made. However, in a more than a decade at the start of electric and magnetic fields. Newtonian universe, there must of the 20th century. It went on to Light, he said, was an oscillation also be changes in the speed inspire Georges Lemaître, Stephen through these fields, and he of light to account for the relative Hawking, and the LIGO team, predicted that the speed of light motion of its source and observer. which searched for the gravitational was always constant, regardless Try as researchers might, no such waves predicted by the theory. of how fast the source was moving. differences could be measured.