The Astronomy Book

ATOMS, STARS, AND GALAXIES 149 See also: Gravitational theory 66–73 ■ Curves in spacetime 154–55 ■ The birth of the universe ■ Dark energy 298–303 ■ Gravitational waves 328–31 At the end of the 19th century, Mirabilis (miracle year) of 1905, when If you can’t explain it to many believed that physicists he presented four papers. These a six year old, you don’t had fully figured out the laws of the included two linked discoveries: understand it yourself. universe. All that was now needed special relativity and the equivalence were more precise measurements. of mass and energy, summed up Albert Einstein However, even as a child, Einstein by the equation E=mc2 (p.150). was not convinced that physics had traveling straight up and down, the been solved. At the age of 16, he Special relativity beam travels diagonally. To Pat on asked himself a question: “What Einstein used thought experiments the platform the light beam has would I see if I were sitting on a to develop his ideas, the most traveled farther, so, since light beam of light?” In the Newtonian significant of which involved two always travels at the same speed, context, young Albert would be men—one on a speeding train and more time must have passed. traveling at the speed of light. Light the other standing on the platform. coming from in front would reach In one version (below), inside the Einstein’s explanation for his eyes at twice the speed of light. train, Bob shines a flashlight at a this took an enormous leap of When looking back, he would see mirror directly above him on the imagination, which became nothing at all. Even though light from ceiling. He measures the time the the basis of special relativity. behind was traveling at the speed light takes to travel to the mirror and Speed is a measure of units ❯❯ of light, it could never catch up. back. At the same time, the train is passing the platform at close to the Annus mirabilis speed of light. From the platform, Einstein’s first job was working as a the stationary observer Pat sees patent clerk in Bern, Switzerland. It the light beam shine to the mirror afforded him a lot of spare time to and back, but in the time it takes devote to private study. The fruit of for the beam to travel, the train has this solitary work was the Annus moved, meaning that, rather than Inside the speeding train, Bob shines a light beam directly up and down. Bob measures the time it takes for the light to be reflected back to him as the distance straight up and down divided by c (the speed of light). On the platform, Pat observes the beam traveling diagonally. It is still traveling at the same speed c, so more time must have passed than for Bob as the light has traveled a longer distance. Pat, stationary observer

150 THE THEORY OF RELATIVITY As an object’s velocity (v) approaches light speed (c), the object of E=mc2, which states that becomes increasingly squashed in the direction of travel when viewed E (energy) is equal to mass (m) by a stationary observer. This is not merely an illusion. In the observer’s multiplied by the square of the frame of reference, the object’s shape really does change. speed of light (c). c2 is a very large number—about 90 million v =0 v =0.3c v =0.6c v =0.9c billion—and so a small amount of mass contains a huge quantity of distance per units of time. which time is measured—such of energy. This is evident in a Therefore, the constancy of the as the swing of a pendulum, the nuclear explosion when mass speed of light must be due to an vibration of a quartz crystal, or the is converted to free energy. inconstancy in the flow of time. behavior of an atom—are physical Objects observed to be traveling phenomena obeying universal laws. Returning to the train thought faster through space are moving According to special relativity, experiment, the two observers more slowly through time. Clocks laws remain unchanged within now throw tennis balls at each on the station and on the train are the reference frame—the moving other. The balls collide and bounce ticking at different rates, depending train, or any other set of objects back to each person (both Pat on the frame of reference from moving together. and Bob have very good aim). which they are observed. On the If both observers were in the same moving train, Bob sees his clock Energy is mass reference frame, the described ticking away as normal, but to the The impact of this dilation of time motion of the balls would occur observer Pat on the platform, the has far-reaching effects, which because the balls had the same train’s clock is moving very slowly. Einstein gradually pieced together mass and were thrown with the into a single general theory same force. But in this experiment The passenger on the speeding of relativity in 1915. One early the balls are in different reference train will not notice any slowing breakthrough was the discovery frames—one stationary, the other of time. The mechanisms by moving at close to the speed of light. Pat would see Bob’s ball moving much more slowly than his own due to the time dilation, yet when they collide, both balls are knocked back to their owners. The only way this could work is if Bob’s slow tennis ball is heavier, or contains more mass, than Pat’s tennis ball. Albert Einstein Einstein was born in Germany He failed to do so, and no one but spent his formative years in else has succeeded yet either. Switzerland. He was an average A leading pacifist voice for many student, and then struggled to years, in 1939 Einstein was find teaching work, ending up instrumental in alerting Allies at the patent office in Bern. After to the dangers that Germany the success of his 1905 papers, might build a nuclear weapon. Einstein took university posts in He declined to be involved in Bern, Zurich, and then in Berlin, the Manhattan Project that built where he presented his general the first atomic bombs. An avid theory in 1915. With the rise of violinist, Einstein stated that Nazism in 1933, Einstein moved he often thought in music. to the United States, where he settled at Princeton University. Key work There he spent the rest of his days trying to link relativity 1915 Relativity: the Special and with quantum mechanics. the General Theory

ATOMS, STARS, AND GALAXIES 151 Each ray of light moves in the To generalize his theory, Einstein The theory of relativity coordinate system “at rest” linked gravity to his ideas about cannot but be regarded as with a definite, constant energy and motion. Taking an a magnificent work of art. velocity independent of object in space and removing all whether this ray of light is reference points, it is not possible Ernest Rutherford emitted by a body at rest to tell if it is moving. There is no or a body in motion. test that can be done to prove that New Zealand physicist Albert Einstein it is. Therefore, from the point of view of any object, or reference Therefore, according to special frame, it stays still while the rest relativity, when matter moves, it of the universe moves around it. becomes more massive. These mass increases can be measured Einstein’s happiest thought Einstein had described motion in on the everyday, human scale, but This is easiest to picture if terms of the links between mass, are negligible. However, they have everything is moving at a constant energy, and time. For a general a marked effect when objects are speed. According to Newton’s first theory, he needed to add space. moving very quickly. For example, law of motion, an object maintains It was not possible to understand the protons accelerated by the its motion unless a force acts to the path of an object through Large Hadron Collider (LHC) accelerate it (change its speed or space without considering its path particle accelerator travel very direction). When Einstein included through time. The result was that close to the speed of light—within the effects of acceleration in his mass moves through spacetime, 99.999 percent. Additional energy theory, it led to an insight that he which has a four-dimensional does very little to this speed, and called his “happiest thought”: it geometry, as opposed to the instead boosts mass. At full power, was not possible to differentiate usual three dimensions (up, down, the protons in the LHC are nearly why an object accelerated—it could and side to side) of the everyday 7,500 times more massive than be because of gravity, or it could be concept of space. When an object ❯❯ they were when stationary. another force. The effect of both was the same and could be described Speed limit by the way the rest of the universe With the relationship between moved around the reference frame. speed and mass, relativity highlights another basic principle: the speed Gravity Acceleration From inside an of light is the upper limit of motion elevator, a person through space. It is impossible for an Mass underneath Lift cannot tell whether object with mass—a nuclear particle, pulls down Force they are being spaceship, planet, or star—to travel accelerated upward at the speed of light. As it approaches pushes up by a force pushing light speed, its mass becomes almost the elevator from below infinite, time slows nearly to a stop, or pulled downward and it would take an infinite amount by the gravity of a of energy to push it to light speed. mass underneath the elevator. Either way, they feel a sense of weight as the floor pushes against them, and objects dropped from a height accelerate down to the floor. This is Einstein’s equivalence principle, which he described as his “happiest thought.”

152 THE THEORY OF RELATIVITY moves through spacetime, the Lift time dimension dilates, and the space dimensions contract. Zero motion Constant velocity Acceleration From the point of view of Pat back at the station, the speeding train’s A light beam shines into an elevator from an observer with a flashlight length is compressed, making it standing outside. The paths of the light beam are shown as they will be look very squashed and stubby. observed from inside the elevator. If the elevator is accelerating, the beam However, it is all normal to Bob; will curve downward. Light is similarly curved toward a source of gravity. anything he measures on board will have the same length as well. Depending on its trajectory, Proof of relativity when the train was stationary. speed, and mass, the meteor might Einstein’s physics were initially This is because his means of collide with the planet or roll back met with bafflement from most of measurement, such as a ruler, up the other side of the well and the scientific community. However, has contracted along with space. escape. If the trajectory is just right, in 1919, the English astronomer the meteor will circle around the Arthur Eddington demonstrated Warping spacetime planet in an orbit. that this new way of describing In Einstein’s universe, gravity the universe was indeed accurate. is recast not as a force but rather The warps created by matter He traveled to the Atlantic island the effect of warps in the geometry also bend time. Two distant of Principe to observe a full solar of spacetime caused by the objects—for this explanation, eclipse and specifically to look presence of mass. A large mass, a red star and a blue star—are not at the background of stars near such as a planet, bends space, moving in relation to one another. to the sun. Light from stars travels and so a smaller object, such as They are in different points of space, to Earth along the most direct a meteor, moving in a straight line but at the same point in time, the route, known as the geodesic. In through space nearby, will curve same “now.” However, if the red Euclidean geometry (the geometry toward the planet. The meteor star moves directly away from the of Newtonian physics), that is a has not changed course—it is blue, its passage through time straight line, but in the geometry still moving along the same line slows compared to the blue star’s. in space; it is just that the planet That means the red star shares Everything must be made has bent that line into a curve. a “now” with the blue star in the as simple as possible. past. If the red star travels directly But not simpler. Warps in spacetime can be toward the blue one, its “now” Albert Einstein visualized as balls deforming a is angled toward the blue star’s rubber sheet, making depressions future. Consequently, events that or “gravity wells.” A large “planet” are observed simultaneously from ball makes a well, and a smaller one reference frame may appear to “meteor” ball will roll into the occur at different times in another. Relativity solved the puzzle of perturbations in the orbit of the planet Mercury (pictured) that could not be explained by Newtonian physics, which had first been noticed in 1859.

ATOMS, STARS, AND GALAXIES 153 of spacetime, a geodesic can be Time is an illusion. The twin paradox curved. So, starlight shining very Albert Einstein close to the edge of the sun passes A result known as the “twin into the warp created by the star’s technology, while the wavelike paradox” is illustrated using mass and follows a bending path. contractions of space predicted a pair of newborn twins. One Eddington photographed the stars by relativity have recently been stays on Earth, while another revealed by the absences of the discovered in the LIGO experiment. is taken on a rocket on a solar glare. These images showed Other ideas from relativity are journey to a star 4 light-years that the apparent position of the also being used in the search for away. The rocket travels at stars had indeed been shifted due possible answers to the mystery an average velocity of 0.8c, to the warping of space, an effect of dark energy. ■ meaning that it returns from now known as gravitational lensing. its 8-light-year journey on the Einstein was proved right. 10th birthday of the twin who stayed on Earth. However, Einstein’s general theory of to the clock on the rocket, it relativity allows astronomers to is only the other twin’s 6th make sense of what they observe, birthday. The clock has been everywhere from the very edge of in a moving time frame, so the visible universe to the event has been ticking more slowly. horizon of a black hole. Today, the time dilations of relativity Relativity insists that the are taken into account in GPS twin on the rocket is also entitled to consider herself at Real Observed rest, which seems to lead to a paradox—from her point of Mass creates a gravity well that causes an effect called gravitational view, the twin on Earth had lensing, first observed in 1919 by Arthur Eddington. The observed position been the one moving. The of a star is changed by the effect of the sun’s gravity, which causes light paradox is resolved by the fact from the star to travel past the sun along a curved path. that only the twin in the rocket has undergone acceleration, with its consequent time dilation, both on the way out and to change direction and come back. The twin on Earth has remained in one frame of reference, while the twin on the rocket has been in two— one on the way out and another on the way back. Thus, the twins’ situations are not symmetrical, and the twin who stayed at home really is now four years older than her sister. The twin paradox has been a popular theme in science fiction. In the film The Planet of the Apes, astronauts return to Earth to find that thousands of years have elapsed, and the planet is now ruled by apes. In the film Interstellar, physics consultants were employed to ensure that the time elapsed for each character was correct according to relativity.

154 BRALENLAEACXTKAIVCHITOTLYSEOPSLRUETDIIOCNTSTO CURVES IN SPACETIME IN CONTEXT The gravitational field This warping can be of a mass is a warping described mathematically KEY ASTRONOMER using the Schwarzschild Karl Schwarzschild of spacetime. (1873–1916) solution. BEFORE The Schwarzschild solution 1799 Pierre-Simon Laplace is an exact solution to relativity develops a theory about black holes, which he calls “corps that predicts black holes. obscures,” or “dark bodies.” Black holes are surrounded 1915 Albert Einstein’s by an event horizon, a boundary beyond general theory of relativity shows that the force of which nothing can be observed. gravity is caused by a warping of space and time. I n 1916, German mathematician how spacetime curved in the Karl Schwarzschild managed presence of mass. This solution AFTER something that even Albert showed how the gravity of objects 1931 Subrahmanyan Einstein had failed to do—he like the sun and Earth was warping Chandrasekhar calculates provided a solution to the field spacetime in accordance with the the mass of stellar cores equations of general relativity theories of relativity. A generation that become neutron stars that could yield precise answers. later, Schwarzschild’s mathematics and black holes. The Einstein field equations are a were used to throw light on the complex set of formulae that link darkest of all objects, the black hole. 1979 Stephen Hawking space and time (or spacetime) with proposes that black holes do the action of gravity. Schwarzschild’s No escape actually emit radiation as a achievement, known as the In the early days of relativity, result of quantum fluctuations. Schwarzschild solution, was to black holes were purely theoretical solve the equations to show exactly objects, although they had been 1998 Andrea Ghez shows that a supermassive black hole sits at the center of the Milky Way.

ATOMS, STARS, AND GALAXIES 155 See also: Gravitational disturbances 92–93 ■ The theory of relativity 146–53 ■ The life cycles of stars 178 ■ Hawking radiation 255 ■ The heart of the Milky Way 297 predicted a century before. The radius is doomed to be pulled into Karl Schwarzschild French astronomer Pierre-Simon the black hole. The points in space Laplace had theorized corps that surround a black hole at a Karl Schwarzschild’s obscures, objects so dense that the distance of the Schwarzschild prodigious mathematical velocity required to escape their radius form its “event horizon,” abilities were obvious from gravity exceeded the speed of light. so-called because it is impossible to an early age. By the age of The modern definition of black observe the events that take place 16, he had published his first holes is similar: objects in space beyond it. Nothing comes out of a scientific paper concerning with such enormous gravity nothing black hole—no mass, no light, and the mechanics of binary orbits, can escape them, not even light. no information about what is inside. and by 28, he was a professor at the University of Göttingen Event horizon The Schwarzschild solution in Lower Saxony. The Schwarzschild solution can allows astronomers to estimate be used to calculate the size of the masses of actual black holes, Schwarzschild made a black hole for a given mass. To although it is not possible to be contributions to the most create a black hole, mass must be exact because black holes rotate and significant sciences of the age: compressed into a volume with a carry an electric charge, and these radioactivity, atomic theory, smaller radius than that predicted factors are not accounted for by the and spectroscopy. In 1914, by the Schwarzschild solution. An mathematics. If the sun became a he joined up to fight in World object so dense that its radius is black hole, its event horizon would War I, but still found time for smaller than the Schwarzschild be 2 miles (3 km) from the center. mathematics. In late 1915, radius for its mass will warp A black hole with Earth’s mass he sent Albert Einstein some spacetime to such an extent would have a 1⁄3-in (9-mm) radius. early calculations, saying: that its gravitational pull will be However, it is not possible to make “As you see, the war treated impossible to resist—it will create black holes from bodies this small; me kindly enough, in spite a black hole. Any mass or light that it is thought that black holes form of the heavy gunfire, to allow comes closer than the Schwarzschild from collapsed stars that are at me to get away from it all and least three solar masses. ■ take this walk in the land of your ideas.” The following Warping spacetime Schwarzschild year, Schwarzschild presented radius the full solution that bears Event horizon Black hole his name. He developed an It is theorized that beyond the Singularity autoimmune disease while event horizon, at the center of the serving on the Russian Front black hole, lies a singularity—a and died in May 1916. point of infinite gravity and infinite density. However, it is impossible to Key work obtain information from beyond an event horizon. In this diagram, one 1916 On the Gravitational of the three dimensions of space has Field of a Mass Point after been removed to aid visualization. Einstein’s Theory

NTHEEBSUPLIRAAEL SARYESSTTEELMLASR SPIRAL GALAXIES



158 SPIRAL GALAXIES I n the 1780s and 1790s, British It seems to me that with this astronomer William Herschel discovery, the great question, IN CONTEXT cataloged large numbers of nebulae and speculated that some if the spirals belong to the KEY ASTRONOMER of these might be comparable in system of the Milky Way or Vesto Slipher (1875–1969) size and nature to the Milky Way. not, is answered with great In his conjectures, Herschel was BEFORE following an earlier suggestion by certainty: they do not. 1842 Austrian physicist German philosopher Immanuel Ejnar Hertzsprung Christian Doppler proposes Kant that nebulae might be large the Doppler effect—a change disks of stars—“island universes” in a letter to Vesto Slipher in the perceived frequency of independent of the Milky Way and waves coming from an object separated from it by vast distances. selected the site because its high moving relative to the viewer. In the 19th century, using improved altitude, at over 6,900 ft (2,100 m), telescopes, British astronomer with few clouds, and its distance 1868 William Huggins Lord Rosse discovered that some from city lights, meant it guaranteed determines the velocity of a nebulae had “arms” arranged in good visibility almost every night. star moving away from Earth a spiral, while his compatriot Lowell’s venture marked the first using the Doppler effect. William Huggins found that many time an observatory had been built nebulae consisted of a mass of stars. deliberately in a remote, high place AFTER However, aside from the fact that for optimal observations. 1929 Edwin Hubble finds a they might contain stars, nebulae link between the recessional were still little understood by the Lowell initially hired him for velocities of spiral galaxies turn of the 20th century, when a a short-term position, but Slipher and their distances. young scientist from Indiana named would remain for his entire career. Vesto Slipher began to study them. Lowell and Slipher worked well 1998 Saul Perlmutter and his together, with the unassuming new colleagues discover that the The Lowell Observatory recruit content to leave the limelight expansion of the universe has From 1901, Slipher worked at the to his flamboyant employer. Slipher been accelerating for the last Lowell Observatory in Flagstaff, was a talented mathematician and 5 billion years. Arizona. The observatory had been had practical mechanical skills, founded by American astronomer which he put to use installing new Percival Lowell in 1894. Lowell had spectrographic equipment. He set to work developing improved techniques in spectrography—the separation of light coming from celestial objects into its constituent wavelengths, and the measurement and analysis of those wavelengths (p.113). Slipher used the 24-in (61-cm) Alvan Clark telescope at the Lowell Observatory to observe the spiral nebulae. Today, people can use the original telescope at the observatory’s visitor center.

ATOMS, STARS, AND GALAXIES 159 See also: The Milky Way 88–89 ■ Examining nebulae 104–05 ■ Properties of nebulae 114–15 ■ Measuring the universe 130–37 ■ The shape of the Milky Way 164–65 ■ The birth of the universe 168–71 ■ Beyond the Milky Way 172–77 Measurements of the If the spiral nebulae lie Spiral nebulae may blueshifts and redshifts within the Milky Way be separate of spiral-shaped nebulae Galaxy, they are moving so show that some are moving fast relative to the rest of the galaxies outside toward Earth while others galaxy that they cannot the Milky Way. remain within it for long. are receding. Studying nebulae were coalescing into new solar Making small adjustments to its Slipher’s initial work and research systems. He asked Slipher to mechanism, Slipher managed were directed at the planets, but record the spectra of the light from to increase the sensitivity of from 1912, at Lowell’s request, he the outer edges of the nebulae, Lowell’s spectrograph, a complex began to study the mysterious to determine if their chemical 450-lb (200-kg) instrument spiral nebulae. Lowell had a theory makeup resembled that of the attached to the eyepiece of that they were spirals of gas that solar system’s gas giant planets. the Observatory’s 24-inch (61-cm) refractor telescope. During the The spectra of galaxies moving toward Earth exhibit “blueshifts” and those fall and winter of 1912, he receding from Earth exhibit “redshifts” because the light waves are squashed obtained a series of spectrograms or stretched when viewed from Earth. These are called Doppler shifts after the from the largest of the spiral Austrian physicist Christian Doppler, who first explained such phenomena. nebulae, which was located in the constellation of Andromeda 400 500 600 700 and known at the time as the WAVELENGTH (nm) Andromeda nebula. This galaxy is not moving relative to Earth. Emission lines in the spectra of stationary The pattern of spectral lines Light waves coming from it are detected on galaxies are consistent with the wavelengths in the nebula’s spectrum (like Earth at their normal, unaffected frequency. of the component gases in the galaxy. a fingerprint of its composition) indicated a “blueshift”—they 400 500 600 700 were unexpectedly displaced WAVELENGTH (nm) toward the short-wavelength/ high-frequency blue end of the This galaxy is moving toward Earth. Light Emission lines in the spectra of approaching spectrum by what is known as waves coming from it are detected on Earth galaxies are shifted toward the shorter, blue a Doppler shift (see diagram, left). as slightly shortened or of a higher frequency. wavelengths: this is a “blueshift.” That could only mean that the light waves coming from the 400 500 600 700 Andromeda nebula were being WAVELENGTH (nm) shortened, or compressed, and their frequency raised, because This galaxy is receding from Earth. Light Emission lines in the spectra of receding the nebula was rushing toward waves coming from it are detected on Earth galaxies are shifted toward the longer, Earth at a considerable speed. as slightly lengthened or of a lower frequency. red wavelengths: this is a “redshift.” Slipher’s calculations revealed that the nebula was approaching at 200 miles per second (300 km per second). Doppler shifts had been measured for astronomical bodies before, but shifts of this size were unprecedented. Slipher ❯❯

160 SPIRAL GALAXIES The galaxy NGC 4565, which Slipher established to be receding at 700 miles/s (1,100 km/s), is also known as the Needle galaxy because of its thin shape when viewed from Earth. asserted that “we have at the most were showing redshifted significantly redshifted. In 1914, present no other interpretation spectra, where the wavelengths Slipher presented his results to the for it. We may conclude that the had stretched, meaning they were American Astronomical Society Andromeda nebula is approaching moving away from Earth. The and received a standing ovation. the solar system.” nebula known as M104 (also called NGC 4594), for example, was flying By the time Slipher had Discovering Doppler shifts away at an astonishing speed of presented his next paper on Over the next few years, Slipher nearly 600 miles per second (1,000 spiral nebulae in 1917, the ratio of studied 14 more spiral nebulae and km per second). Another called redshifted to blueshifted nebulae found that nearly all were traveling M77, or NGC 1068, was receding had risen to 21:4. In this paper, at incredible speeds relative to at 680 miles per second (1,100 km Slipher noted that the average Earth. Most remarkably, whereas per second). Altogether, out of velocity at which they were some were moving toward Earth, the 15 galaxies observed, 11 were approaching or receding— scientifically known as “radial velocity”—was 450 miles per second (700 km per second). This was much faster than any star had ever been measured moving relative to Earth. Slipher found it almost inconceivable that the spiral nebulae could be passing through the Milky Way at such speeds, and he began to suspect that they were not moving through the Milky Way at all, stating: “It has for a long time been suggested that the spiral nebulae are stellar systems seen at great distances … This theory, Vesto Slipher Vesto Slipher was born on a farm of spiral galaxies, measuring in Mulberry, Indiana, in 1875. spin rates of hundreds of Soon after graduation, he started miles per second. He also working at the Lowell Observatory demonstrated that gas in Arizona, where he would remain and dust exist in interstellar for more than half a century. Most space. Slipher was director of of Slipher’s major discoveries the Lowell Observatory from occurred in the earlier part of his 1926 to 1952. During this time, career. He began by investigating he supervised a search for the rotational periods of planets, trans-Neptunian planets, finding evidence, for example, which ledin 1930 to Clyde that Venus’s rotation is very slow. Tombaugh’s discovery of Pluto. Between 1912 and 1914, he made his most significant discovery— Key work that some spiral nebulae are moving at high speed. In 1914, 1915 Spectrographic Slipher discovered the rotation Observations of Nebulae

ATOMS, STARS, AND GALAXIES 161 it seems to me, gains favor in the In the great majority of cases 39 spiral nebulae, the majority present observations.” Slipher was the nebula is receding; the of which showed high velocities echoing Kant’s suggestion that largest velocities are positive. of recession—as much as 775 miles some nebulae, in particular the The striking preponderance per second (1,125 km per second). spiral ones, could be separate of [these positive velocities] Hubble used Slipher’s measurements galaxies from the Milky Way. indicates a general fleeing of redshifts in galaxies that he had from us or the Milky Way. proved were outside the Milky In 1920, partly prompted by Way to find a relationship between Slipher’s findings, a formal debate Vesto Slipher galaxy redshifts and distances. took place in Washington, D.C., to discuss whether the spiral nebulae like it were far too distant to be By the late 1920s, Hubble were separate galaxies outside part of the Milky Way and so must had used this result to confirm the Milky Way. Now referred to be galaxies outside it. Slipher’s that the universe is expanding. as the “Great Debate,” two eminent suspicions dating back to 1917 Thus, Slipher’s work in the years American astronomers advocated had been proved right. By the 1912–25 played a crucial role in opposing positions—Harlow time of Hubble’s paper, Slipher had what today is often considered the Shapley that the spiral nebulae measured the radial velocities of greatest astronomical discovery were part of the Milky Way; and of the 20th century, paving the Heber D. Curtis that they were far way for further investigations beyond it. Neither astronomer into the motions of galaxies and changed his position as a result cosmological theories based on of the debate, but many perceptive an expanding universe. As for the figures were concluding by this Andromeda galaxy, it is expected time that the spirals had to be to collide with the Milky Way in outside the Milky Way. about 4 billion years, and together the two are likely to form a new Slipher’s legacy elliptical galaxy. ■ Despite an enthusiastic response from many in the astronomical community, some still questioned Slipher’s findings. For more than a decade, until others began to believe Slipher’s ideas and understand the implications arising from them, he was virtually the only person investigating the Doppler shifts of spiral nebulae. In 1924, a new paper by American astronomer Edwin Hubble put a decisive end to the debate about the nature of spiral nebulae. Hubble had observed a class of stars called Cepheid variables in some nebulae, including the Andromeda nebula. As a result of his observations, Hubble was able to announce that the Andromeda “nebula” and others Some 4 billion years into the future, the night sky will look like this, as the Andromeda galaxy collides with the Milky Way.

162 BSAYNTADHRYHSDEARLRIOUEGMEDNOMINATED STELLAR COMPOSITION IN CONTEXT I n 1923, the consensus among established view would be astronomers was that the sun overturned with the arrival that KEY ASTRONOMER and other stars had a similar year of British graduate student Cecilia Payne-Gaposchkin chemical composition to Earth. Cecilia Payne at the Harvard (1900–1979) This belief was based on the College Observatory (HCO) analysis of dark lines (Fraunhofer in Massachusetts. BEFORE lines) in star spectra, which are 1850s Gustav Kirchhoff caused by the absorption of light Star spectra shows that dark lines in the by chemical elements in star Payne set to work analyzing the sun’s spectrum are due to atmospheres. The spectra contain HCO’s photographic collection of light absorption by elements. strong lines for elements that are star spectra. She wanted to clarify common on Earth, such as oxygen the relationship between star 1901 Annie Jump Cannon and hydrogen, and metals such spectra and temperatures. Also, classifies stars by the strength as magnesium, sodium, and iron, because the pattern of absorption of the dark lines in their spectra. and therefore it was assumed that lines seemed to vary between the Earth and stars were made of the spectra of different classes of star, 1920 Indian physicist Meghnad same chemical elements, in more she wanted to see what differences Saha demonstrates how or less the same proportions. This in chemical composition might temperature, pressure, and exist between these classes. ionization are linked in a star. The reward of the old scientist is the sense of having seen Since 1901, astronomers at AFTER a vague sketch grow into the HCO had classified stars 1928–29 Albrecht Unsöld and a masterly landscape. into a sequence of seven main William McCrea independently spectral types, and believed that find that hydrogen is a million Cecilia Payne-Gaposchkin the sequence was related to the times more abundant in the stars’ surface temperatures. In her sun’s atmosphere than any doctoral thesis, however, Payne other element. applied an equation formulated by Indian physicist Meghnad Saha in 1933 Danish astrophysicist 1920. The equation related a star’s Bengt Strömgren shows that spectrum to the ionization (electrical stars are mainly hydrogen charge separation) of chemical all the way through, not just elements in its atmosphere and the in their atmospheres. ionization of its surface temperature. Payne demonstrated a link between

ATOMS, STARS, AND GALAXIES 163 See also: The sun’s spectrum 112 ■ The characteristics of stars 122–27 ■ Nuclear fusion within stars 166−67 ■ Energy generation 182–83 the spectral classes of stars and If you are sure of Cecilia Payne- their surface temperatures. She your facts, you should Gaposchkin also showed that the variation defend your position. in absorption lines between Cecilia Payne-Gaposchkin Cecilia Payne was born star spectra was due to varying in Wendover, England, in amounts of ionization at different statement saying that the levels of 1900. At age 19, she won temperatures, and not to varying hydrogen and helium she had found a scholarship to Newnham abundances of chemical elements. were “almost certainly not real.” College, Cambridge, where Four years later, however, Russell she studied botany, physics, Payne knew that the intensity conceded that Payne was right. and chemistry. After attending of absorption lines in star spectra a lecture by Arthur Eddington, could give only crude estimates of Payne’s discoveries were she switched to astronomy. chemical elements, so other factors revolutionary. First, she established In 1923, she left for the US to needed to be taken into account, that most stars are chemically join a new graduate course in such as the ionization states of the similar. Second, she demonstrated astronomy at Harvard College atoms of different elements. Using how to determine the temperature Observatory. Within two her knowledge of atomic physics, of any star from its spectrum. Third, years, she had produced her she determined the abundances she showed that hydrogen and revolutionary doctoral thesis, of 18 elements found in the spectra helium are dominant elements in Stellar Atmospheres. Much of many different stars. She found the universe—a key step toward of her research focused on that helium and hydrogen were the Big Bang theory. ■ variable stars and novae vastly more abundant than on (exploding white dwarfs). Earth, making up nearly all the This work helped explain the matter in stars. Milky Way’s structure and the paths of stellar evolution. Astronomers’ reaction In 1931, she became a US In 1925, Payne’s thesis was sent citizen and in 1934 married to the astronomer Henry Russell for Russian astronomer Sergey review. Russell declared that Payne’s Gaposchkin. In 1956, she was results were “clearly impossible,” made professor of astronomy and pressured her to include a at Harvard University, the first female professor at Hydrogen and helium Hydrogen: Harvard. She died in 1979. together make up 98 74 percent percent of all of the Key works matter throughout the galaxy that is made 1925 Stellar Atmospheres of atoms (this omits 1938 Variable Stars dark matter, the 1957 Galactic Novae nature of which is not understood). Other 1 percent: Helium: carbon 0.45; neon 0.13; 24 percent iron 0.11; nitrogen 0.096; Oxygen: 1 percent silicon 0.065; magnesium 0.058; sulfur 0.044

164 IOSURROGTAALTAINXGY THE SHAPE OF THE MILKY WAY IN CONTEXT I n the 1920s, there were two The solar system is orbiting the opposing views of the universe. center of the Milky Way at a speed KEY ASTRONOMER Some astronomers thought of 140 miles/s (230 km/s). Stars closer Bertil Lindblad (1895–1965) the Milky Way was itself the entire to the center orbit at a faster speed. universe. Others argued that the BEFORE spiral nebulae observed were not astronomers. The first was the 1904 Jacobus C. Kapteyn cloudy masses on the edge of the American Harlow Shapley, who shows how stars can be Milky Way, but galaxies in their believed that the Milky Way made divided into two streams own right a vast distance away. up the whole universe. Shapley moving in opposite directions. suggested that the edge of the In 1926, a Swedish astronomer galaxy could be plotted using the 1917 Vesto Slipher shows that named Bertil Lindblad considered many globular clusters of stars that spiral nebulae are moving the likely shape of the Milky Way, had been observed, and that the faster than any star. concluding that it took the form center lay in Sagittarius. The second of a rotating spiral. Lindblad was was the Dutchman Jacobus C. 1920 Harlow Shapley predicts building on the work of two other that the center of the galaxy is in Sagittarius, estimating it at 50,000 light-years away (now known to be 26,100 light-years). AFTER 1927 Jan Oort confirms that the galaxy is rotating and proposes that a large mass of stars forms a bulge at its core. 1929 Edwin Hubble shows that other galaxies lie far beyond the Milky Way. 1979 Vera Rubin uses galactic rotation to show that galaxies contain invisible dark matter.

ATOMS, STARS, AND GALAXIES 165 See also: Spiral galaxies 156–61 ■ Beyond the Milky Way 172–77 ■ The Oort cloud 206 ■ Dark matter 268–71 Kapteyn, who had described Stars in the same subsystem appear to move a phenomenon he called star in the same direction and at the same speed. streaming. Stars did not move in random directions, Kapteyn said, If stars in other subsystems move in the but appeared to move in groups, opposite direction, it is because they are lagging behind, going either one way or in the opposite direction. Lindblad but they are all moving in the same direction. himself was a leading expert in The galaxy is shaped like a spiraling disk with measuring the absolute magnitude outer regions moving more slowly than inner ones. of stars from their spectra, and was able to calculate their distances from Earth. He combined this data with his observations of the motion of globular clusters and made an interesting discovery. Spinning in subsystems they were lagging behind. As Lindblad had offered no evidence of Lindblad saw that stars move in Shapley had predicted, Lindblad bodies lying outside the Milky Way, subsystems, and each subsystem placed the galactic center in his disk-shaped galactic model with moves at a different speed. From Sagittarius. He supposed that a bulging core gave credence to the this, he deduced that Kapteyn’s star subsystems farther from the galactic idea that similar-looking objects streaming was in fact evidence of center orbited more slowly than the were also galaxies. However, Oort’s the galaxy rotating, which meant ones closer in. This was confirmed observations would also reveal a that the Milky Way’s stars were in 1927 by the observations of Jan new puzzle. The galaxy appeared all moving in the same direction Oort, one of Kapteyn’s students. to be rotating faster than could be around a central point. Stars that accounted for by the mass of its streamed ahead of the solar system The Milky Way was revealed to visible matter. Here was the first were nearer to the center, and stars be a swirling disk that spun, albeit hint of a mystery that endures that were farther from it appeared to very slowly, taking 225 million years today: dark matter. ■ stream in the opposite way because to complete one orbit. Although Bertil Lindblad Bertil Lindblad grew up in Örebro, that position until his death, Sweden. He did his undergraduate overseeing many improvements. degree at Uppsala University, In later years, he was a leading north of Stockholm, and became organizer for the European an assistant at the observatory Southern Observatory, which there. It was while working at has been located in the high Uppsala that Lindblad made his desert of Chile since 1962, and observations of the motion of president of the International globular clusters that led to his Astronomical Union. theory of galactic rotation, which was published in 1926. The Key works following year, still barely into his 30s, Lindblad was offered 1925 Star-Streaming and the the directorship of the Stockholm Structure of the Stellar System Observatory and became the chief 1930 The Velocity Ellipsoid, astronomer for the Royal Swedish Galactic Rotation, and the Academy of Sciences. He held Dimensions of the Stellar System

166 OOAFFSAMLNOANWTITHPEIRRLOATCIEOSNS NUCLEAR FUSION WITHIN STARS IN CONTEXT The sun is composed I n the 1920s, British astronomer mostly of hydrogen gas. Arthur Eddington was the first KEY ASTRONOMER person to explain the processes Arthur Eddington At its center, the sun at work inside stars. He championed (1882–1944) is hot and dense. the idea that their source of energy is nuclear fusion. BEFORE Conditions are right for 1890s Briton Lord Kelvin nuclear fusion, slowly A stable sun and German Hermann von When looking at the sun from Earth, Helmholz suggest that the sun turning mass into what can actually be seen is the gets its energy by shrinking. energy according to gaseous surface layer in the top the equation E = mc 2. 300 miles (500 km) or so, which 1896 Physicist Henri Becquerel has a temperature of about 9,900°F discovers radioactivity. Stars are fueled (5,500°C). The sun appears to be by a slow process in equilibrium, meaning that, over 1906 Karl Schwarzschild the centuries in which astronomers shows that energy can travel of annihilation have observed it (a tiny fraction of through a star by radiation. of matter. time in the lifespan of the sun), it has always appeared to be the same AFTER size and show the same luminosity. 1931 Robert Atkinson sets out Eddington realized that the the process by which protons gravitational force pulling inward can combine to release energy would be balanced not only by the and build new elements. gas’s tendency to expand outward but also by the pressure produced by 1938 German physicist Carl the radiation pouring out of the star. von Weizsäcker discovers that protons can combine into Eddington was able to show helium in stars by the carbon- convincingly that all stars are giant nitrogen-oxygen (CNO) cycle. balls of hot gas. He calculated how luminous stars of different masses 1939 Hans Bethe details how would appear if the gas in their the proton-proton chain and centers, where the temperature and CNO cycle processes work. density are very high, followed the same physical laws as cooler, less

ATOMS, STARS, AND GALAXIES 167 See also: The theory of relativity 146–53 ■ Stellar composition 162–63 ■ Energy generation 182–83 ■ Nucleosynthesis 198–99 dense gas. The answers he obtained Since all gases obey the same laws, went into Einstein’s equation. In were a good match for observations assuming that the sun is not only 1931, Welsh astrophysicist Robert of both giant and dwarf stars. gaseous at the surface but throughout Atkinson showed that a process in allows the calculation of the temperature which four hydrogen atoms were Gas laws and relativity and pressure at the center. fused into one slightly less massive The physical laws governing the helium atom fit the data from the relationships between the pressure, Eddington used these laws to sun. This process is very slow and volume, and temperature of a gas calculate that the temperature produces energy to fuel the sun for were well understood. Since they at the center of the sun is about billions of years. Here also was all have widely spaced molecules, 29,000,000°F (16,000,000°C), with evidence of the transmutation gases behave in similar ways—for a density 150 times that of water. of the elements, showing how instance, Boyle’s law (formulated by the composition of the universe Irish chemist Robert Boyle) states To understand what was going changes with time. ■ that, at a constant temperature, the on in the sun’s center, Eddington product of the pressure and volume now needed Einstein’s equation It is sound judgment to hope of a given mass of gas is constant. E = mc 2 (pp.149–50). This equation that, in the not too distant states that energy is equal to mass future, we shall be competent multiplied by the square of the to understand so simple velocity of light. It was the key to unlocking the mystery of the source a thing as a star. of the sun’s energy, as it showed Arthur Eddington how mass could be turned into energy. The conditions at the solar center were sufficiently hot and dense to allow nuclear reactions to take place and mass to be destroyed, thus producing the energy that Einstein’s equation predicted. At first, physicists suggested that individual electrons or hydrogen atoms might be the mass that Arthur Eddington Arthur Eddington was born into a about the bending of starlight Quaker family, and was educated by the sun. He was a brilliant in mathematics and physics at astronomer and mathematician, the universities of Manchester and able to communicate the and Cambridge. In 1905, he most difficult physical idea in joined the Royal Observatory, a simple and elegant language. Greenwich, but a few years later This made his books extremely he returned to Trinity College, popular, especially his Cambridge, becoming Plumian explanations of relativity Professor in 1913 and director and quantum mechanics. of the Cambridge University Observatory in 1914. He lived Key works there for the rest of his life. 1923 The Mathematical Theory In 1919, Eddington sailed of Relativity to Príncipe Island, West Africa, 1926 The Internal Constitution to observe a total solar eclipse of the Stars and test Einstein’s prediction

168 IN CONTEXT WYAEDISTATHYEORUDTAY KEY ASTRONOMER Georges Lemaître  THE BIRTH OF THE UNIVERSE (1894–1966) BEFORE 1915 Albert Einstein publishes his general theory of relativity, which includes equations that define various possible universes. 1922 Alexander Friedmann finds solutions to Einstein’s equations, indicating that the universe could be expanding, contracting, or static. AFTER 1929 Edwin Hubble observes that distant galaxies are moving away from Earth at a rate proportional to their distance. 1949 Fred Hoyle coins the term “Big Bang” for Lemaître’s theory. T he idea that the universe originated from a tiny object in the form of an egg appears in The Rigveda, a collection of Hindu hymns from the 12th century bce. However, there were few scientific clues to the universe’s true origins until Albert Einstein provided a new way of conceiving time and space with his general theory of relativity in 1915. Einstein’s insight led many to revisit the idea that the universe started small, among them the Belgian priest Georges Lemaître, whose 1931 proposal would carry echoes of The Rigveda. In the 17th century, Johannes Kepler, observing that the night sky is dark, argued that the universe

ATOMS, STARS, AND GALAXIES 169 See also: Gravitational theory 66–73 ■ The theory of relativity 146–53 ■ Spiral galaxies 156–61 ■ Beyond the Milky Way 172–77 ■ The primeval atom 196–97 Olbers’ paradox is the argument that, if the universe is infinite, not expanding, has always existed, and everywhere contains roughly the same density of stars, then any sight line from Earth must end at the surface of a star. The night sky should be uniformly bright, but this contradicts the observed darkness of the night. What an observer would see Georges Lemaître cannot be infinite in both time and exact relationship between mass, Georges Lemaître was born space, as otherwise the stars shining space, and time was explained in 1894 in Charleroi, Belgium. from every direction would make in a series of 10 complex equations. Following distinguished the whole sky bright. His argument These were called Einstein’s field service in World War I, in 1920 was restated in 1823 by German equations. Einstein found an he was awarded a doctoral astronomer Wilhelm Olbers and initial solution to his equations degree in engineering. He became known as Olbers’ paradox. that suggested the universe is subsequently entered a Despite this problem, Isaac Newton contracting. Since he could not seminary, where, in his leisure stated that the universe was static believe this, he introduced a “fix”— time, he studied mathematics (not getting any bigger or smaller) an expansion-inducing factor called and science. and infinite in time and space, with the cosmological constant—to its matter distributed more or less balance the inward pull of gravity. After his ordination in 1923, uniformly over a large scale. At the This allowed for a static universe. Lemaître studied mathematics end of the 19th century, this was and solar physics at Cambridge still the prevailing view, and one In 1922, Russian mathematician University, studying under that Einstein himself initially held. Alexander Friedmann attempted Arthur Eddington. In 1927, to find solutions to Einstein’s field he was appointed professor of An unchanging universe? equations. Starting with the astrophysics at the University Einstein’s general theory of assumption that the universe of Leuven, Belgium, and relativity explains how gravity is homogenous (made of more or published his first major paper works at the largest scales. He less the same material everywhere) on the expanding universe. In realized that it could be used to and spread out evenly in every 1931, Lemaître put forward his test whether the Newtonian model direction, he found several solutions. theory of the primeval atom in of the universe could exist long- These allowed for models in which a report in the journal Nature, term without becoming unstable, the universe could be expanding, and his fame soon spread. and to explore which other types contracting, or static. Friedmann He died in 1966, shortly after of universe might be feasible. The was probably the first person to learning of the discovery of use the expression “expanding ❯❯ cosmic microwave background radiation, which provided evidence for the Big Bang. Key works 1931 The Beginning of the World from the Point of View of Quantum Theory 1946 The Primeval Atom Hypothesis

170 THE BIRTH OF THE UNIVERSE Galaxies outside the Milky General relativity allows but your grasp of physics is Way are shown to be for a universe that is abominable.” However, the British expanding. astronomer Arthur Eddington later moving away from it published a long commentary on at tremendous speed. Lemaître’s 1927 paper, describing it as a “brilliant solution.” Space is expanding. In 1929, Hubble released Running the clock backward, in the distant past, findings showing that there was galaxies must have been closer in a small, dense region. indeed a relationship between the remoteness of a galaxy and how fast it was receding, confirming for many astronomers that the universe was expanding, and that Lemaître’s paper had been correct. For many years the credit for the discovery of the expansion of the universe was given to Hubble, but today most agree it should be shared with Lemaître and possibly also with Alexander Friedmann. The universe began from an explosion of The primeval atom matter on “a day without yesterday.” Lemaître reasoned that, if the universe is expanding and the universe.” Einstein first called his expanding universe. Putting these clock is run backward, then far work “suspicious,” but six months various threads together, in 1927, back in time, all the matter in the later acknowledged that his results Lemaître published a paper that universe must have been much were correct. However, this was proposed that the whole universe closer. In 1931, he suggested that Friedmann’s final contribution as he is expanding and carrying galaxies the universe was initially a single, died two years later. In 1924, Edwin away from each other and from extremely dense particle containing Hubble showed that many nebulae Earth. He also predicted that all its matter and energy—a were galaxies outside the Milky galaxies that are more distant from “primeval atom” as he called it, Way. The universe had suddenly us would be found to be receding about 30 times the size of the sun. become a lot bigger. at a faster rate than closer ones. This disintegrated in an explosion, The expanding universe Lemaître’s paper was published The radius of space began at Later in the 1920s, Lemaître entered in an obscure Belgian journal, and zero, and the first stages of the debate about the large-scale as a result, his hypothesis failed to the expansion consisted of a organization of the universe. attract much attention at the time. rapid expansion determined He had worked at institutions He did, however, communicate by the mass of the initial atom. in the United States, becoming his findings to Einstein, telling aware of Vesto Slipher’s work on him of the solution he had found Georges Lemaître receding galaxies and Hubble’s to the field equations allowing measurements of galaxy distances. for a universe that expands. A competent mathematician, he had Einstein introduced Lemaître to also studied Einstein’s field equations Friedmann’s work, but remained and found a possible solution to ambivalent about Lemaître’s idea. the equations that allowed for an Famously, Einstein is said to have said: “Your calculations are correct,

ATOMS, STARS, AND GALAXIES 171 giving rise to space and time on model, the universe has a finite age, A parallel exists between the “a day without yesterday.” Lemaître and because the speed of light is Big Bang and the Christian described the beginning of the also finite, that means that only notion of creation from nothing. universe as a burst of fireworks, a finite number of stars can be comparing galaxies to the burning observed within the given volume George Smoot embers spreading out from the of space visible from Earth. The center of the blast. density of stars within this volume term “Big Bang” for the model of is low enough that any line of sight the universe Lemaître and Gamow The proposal initially met from Earth is unlikely to reach a star. had been developing. Lemaître’s with scepticism. Einstein found hypothesis now had a name. it suspect but was not altogether Refining the idea dismissive. In January 1933, Compressed into a tiny point, Lemaître’s idea about the however, Lemaître and Einstein the universe would be extremely original size of the universe is traveled together to California for hot. During the 1940s, Russian- now considered incorrect. Today, a series of seminars. By this time, American physicist George Gamow cosmologists believe it started from Einstein (who had removed the and colleagues worked out details an infinitesimally small point of cosmological constant from his of what might have happened infinite density called a singularity. ■ general theory of relativity because during the exceedingly hot first it was no longer needed) was in full few moments of a Lemaître-style agreement with Lemaître’s theory, universe. The work showed that calling it “the most beautiful and a hot early universe, evolving satisfactory explanation of creation into what is observed today, to which I have ever listened.” was theoretically feasible. In a 1949 radio interview, the British Lemaître’s model also provided astronomer Fred Hoyle coined the a solution to the long-standing problem of Olbers’ paradox. In his Lemaître’s model of a universe expanding from an initial extremely dense concentration of mass and energy is today called the Big Bang model of the universe. Although Lemaître described the initial stages of the process as an “explosion,” the prevailing view today is that expansion is a fundamental quality of space itself and this carries galaxies away from each other, rather than being projected by the initial explosion into a preexisting void. Lemaître’s Galaxies form in primeval atom the early universe Galaxies are carried farther and farther apart as the universe expands TIME

IN ALLETHXEPUANINVEDRISNE GIS DIRECTIONS BEYOND THE MILKY WAY



174 BEYOND THE MILKY WAY IN CONTEXT I n the early 1920s, American Observations indicate that astronomer Edwin Hubble the universe is expanding KEY ASTRONOMER provided proof of the true at an ever-increasing rate. Edwin Hubble (1889–1953) size of the universe. Working at the Mount Wilson Observatory It will expand forever, BEFORE near Pasadena, California, Hubble getting emptier and darker. 1907 Henrietta Swan Leavitt used the newly constructed 100-in shows the link between period (2.5-m) Hooker Telescope, at that Stephen Hawking and luminosity of Cepheid stars. time the largest in the world, to settle the greatest argument rotating, reasoning that this must 1917 Vesto Slipher publishes then raging in astronomy. His make them relatively small because a table of 25 galactic redshifts. observations would lead to the otherwise the outer regions would startling revelation that the be spinning at speeds faster than 1924 Hubble shows that the universe is not only far, far the speed of light (these reports Andromeda galaxy lies well larger than previously thought, were later shown to be wrong). outside the Milky Way. but is also expanding. Opposed to Shapley was Heber D. Curtis, who supported the idea 1927 Georges Lemaître Settling the Great Debate that each nebula was far beyond proposes that the universe At the time, the question of the Milky Way. Curtis cited as may be expanding. whether spiral nebulae were evidence the discovery by Vesto galaxies beyond the Milky Way Slipher that light from most “spiral AFTER or a special kind of nebula was nebula” galaxies was shifted to 1998 The Supernova Cosmology the subject of a “Great Debate.” the red part of the electromagnetic Project and High-Z Supernova In 1920, a meeting was held at spectrum, indicating that they Search prove that cosmos the Smithsonian Museum in were moving away from Earth expansion is accelerating. Washington, D.C., in an attempt to settle the question. Speaking for 2001 Hubble Space Project the “small universe” was Princeton measures the Hubble Constant astronomer Harlow Shapley, who (H0) to within 10 percent. contended that the Milky Way comprised the entire universe. 2015 The Planck Space Shapley cited as evidence reports Observatory puts the age of the that the spiral nebulae were universe at 13.799 billion years. Edwin Hubble Born in 1889 in Missouri, Edwin anyone encroached on his field Hubble was a gifted athlete as of research. It is to Hubble’s a youth, leading the University discredit, therefore, that he did of Chicago’s basketball team. not acknowledge that 41 of the After graduating with a science 46 redshifts used to formulate degree, he studied law at Oxford his famous law were measured University. Returning from not by him but by Vesto Slipher. England dressed in a cape and Hubble spent his last years behaving like an aristocrat, he campaigning for a Nobel Prize was described by Harlow Shapley to be awarded in astronomy. as “absurdly vain and pompous.” He died in 1953. Despite his flair for self- Key work publicity, Hubble was a cautious scientist. He described himself 1929 A relationship between as an observer, and reserved distance and radial velocity judgment until he had sufficient among extra-galactic nebulae evidence. He reacted furiously if

ATOMS, STARS, AND GALAXIES 175 See also: Measuring the universe 130–37 ■ The theory of relativity 146–53 ■ Spiral galaxies 156–61 ■ The birth of the universe 168–71 ■ Space telescopes 188–95 ■ Curtis (Directory) 337 ■ Arp (Directory) 339 at enormous speeds—speeds far Hubble is seen here looking through “true” size. Thanks to Slipher’s too high for them to be contained the lense of the Hooker Telescope measurements, Hubble already within the Milky Way. at Mount Wilson. It was here that he knew that light from most spiral measured galaxy distances and a nebulae was redshifted. In addition, Hubble set out to see whether value for the expansion of the universe. the fainter spirals had higher values there was a relationship between of redshift, showing that they the distances of spiral nebulae and the Milky Way. Just as Curtis had were moving faster through space. their velocity. His strategy was to maintained, the spiral nebulae Hubble realized that, if there were search for Cepheid variable stars were “island universes,” or “extra- indeed a relationship between (p.138)—stars whose luminosity galactic nebulae” as Hubble termed a galaxy’s distance from Earth changes predictably—within them. Over time, the term “spiral and its recession velocity, these nebulae and to measure their nebulae” fell into disuse and they redshifts would serve as a cosmic distances from Earth. This are now simply called galaxies. yardstick, enabling the distances provided Hubble with his first big of the very farthest and faintest discovery in the winter of 1923. In the realm of the nebulae galaxies to be calculated, and Hubble pressed ahead with a ballpark figure to be put on the Beginning with photographic his program of measuring the size of the universe as a whole. plates of the closest and clearest distances to galaxies beyond Meanwhile, Milton Humason, nebulae, Hubble spotted a Cepheid the Milky Way. Farther out, the assistant astronomer at Mount variable on one of the first plates however, it became impossible Wilson, checked Slipher’s redshifts he reviewed. The distances he to pick out individual Cepheid and collected new spectra from calculated for even relatively variables in such faint and fuzzy distant galaxies. It was hard, nearby nebulae were so vast distant galaxies. He was compelled punishing work, and he and Hubble that it effectively killed the Great to fall back on indirect methods, spent many bitterly cold nights Debate immediately: NGC 6822 such as the so-called “standard in the observer’s cage at the top was 700,000 light-years away, while ruler” assumption: reasoning that of the tube telescope on Wilson M33 and M31 were 850,000 light- all galaxies of a similar type are Mountain in California. years away. It was instantly clear the same size allowed him to that the universe extended beyond estimate the distance to a galaxy Hubble’s landmark paper, by measuring its apparent size “A relationship between distance and comparing it to the expected and radial velocity among extra-galactic nebulae,” was ❯❯ Distant galaxies are all moving away The farther away from us. the galaxy, the greater The universe its velocity. is expanding in all directions. There is a linear relation between velocities and distance.

176 BEYOND THE MILKY WAY published in a journal called Equipped with his five had proposed the expansion of the Proceedings of the National senses, man explores the universe from a “primeval atom” Academy of Science in 1929. It universe around him and calls in 1927. However, Hubble’s result contained a straight-line graph the adventure Science. provided a simple link between that plotted 46 galaxies from his redshift-measured velocities near to far against their redshifts. Edwin Hubble and distance, and with it, the Although there was a considerable convincing proof that the scientific scatter, Hubble managed to fit a to assume that Earth occupies community needed. “Hubble’s straight line through the majority. a unique position. Instead, the law,” stating that the redshift of The graph shows that, with the light from distant nebulae showed galaxies is proportional to their exception of the nearest galaxies, that the universe was not static. distance from Earth, was accepted Andromeda and Triangulum, Many astronomers quickly reached almost unanimously. which are encroaching on the Milky the conclusion that this was due Way, all other galaxies are receding. to the expansion of the universe, Einstein’s blunder What is more, the farther away they although Hubble never stated The revelation that the universe are, the faster their movement. this explicitly. might be expanding made news all over the world—not least for the fact Toward an interpretation In reality, Vesto Slipher had that it directly contradicted a theory If, from Earth’s perspective within indicated the trend in 1919, four of Albert Einstein’s. Einstein saw the universe, all galaxies are seen years before Hubble made his that gravity could eventually cause flying away, then the potential observations, and Georges Lemaître the universe to collapse under its explanations are that (a) Earth own weight, so he used a value he lies at the center of the universe; called the cosmological constant— or (b) the universe itself originated a kind of negative pressure—to from a single point and is prevent this from occurring in the expanding as a whole. field equations of general relativity. He abandoned the idea in the wake Objectivity—a kind of of Hubble’s discovery. foundational law in science— requires that there is no reason Einstein and others assumed that the observed velocities were Here, the galaxies of the universe are imagined as dots stuck to Doppler effects caused by the an inflating balloon (for visualization, the dimensions are reduced galaxies’ speed of recession, but from the three of space to the two of the balloon’s surface). As the there were some dissenting voices. balloon doubles in diameter, the distances Swiss astronomer Fritz Zwicky between dots also double. The farther suggested that the redshift might away a dot is from another dot, the be due to “tired light” reaching farther, and so more quickly, it moves. Earth—caused by the interaction C has moved 2 cm from A, but B C of photons with the intervening has moved just 1 cm from A. matter. Hubble himself found it hard to believe that the velocities C 4 cm indicated by the redshifts were actually real, and was happy to use 2 cm them solely as distance indicators. In fact, the velocities of galaxies B A B observed by Hubble are due to A 1 cm the expansion of spacetime itself. 2 cm K-factor Galaxies Spacetime between Hubble showed how fast spacetime galaxies expands is expanding by plotting a straight- line graph—which he grandly

ATOMS, STARS, AND GALAXIES 177 called the “K-factor.” The gradient distance measurements. Many The ESA’s Planck Observatory is described mathematically by a were out by a factor of seven operated between 2009 and 2013. It value now known as the Hubble due to his method of taking the produced data that helped to measure Constant (H0). This important brightest star in any galaxy—or many cosmological parameters, number determines not only the even the luminosity of the galaxy including the Hubble Constant. size of the observable universe itself—and assuming it to be a but also its age. The Hubble Cepheid variable star. Luckily for light curves. The final result, Constant allowed astronomers Hubble, the inaccuracies were delivered in 2001, gave an age for to work backward and calculate fairly consistent throughout the the universe of 13.7 billion years. the moment in time of the Big dataset, allowing him to plot This figure was fine-tuned to 13.799 Bang itself, when the radius of the trend in spite of them. billion years (with an error of 21 the universe was zero. million years either way) by data Hubble Key Project from the Planck Space Observatory The initial calculation of H0 Calculating the rate of expansion in 2015. The most dramatic revision was 300 miles (500 km) per second of the cosmos drove the decision to Hubble’s law, however, came in per megaparsec (one megaparsec to develop the Hubble Space 1998 when astronomers discovered is approximately 3.26 million light- Telescope from its inception that the universe’s expansion is years). This presented a problem, in the 1970s to its 1990 launch. accelerating due to a mysterious since it gave a figure of 2 billion NASA made one of the telescope’s and unknown agent known as years for the age of the universe, “Key Projects” determining the dark energy, which has led to a less than half the accepted age Hubble Constant to within 10 renewal of interest in Einstein’s of Earth. The discrepancy was percent. As a result, the instrument so-called blunder, the cosmological found to have been caused by spent years measuring Cepheid constant (pp.298–303). ■ systematic errors in Hubble’s

178    HMWAAHVSIETSEA DMWAXAIRMFUSM THE LIFE CYCLES OF STARS IN CONTEXT I n 1930, a young Indian student very dense and made of compact named Subrahmanyan “degenerate” matter composed of KEY ASTRONOMER Chandrasekhar calculated atomic nuclei and free electrons. Subrahmanyan that a star that ends its life with White dwarfs were prevented from Chandrasekhar (1910–1995) a little more mass than the sun collapsing by a phenomenon known cannot hold itself up against the as electron degeneracy pressure. BEFORE pull of its own gravity. This was This meant that, when electrons 1914 Walter Adams details key to understanding the life cycles were packed very close together, the spectrum of 40 Eridani B, of stars, and in particular the dim, their movement was limited, an unusually faint white star. very hot stars called white dwarfs. creating outward pressure. This type of star was known to be 1922 Dutch astronomer The Chandrasekhar limit Willem Luyten coins the term The black holes of nature are Chandrasekhar figured out that “white dwarf” for low-mass the most perfect macroscopic electron degeneracy pressure white stellar remnants. can prevent a white dwarf from objects there are in the collapsing only up to an upper limit 1925 Austrian physicist universe: the only elements for the white dwarf’s mass, which is Wolfgang Pauli formulates the in their construction are our about 1.4 times the mass of the sun. Pauli Exclusion Principle, which concepts of space and time. Today, it is known that the core of states that no two electrons a giant star at the end of its life will can occupy the same quantum Subrahmanyan collapse into a white dwarf if its state. This leads to recognition Chandrasekhar mass is below the Chandrasekhar of the phenomenon of electron limit, but will collapse to an even degeneracy pressure. denser object—a neutron star or a black hole—if its mass exceeds AFTER the limit. This insight was largely 1937 Fritz Zwicky ignored by the scientists of the characterizes a type 1a day because neutron stars and supernova as the explosion of black holes were, at this time, a white dwarf that exceeded still purely theoretical. ■ its Chandrasekhar limit. See also: Discovering white dwarfs 141 ■ Nuclear fusion within stars 166–67 ■ 1972 Astronomers find the first Supernovae 180–81 stellar black hole candidate.

ATOMS, STARS, AND GALAXIES 179 UTHNEIVRERADSIEO  RADIO ASTRONOMY IN CONTEXT W hile working for Bell Jansky is pictured here with his Telephone Laboratories in hand-built antenna. He published a KEY ASTRONOMER 1930s America, telephone paper on his work in 1933, but soon Karl Jansky (1905–1950) engineer Karl Jansky was given the after was reassigned by Bell Labs task of plotting the natural sources and did no more astronomical work. BEFORE of static that might interfere with 1887 German physicist long-wave radio voice transmissions. following the sidereal day (relative Heinrich Hertz demonstrates To conduct his investigations, to the stars), not the solar day, and the existence of radio waves. Jansky hand-built a directional radio Jansky realized that the radio waves antenna that was 100 ft (30 m) wide were coming from the constellation 1901 Italian inventor and 20 ft (6 m) high. The contraption Sagittarius, at the heart of the Milky Guglielmo Marconi sends a rotated on four tires salvaged from Way: radio waves were “shining” radio signal across the Atlantic, an old Model T Ford. His colleagues from space just like visible light. unknowingly bouncing the dubbed the device “Jansky’s merry- waves off the ionosphere. go-round” because the young The newspapers reported the engineer would systematically discovery of “extraterrestrial radio,” AFTER rotate the antenna to pinpoint and astronomers soon began to 1937 American amateur sources of atmospheric radio waves. copy Jansky’s device—in effect the astronomer Grote Reber makes first radio telescope. This opened the first radio survey of the sky. Radio astronomy up the possibility of viewing the Jansky matched most sources universe in a new way—not from its 1965 James Peebles proposes of radio waves to approaching light but from its radio emissions. ■ that universal background thunderstorms, but there was microwave radio waves are the a persistent hiss that remained last remnant of the Big Bang. unidentified. The intensity of this static rose and fell once a day, and 1967 Antony Hewish and initially Jansky thought he was Jocelyn Bell Burnell detect a detecting radio from the sun. repeating stellar radio signal, However, the “brightest” spot of the first pulsar. radio waves moved through the sky 1998 Sagittarius A* is shown See also: Searching for the Big Bang 222–27 ■ Quasars and pulsars 236–39 ■ to be a supermassive black hole Reber (Directory) 338 ■ Ryle (Directory) 338−39 at the center of the Milky Way.

180 TAARNNAEENUXSPTILRTOOIOSNNIVSTETOAR SUPERNOVAE IN CONTEXT I n 185 ce, Chinese astronomers intensity for short periods. It was not recorded a phenomenon they until the 1930s that two astronomers KEY ASTRONOMERS called a “guest star.” The star at Caltech in California, Walter Baade Walter Baade (1893–1960) had appeared in the direction of and Fritz Zwicky, calculated that Fritz Zwicky (1898–1974) Alpha Centauri, the closest star some novae released much more system to Earth, and had shone energy than others. For example, they BEFORE brightly for eight months before calculated that S Andromedae, a 1914 American astronomer vanishing. This is probably the nova seen in 1885, had released the Walter Adams first describes first recording of a supernova. equivalent of 10 million years of the white dwarf stars, which are sun’s output all at once. Baade and now known to be involved Mysterious new stars have Zwicky dubbed these incredibly in common novae. appeared several times across energetic events “super-novae.” the centuries. In 1572, Danish 1931 Subrahmanyan astronomer Tycho Brahe named Core collapse Chandrasekhar calculates one a nova, meaning “new.” With In 1934, Baade and Zwicky suggested the greatest mass a white the development of telescopes, that a supernova was the core of a dwarf can have. novae became subject to closer large star, collapsing under its own scrutiny and were found to be gravity after running out of fuel. AFTER faint stars that lit up with a great 1967 Antony Hewish and Jocelyn Bell Burnell discover Faint stars can become Some novae release pulsars, which are found to be much brighter for short vastly more energy fast-spinning neutron stars. periods, forming novae. than others. 1999 A survey of the light from type 1a supernovae The core of the collapsed Some of these shows that the universe’s star is crushed into a supernovae are formed expansion is accelerating neutron star, made due to an unknown quantity of material containing by the collapse of a known as dark energy. only neutrons. star that annihilates its own matter.

ATOMS, STARS, AND GALAXIES 181 See also: The Tychonic model 44–47 ■ Quasars and pulsars 236–39 ■ Dark matter 268–71 ■ Dark energy 298–303 The collapse was so powerful that it A supernova in the Large Magellanic Fritz Zwicky annihilated matter, releasing a huge Cloud blew out this cloud of shrapnel, amount of energy in accordance captured by the Chandra X-ray Space Born in Bulgaria to a Swiss with Einstein’s equation E=mc2 Observatory. The explosion was caused father and Czech mother, (p.149). What was left was a neutron by the collapse of a massive star. Fritz Zwicky emigrated to star—a body composed of only the United States in 1925 to neutrons that were packed together stars. In 1979, a powerful burst of work at Caltech with Robert like the particles in an atomic gamma rays was detected. This Millikan, a leading particle nucleus, only on a much larger has since been attributed to a physicist. In 1931, he began scale. A neutron star is only about “magnetar”—a kind of neutron a collaboration at the Mount 7 miles (11 km) across, but has star with a magnetic field billions Wilson Observatory near Los huge density and gravitational of times greater than Earth’s. Angeles with Walter Baade, pull. Neutrons can be packed more a German astronomer, who closely than atomic nuclei, meaning Many mysteries remain had just arrived from Europe. that a teaspoon of neutron star regarding collapsing stars. Only It was this partnership weighs 10 million tons. The star’s stars above the Chandrasekhar that led to the discovery of escape velocity (the velocity Limit of 1.4 solar masses (p.178) supernovae and neutron stars, required to escape its gravitational will become supernovae and form but Zwicky’s work around this pull) is nearly half the speed of light. neutron stars. A star above 3 solar time was also instrumental masses goes further and becomes in another great discovery. First detection a black hole. There may be a halfway Zwicky calculated that the The concept of a neutron star stage in which neutron matter mass of galaxies, as indicated remained purely hypothetical degenerates even more into quark by their gravitational effects, until 1967, when pulsars were particles—the particles from which was much greater than the discovered. Pulsars were shown neutrons and protons are made. matter that could be measured to be rapidly spinning neutron Quark stars remain hypothetical, through observations. He but the search for them is on. ■ named the missing material dunkle Materie, now better known as dark matter. In addition to his theoretical work, Zwicky worked on the development of jet engines and took out more than 50 patents on his inventions. Key works 1934 On Supernovae (with Walter Baade) 1957 Morphological Astronomy

182 IETSNHENERUSGCOYLUEIRNACRSETFOAUFRSSION ENERGY GENERATION IN CONTEXT U ntil a brilliant young of this discovery was that a small German-born physicist loss of mass could be accompanied KEY ASTRONOMER named Hans Bethe by a very large release of energy. Hans Bethe (1906–2005) figured it out in 1938, no one knew for sure why the sun and In 1919, British chemist Francis BEFORE other stars emitted so much Aston found that an atom of helium 1919 Francis Aston discovers light, heat, and other radiation, (the second lightest element) had a that four hydrogen nuclei or where their energy came from. mass slightly less than that of four (protons) have more mass atoms of hydrogen (the lightest than a helium nucleus. A step toward the correct element). Soon afterward, British answer had been made in 1905 astrophysicist Arthur Eddington 1929 Welsh astronomer Robert with Albert Einstein’s special and French physicist Jean Baptiste Atkinson and Dutch physicist theory of relativity, which proposed Perrin independently proposed that Fritz Houtermans calculate that mass and energy have an stars might obtain their energy by how the fusion of light nuclei equivalence. The significance combining four hydrogen nuclei to within stars could release energy in accordance with Low- to medium-mass mass−energy equivalence. stars are fueled by the proton−proton chain, turning hydrogen into helium. AFTER 1946 Ralph Alpher and High-mass stars The fusion of hydrogen George Gamow describe are fueled by the CNO nuclei to form helium how nuclei of helium and cycle, which turns hydrogen some other nuclei could have into helium in the presence turns mass into energy. been synthesized during of carbon and nitrogen the Big Bang. as catalysts. 1951 Ernst Öpik describes the triple-alpha process, The source of energy which converts the nuclei in stars is nuclear fusion. of helium-4 into carbon-12 in the cores of red giant stars.

ATOMS, STARS, AND GALAXIES 183 See also: The theory of relativity 146–53 ■ Nuclear fusion within stars 166–67 ■ The primeval atom 196–97 Deuterium Helium-3 nucleus nucleus The proton−proton chain Proton Helium-4 Hans Bethe joins protons to eventually Positron nucleus form helium-4 atoms, releasing Gamma ray Hans Bethe was born in 1906 energy as gamma rays. Neutron in Strasbourg, then part of Neutrino the German Empire. From a Fusion very early age, he displayed a high ability in mathematics. form a helium nucleus with some He saw the entire sequence of By 1928, he had completed a loss of mass, which was then reactions, called the proton−proton doctorate in physics. With the converted to energy. Eddington chain, as the main source of energy rise of the Nazi regime, Bethe thought this might allow the sun production in stars up to about the emigrated first to Britain to shine for tens of billions of years. size of the sun. and then to the US. His work In 1929, Robert Atkinson and Fritz during World War II included Houtermans calculated how the The CNO cycle three years at the Los Alamos fusion of light nuclei, rather than Whereas the core temperature Scientific Laboratory, which atoms, could create energy in stars, of a star rises slowly as star size was engaged in assembling but the reactions were unknown. increases, the amount of energy it the first atomic (fission) bomb. produces rises much more rapidly. After the war, Bethe played The proton−proton chain The proton−proton chain could not an important role in the In 1938, Bethe attended a physics explain this, so Bethe investigated development of the hydrogen conference in Washington, D.C., to reactions involving heavier atomic (fusion) bomb. He later discuss how energy is generated nuclei. After hydrogen and helium, campaigned against nuclear in stars. During the conference, the next heaviest element present testing and the arms race. he realized that, given the in appreciable amounts in higher In addition to his work in abundance of hydrogen in stars, mass stars is carbon, so Bethe astrophysics and nuclear the most likely first step in energy looked at possible reactions of physics, Bethe made major generation was one in which two carbon nuclei with protons. He contributions to other fields hydrogen nuclei—which are single found a cycle of reactions, called of physics, including quantum protons—join to form the nucleus the CNO (Carbon–Nitrogen– electrodynamics (QED). of a deuterium (heavy hydrogen) Oxygen) cycle, during which He continued to work in all atom. Bethe knew that this reaction hydrogen nuclei fuse to form these fields until his death generated energy. He then figured helium in the presence of heavier in 2005, at age 98. out how two further reaction steps elements, which seemed to work. could produce a nucleus of helium-4 Bethe’s findings were quickly Key works (the most common form of helium). accepted by other physicists. ■ 1936–37 Nuclear Physics (with Robert Bacher and Stanley Livingston) 1939 Energy Production in Stars

184  BCAEORYMEOSENTEDSRVTEOHXIEIRSPTOLSFANETS THE KUIPER BELT IN CONTEXT I n 1943, Irish astronomer needed to supply the numbers of Kenneth Edgeworth suggested short-period comets seen in the KEY ASTRONOMER that beyond Neptune and Pluto, inner solar system. Photographs Kenneth Edgeworth there existed a disk of icy bodies of the region were taken, a few (1880–1972) that were formed at the dawn of the hours apart, and these were then solar system, but were too small examined to see if any of the BEFORE and widely spaced to accrete into objects had moved, indicating 1781 and 1846 The discovery a planet. From time to time, they that they were much closer than of Uranus and Neptune leads to were nudged into the inner solar the stars. More than 1,000 objects discussion of where the outer system, where they appeared as have now been found in the Kuiper edge of the solar system lies. comets. He published his idea in belt. Most are larger than 60 miles Journal of the British Astronomical (100 km) across, since anything 1930 Pluto is discovered. Association, a periodical not widely smaller is too faint to detect. ■ Astronomers Frederick C. read in the US. Leonard and Armin O. Leuschner suggest there may Kuiper belt The comets have remained be similar bodies out there. In 1951, in the more prestigious what they were from the Astrophysical Journal, a Dutch- beginning—astronomical AFTER American astronomer named heaps of gravel without 1977 Charles Kowal discovers Gerard Kuiper suggested that any cohesion. Chiron, an icy centaur (minor such a disk once existed but had Kenneth Edgeworth planet) beyond Saturn. long since been dispersed by the effects of Pluto’s gravity. It came 1992 A Trans-Neptunian to be called the Kuiper Belt, Object (TNO—an object though some astronomers now orbiting at a distance greater use “Edgeworth-Kuiper Belt.” than Neptune’s) is discovered by David Jewitt and Jane Luu. Then, in 1980, Uruguayan astronomer Julio Fernández 2005 The discovery of Eris, realized that a belt of cometary a TNO of a similar size to nuclei beyond Neptune was Pluto, and TNOs Haumea and Makemake, lead to Pluto being See also: The Oort cloud 206 ■ Exploring beyond Neptune 286–87 demoted to a dwarf planet.

ATOMS, STARS, AND GALAXIES 185 SAHTOAVMTEHEEAGICRATLCIAVEXENITREESERGSIONS   NUCLEI AND RADIATION IN CONTEXT B etween 1940 and 1942, Spiral galaxy NGC 1068 (M 77) is American astronomer Carl the archetypal Seyfert galaxy. It has KEY ASTRONOMER Seyfert studied a number an intensely bright active center Carl Seyfert (1911–1960) of spiral galaxies that had compact, surrounded by swirls of ionized gas. particularly bright centers, often BEFORE bluish in color. His investigations produce so much energy that they 1908 Edward Fath and Vesto revealed that there were distinctive outshine their host galaxies, which Slipher observe peculiarities emission lines in these galaxies’ cannot be seen. AGNs are thought in the spectrum of the nebula spectra. He published a paper to be powered by matter spiraling NGC 1068 (now recognized describing galaxies of this type, into massive black holes at their as a typical Seyfert galaxy). which later came to be known centers. In addition to emitting as Seyfert galaxies. They are radiation, many AGNs also send 1936 Edwin Hubble classifies usually spiral galaxies with nuclei powerful jets of particles into space the shapes of galaxies. that produce large amounts of from the vicinity of their central radiation over a broad range of black holes. Some are associated AFTER wavelengths, often most strongly with vast lobes of material that 1951 Cygnus A, one of the in the infrared region, but also emit radio waves—active galaxies strongest sources of radio often including visible light, radio that feature these “radio lobes” are waves in the sky, is identified waves, ultraviolet radiation, X-rays, called radio galaxies. ■ as the first radio galaxy. and gamma rays. 1963 Dutch astronomer Violent centers Maarten Schmidt coins the Seyfert galaxies are just one variety term quasi-stellar radio source of a class of galaxies called active (later “quasar”) for an object galaxies. These have central regions, with a starlike appearance termed active galactic nuclei (AGNs), that is actually a distant, very in which an extraordinary amount bright source of radio waves. of violent activity occurs. Quasars are another type of AGN. These are 1967 Armenian astrophysicist always a vast distance away and Benjamin Markarian begins publishing a list of galaxies with See also: Spiral galaxies 156–61 ■ Beyond the Milky Way 172–77 ■ strong ultraviolet emissions, Quasars and black holes 218–21 many of them Seyfert galaxies.

186 ITASHNTEDOMEOAAPRTETCRHHFMOEFCATTLEURNIAARL THE ORIGIN OF THE MOON IN CONTEXT G eologists had broadly put orbiting satellite. He suggested together a story for Earth’s that the moon had once been KEY ASTRONOMER billions of years’ existence much closer to Earth and was Reginald Daly (1871–1957) by the early 20th century. But the slowly drifting away. This has since origin of the moon remained open been confirmed by measurements BEFORE to speculation. Up until the 1940s, showing that the moon is moving 1913 British geologist Arthur most astronomers subscribed to away by about 1½ in (3.5 cm) a year. Holmes produces the first a theory put forward by George modern geologic timescale, Darwin, son of the naturalist Astronomers have found evidence which proposes that Earth Charles. In 1898, Darwin proposed that two small planets orbiting the star is at least 1.5 billion years old. that the moon formed when a hot HD 172555 collided a few thousand and fast-spinning Earth threw out years ago. A similar collision involving AFTER molten rock that coalesced into an Earth probably formed the moon. 1969–72 The Apollo missions bring back moon rock to be analyzed on Earth. 1975 Following analysis of moon rock, US astronomer William Hartmann and others return to the giant impact theory to explain the new evidence. 2011 Norwegian−American planetary scientist Erik Asphaug and Swiss astrophysicist Martin Jutzi suggest that the moon formed with a tiny companion moon and the two later collided.

ATOMS, STARS, AND GALAXIES 187 See also: The discovery of Ceres 94–99 ■ The composition of comets 207 ■ Investigating craters 212 ■ The Space Race 242–49 Lunar rocks match The moon may have the material in formed in a giant Earth’s mantle. impact that knocked Earth’s magma At bottom each “exact” into orbit. science is, and must Computer modeling suggests that a smaller be, speculative. Its chief planet hit Earth 4.3 billion years ago, tool of research, too rarely used with both courage and that the collision created the moon. and judgment, is the regulated imagination. Reginald Daly An alternative theory, championed core. However, the rock evidence seething balls of magma. Most of in the 1940s by American chemist points to the moon being made from Theia merged with Earth (which Harold Urey, was the “capture” material gouged from Earth’s surface explains why the planet has an model, in which the moon formed after the planet had solidified. oversized metallic core) and the elsewhere in the solar system and “splash” hurled magma into orbit, fell under the control of Earth’s Big Splash mostly from the rocky outer region gravity. However, the moon is so In the last decade, computer of the planet. This material formed big compared to Earth that most modeling of possible impacts has the moon. Although the Big Splash thought such an event unlikely. suggested an event now dubbed idea is very much a hypothesis at the Big Splash. In this scenario, a present, it remains the best guess In 1946, Canadian geologist Mars-sized planet (named Theia, as to the origins of the moon. ■ Reginald Daly presented a third after the mother of the moon in idea. While Daly agreed with Darwin Greek mythology) hit Earth 4.3 him to become a leading voice that the moon and Earth were billion years ago—about 200 million on the origins of different rock formed from the same material, years after Earth’s formation. The types. As early as the 1920s, he hypothesized that the driving impact turned both bodies into Daly proposed that material force was an impact between ejected from Earth to form the Earth and another body, which Reginald Daly moon was a primary cause for had thrown material into orbit. the dynamic character of Earth’s Geologist Reginald Daly’s crust. The impact theory was Matching rock contributions to the theories of a late addition to Daly’s work, Daly’s idea was more or less continental drift, plate tectonics, coming after he had retired as ignored until the 1970s, when the and the rock cycle have proved the head of Harvard University’s analysis of moon rocks showed that invaluable in understanding geology department. their mineral content is a very close the similarities and differences match to that of Earth’s mantle (the between Earth and other rocky Key work layer between the crust and the bodies in the solar system. core). Both are high in silicates but 1946 Origin of the Moon and have low levels of metals. If the moon Daly’s abilities as a geologist its Topography had formed elsewhere, its rocks became clear when he was would be very different from Earth’s. surveying the southern border of If it had formed from the same molten Canada, from the Pacific Coast, raw materials as Earth, it would be through the Rockies to the Great expected to be a mini version of Plains. The rock samples he Earth, and have a larger metallic collected during this survey led

IMPORTANT NEW DISCOVERIES TELESCOPESWILL BE MADE WITH FLYING SPACE TELESCOPES



190 SPACE TELESCOPES I n 1946, a full 11 years before Astronomy may be Sputnik 1, the first satellite, revolutionized more than IN CONTEXT was launched into Earth’s any other field of science orbit, a 32-year-old astrophysicist by observations from above KEY ASTRONOMER named Lyman Strong Spitzer Jr. the atmosphere. In a new Lyman Spitzer Jr. (1914–1997) conceived of a powerful telescope adventure of discovery, no one that would one day operate not on can foretell what will be found. BEFORE Earth’s surface but in orbit. High 1935 Karl Jansky reveals that above the opaque atmosphere Lyman Spitzer Jr. celestial objects produce radio and the light pollution, this space waves, offering new ways to telescope would have a clear and view the universe beyond unprecedented view of the universe. visible light. It would be more than four decades before Spitzer’s dream was realized, 1970 NASA launches Uhuru, but his patience and tenacity would an orbiting X-ray observatory. eventually pay off. 1978 The International More than light researchers found that meteors Ultraviolet Explorer, the first The discovery of extraterrestrial and sunspots produced radio telescope operated in real radio sources by Karl Jansky in waves of their own, this time time, is launched. 1935 revealed that there were in the microwave band used in ways of observing the universe radars. If it was possible to discover AFTER other than by visible light. The new objects using radio, then it 1990 The Hubble Space outbreak of World War II in 1939 stood to reason that other forms Telescope is launched. interrupted research into this new of electromagnetic radiation, such and exciting field. It was left to an as infrared, ultraviolet (UV), and 2003 The infrared Spitzer amateur astronomer from Illinois X-rays, could be harnessed as Space Telescope is launched. named Grote Reber to take the first tools of observation. steps in radio astronomy. In 1937, 2009 The Kepler Telescope Reber had made the first survey of There was a problem, however. is launched to search for the radio universe using homemade Earth’s atmosphere, transparent extrasolar planets. antennae he had built in his to visible light, is opaque to many backyard. Soon afterward, wartime 2018 Scheduled launch of the infrared James Webb Lyman Spitzer Jr. was born in triumphed in another field Space Telescope. Toledo, Ohio, in 1914. He received entirely. With his friend Donald a Ph.D. in astrophysics from Morton, Spitzer became the first Lyman Spitzer Jr. Princeton under the supervision to climb Mount Thor, a 5,495-ft of Henry Norris Russell. After (1,675-m) peak in the Canadian World War II, he became head Arctic. In 1977, his campaigning of the astrophysics department, for a space telescope paid off, and began his 50-year devotion and funding was granted to to space telescopes. the Hubble Space Telescope. He lived long enough to see his As an expert in plasma, Spitzer dream become a reality in 1990. invented the stellarator in 1950. This device contained hot plasma Key work within a magnetic field and started the search for fusion power that 1946 Astronomical Advantages continues today. In 1965, Spitzer of an Extra-terrestrial joined NASA to develop space Observatory observatories, but that year he

ATOMS, STARS, AND GALAXIES 191 See also: Beyond the Milky Way 172–77 ■ Radio astronomy 179 ■ Studying distant stars 304–05 ■ Gravitational waves 328–31 The level of the orange curve in this graph represents how opaque the atmosphere is at the given wavelength of radiation. ATMOSPHERIC OPACITY (%) 150 The major windows are around visible wavelengths (marked by the rainbow) and radio wavelengths from about 1 mm to 10 m. 100 50 0 10m 100m 1km 0.1nm 1nm 10nm 100nm 1μm 10pm 100pm 1mm 1cm 10cm 1m WAVELENGTH of these kinds of radiation. The from Earth, the stars appear to The scientific term for twinkling is waves are absorbed by the air’s twinkle. This effect is caused by the scintillation. It is caused when light molecules, reflected back into star’s light shifting back and forth, passes through layer upon layer space, or scattered in all directions and rising and falling in brightness. of turbulent air in the atmosphere. into a meaningless hodgepodge. This is not a property of starlight, The turbulence itself has no effect As a result, it is almost impossible but is caused by Earth’s thick on the light, but the density and to garner information about most atmosphere. The twinkle becomes temperature differences that are kinds of non-visible radiation more marked as magnification making the air churn and swirl from terrestrial observatories. increases, making the objects do have an effect. As the starlight appear shaky and fuzzy in the passes through one pocket of air Spitzer’s 1946 paper, entitled eyepiece of a telescope or as diffuse to another of a different density, Astronomical Advantages of smears of light in photographs. it refracts slightly, with some ❯❯ an Extra-terrestrial Observatory, highlighted the problem of detecting Earth’s atmosphere Many kinds of non-visible radiation. His solution causes astronomical electromagnetic radiation was to put a telescope into space. objects to twinkle, so they But Spitzer also highlighted the cannot be captured with cannot pass through obstacles to such a proposal: first, the atmosphere. the technological challenge of sharp definition. inventing space travel, and second, that of designing an instrument capable of operating in space by remote control from the ground. Twinkle, twinkle little star The solution to both problems The rest of Spitzer’s paper was is to put telescopes in space. focused on solving a problem that had frustrated astronomers for centuries—the sky itself. Viewed

192 SPACE TELESCOPES wavelengths bending more is smeared out into a larger disk. Our knowledge of stars and than others. As a result, the The effect is similar to the interstellar matter must be straight beam of light that traveled telescope being out of focus. to Earth across the cosmos starts based primarily on the to follow an ever-changing and Improving the view electromagnetic radiation haphazard zigzagging path Observational conditions change through the air. A telescope or constantly with the atmosphere. which reaches us. a naked eye focused on it will Before the 1990s, observers simply Lyman Spitzer Jr. see a fluctuation in brightness waited until distortions were at as some of the light is directed a minimum. For instance, high With the advent of powerful in and out of that line of sight. winds clear away turbulence, computers in the 1990s, earthbound creating near-perfect viewing astronomers began using adaptive The impact twinkling has conditions. In the late 1940s, optics (AO) to correct the problems on capturing sharp astronomical astronomers started to use of astronomical seeing. AO images is called “seeing.” When movie cameras to film the sky measures distortions in the arriving the atmosphere is very still and in the hope that, among the light and evens it out, just as a seeing is good, the image of a thousands of frames filmed over distorted mirror might be used to distant star in a telescope is a time, there would be the odd “lucky correct a deformed image to make small steady disk. When seeing image” that captured the sky in it look like the original image prior is poor, the image breaks up into crystal clarity. Another solution to deformation. AO systems use a squirming cluster of dots. An was to go higher. Today, the minutely adjustable mirrors and image taken over a period of time world’s most effective terrestrial other optical devices, but they observatories are invariably built also rely heavily on computers to Adaptive optics requires a clear star at the top of high, arid mountains, filter out the atmospheric “noise” as a reference point. As these are hard where cloud cover is minimal, and from images. Despite the dramatic to find, a sodium laser creates a “star” by the air above is generally calm. improvements brought about by lighting up dust in the high atmosphere. AO, however, a large telescope in orbit, which could observe in multiple wavelengths of the spectrum, including visible light, was the ultimate goal for astronomy. The road to Hubble As the leading voice in the field, Spitzer had been made head of NASA’s task force for developing the Large Space Telescope (LST) program in 1965. In 1968, NASA scored its first space- telescope success with the Orbiting Astronomical Observatory (OAO-2), which took high-quality images in ultraviolent (UV)

ATOMS, STARS, AND GALAXIES 193 A machine polishes the Hubble’s mirror. Its 100-in (2.4-m) aperture may seem small today, yet it is the same size as the Hooker Telescope, which was the world’s largest telescope until 1948. light, doing much to raise held on a support that emulated of Space Shuttle Challenger on awareness of the advantages weightlessness to ensure they January 28, 1986, with the result of space-based astronomy. did not warp in space. The glass that NASA’s shuttle fleet was had to be polished into a curve grounded for two years. Spitzer’s LST aimed to achieve with an accuracy of 10 nanometers. more dramatic results than the This would make it possible for Finally, on April 24, 1990, OAO-2, observing near and far HST to view everything from Space Shuttle Discovery hauled objects with the visible light UV light to the upper end of the the 11-ton HST to its orbit spectrum. His team settled on a infrared spectrum. 335 miles (540 km) above Earth. 10-ft 5-in (3-m) reflecting telescope Spitzer had finally realized the and a launch was scheduled for Further delays pushed the dream of his career—a telescope 1979. However, the project became launch of HST to 1986, but then in space unencumbered by the too expensive for its budget. The tragedy struck with the explosion problems of poor seeing and an aperture was reduced to a less atmosphere partly opaque to costly 100 in (2.4 m), and LST was ultraviolet and infrared rays. postponed to 1983. As that year came and went, no launch occurred, Hubble trouble but Spitzer persisted and the project The problems that had beset the continued. In the meantime, LST mission on the ground, however, was renamed the Hubble Space continued in space. The first Telescope (HST) after Edwin Hubble, images sent back by HST were who had first grasped the true scale so badly distorted that they were of the universe (pp.172–77). By now, almost worthless. Was HST going the telescope’s mirrors had been to be a worse observational tool constructed. To help reduce weight, than a ground-based telescope? ❯❯ a top layer of low-expansion glass sat on a honeycomb support. The shape of the mirrors was crucial. During construction, they were The Hubble Space Telescope is the realization of Spitzer’s vision. It remains one of the finest scientific instruments ever made.

194 SPACE TELESCOPES Analysis of the images revealed US astronaut Andrew Feustel uses Ultra deep, ultra clear that the mirror was the wrong a power tool to repair the Hubble Despite its shaky start, HST has shape around the edge. The error Space Telescope during a servicing surpassed all expectations. The was tiny—about 2 millionths of mission in 2009. telescope has made 1.2 million a meter—but enough to send the observations to date during its light captured by the outer part mirrors in front of Hubble’s 3-billion-mile (5-billion-km) journey of the primary mirror to the wrong instruments so that the light around Earth. Despite traveling at area of the secondary mirror, creating entering them from the main mirror 17,000 mph (27,000 km/h), it can serious aberrations in the images. was correctly focused. Two sets pinpoint a position in space to an This was a worrying moment for of these mirrors were fitted during accuracy of 0.007 arc seconds— Spitzer and his team, as it seemed a crucial service mission to HST which is like hitting a penny coin as if HST might be about to prove in 1993. They worked perfectly. from 200 miles (300 km) away. an embarrassing failure. HST could at last be put to work, It can resolve an object that is and the results were astonishing. 0.05 arc seconds. NASA likened Corrective vision this to standing in Maryland and If Hubble was to fulfill its potential, Astronauts serviced HST four viewing two fireflies in Tokyo, it needed corrective elements more times after 1993 and for the Japan. Astronomers worldwide added to its optical system. In last time in 2009, in one of the began booking HST’s time to see effect, it was given a pair of final shuttle missions. The shuttles objects of interest. The archive of eyeglasses. The problem with were retired in 2011, after which everything it has seen—totaling the primary mirror was precisely it would not be possible to service 100 terabytes and counting—can calculated by analyzing the HST again. However, that final be viewed on a public website. telescope’s images. The solution service added significant upgrades, was to add carefully designed which mean that HST may remain Many of HST’s observations in use until 2040. have looked deep into space— and far back in time. In 1995, the Deep Field image focused on an empty patch of space, one 24-millionth of the total sky. Combining 32 long exposures revealed a number of unknown galaxies that were 12 billion light- years away—light that began its Nature has thoughtfully provided us with a universe in which radiant energy of almost all wavelengths travels in straight lines over enormous distances with usually rather negligible absorption. Lyman Spitzer Jr.

ATOMS, STARS, AND GALAXIES 195 Taken in 2004, the Ultra Deep Field reveals thousands of jewel-like galaxies in a variety of shapes, colors, and ages. The red galaxies are the most distant. journey just 1.5 billion years occur at the edge of the visible into nebulae to pick out the hot after the Big Bang. In 2004, the universe. The Chandra X-ray zones where stars are forming. Ultra Deep Field showed objects Observatory was launched in In 2009, the liquid helium that 13 billion light-years away, and 1999, and is tasked with finding kept its heat-sensitive detectors in 2010, HST used infrared black holes, infant solar systems, cool finally ran out. radiation to make the eXtreme and supernovae. The final member Deep Field of objects that existed is the Spitzer Space Telescope, Observatories can be placed just 480 million years into the which entered space in 2003. in orbit around the sun rather than history of the universe. To see One of its tasks was to peer Earth, where it is easier to shield farther than this will require them from the sun’s heat and light the infrared James Webb Space and they have a wide, unobstructed Telescope in 2018. view of the sky. Today, there are about 30 observatories in orbit, Spitzer in space sending back images. NASA’s HST is the most famous of the four Kepler, which searches for great observatories that are Lyman extrasolar planets, and two ESA Spitzer Jr.’s legacy. Between 1991 missions, Herschel and Planck, and 2000, the Compton Gamma are examples. All were launched Ray Observatory looked at gamma- in 2009. Herschel was the largest ray bursts, energetic events that infrared telescope ever put into space, while Planck studied the cosmic microwave background. In 2015, ESA launched the LISA Pathfinder to test the technology for a space observatory that would detect not electromagnetic waves, but gravity waves. Not even Lyman Spitzer Jr. could have predicted such an advance. ■ The Spitzer Telescope was named by NASA to honor the vision and contributions of Lyman Spitzer Jr. It was initially called the Space Infrared Telescope Facility.

196 ITATHNTEHOAOOTUKORMLTEICOSSNMUTACHKLAEENI THE PRIMEVAL ATOM IN CONTEXT If the Big Bang theory is right, during the universe’s first few moments, temperatures were exceedingly high. KEY ASTRONOMERS George Gamow (1904–1968) For a short window of time, conditions were right for Ralph Alpher (1921–2007) protons and neutrons to combine to form atomic nuclei. BEFORE It took less than an hour 1939 Hans Bethe describes to make the atomic nuclei. two ways in which helium can be made from hydrogen in stars. I n 1931, Georges Lemaître it would have been unimaginably suggested that the universe hot. Matter would have consisted AFTER originated from the explosion of a frenzy of elementary particles 1957 Fred Hoyle and of an initial, extremely dense, (particles that cannot be broken colleagues set out eight “primeval atom” and has been down into smaller particles)— processes by which chemical expanding ever since—now known considered at that time largely to elements can be synthesized as the Big Bang theory. However, comprise protons, neutrons, and from other elements in stars. by the mid 1940s, the theory was electrons. Temperatures would have in need of additional evidence to been too high for these particles 1964 German−US physicist sustain its credibility. to join up, except very briefly. Arno Penzias and US However, after several seconds of astronomer Robert Wilson A Ukrainian physicist named existence, the universe would have discover the cosmic microwave George Gamow began thinking expanded and cooled to the point background radiation. about conditions at the start of where protons and neutrons might the universe as it was proposed by be held together by an interaction 1970s The mass of atom- Lemaître. He quickly realized that based matter (made of protons and neutrons) as calculated by Big Bang nucleosynthesis is found to be much less than the observed mass of the universe. This puzzle is largely resolved by suggesting the existence of dark matter.

ATOMS, STARS, AND GALAXIES 197 See also: The birth of the universe 168–71 ■ Energy generation 182–83 ■ Nucleosynthesis 198–99 Deuterium Helium-3 Tritium numbers of free protons (hydrogen nucleus nucleus nucleus nuclei) left at the end of the process, plus some unstable nuclei, which Lithium-7 nucleus would have quickly decayed. Helium-4 nucleus Their calculations showed that the universe would have consisted of about 25 percent helium, with the rest mainly hydrogen. The paper that Alpher and Gamow published also argued that other heavier nuclei might have been created in the Big Bang through successive additions of neutrons. Atomic nuclei formed from protons and neutrons in Proton Correct predictions the first minutes of the universe. Most neutrons ended Neutron It was eventually recognized up in helium-4 nuclei. Small amounts of helium-3 and Gamma ray through the work of scientists such deuterium (an isotope of hydrogen) and tiny amounts as Fred Hoyle that heavier elements of lithium-7 were also made. Another isotope of such as carbon were created in hydrogen—tritium—formed, and decayed to helium-3. stars and supernova explosions. Energy was released in the form of gamma rays. Nevertheless, the Alpher–Gamow theory correctly explained the called the strong nuclear force, thus would have ended up combined relative abundances of hydrogen creating an array of atomic nuclei. with protons in an isotope (one and helium, lending considerable Gamow believed that, after a few of the possible alternative forms) support to the theory that the initial “seed” nuclei had assembled of helium, helium-4. A small universe began with a Big from protons and neutrons, others number would have become other Bang. It also correctly predicted might have been built up by the small atomic nuclei. In addition, the existence of the cosmic successive addition of neutrons, there would have been large background radiation that was with some decaying to protons. discovered in 1964 (pp.222–27). ■ At a later time, all the nuclei might George Gamow have captured electrons to form the a conference in Brussels. At atoms of the chemical elements. George Gamow was born in George Washington University Odessa, Ukraine, in 1904. From in the US, he turned his Doing the math 1923, he attended the University attention to the evolution Gamow asked an American of Leningrad, studying under of stars. From 1954, Gamow graduate student, Ralph Alpher, Alexander Friedmann. In 1928, became interested in genetics to work out the details of his idea. Gamow briefly stayed at the and biochemistry. He also wrote Alpher and a colleague, Robert University of Göttingen in a number of popular science Herman, performed extensive Germany, where he developed books and sci-fi novels. mathematical calculations. They a theory called quantum found that the right conditions tunneling. This theory was Key works for protons and neutrons to come used by others to explain how together existed only for a short the fusion of light atomic nuclei 1948 The Origin of Chemical window of time of a few minutes. might create energy inside stars. Elements (also called the Their calculations showed that In 1933, he defected from the Alpher–Bethe–Gamow paper) most of the universe’s neutrons Soviet Union while attending 1952 The Creation of the Universe

198 FESLOTERAMRTSEHNEATRCSEHFEAMCICTOALRIES NUCLEOSYNTHESIS IN CONTEXT Heavier elements require U ntil the late 1940s, it was high temperatures not known where the atoms KEY ASTRONOMER to be created. of most chemical elements Fred Hoyle (1915–2001) Suitable conditions in the universe—for example, carbon, oxygen, and iron—had BEFORE for the creation of many come from, nor how they had been 1928 George Gamow elements occur in the made. It had been established in constructs a formula based evolution of giant stars. the 1920s that the two lightest on quantum theory that can Extreme conditions for elements, hydrogen and helium, be used to determine how other elements occur when made up most of the universe’s various atomic nuclei join. giant stars disintegrate in matter, and in 1948, George Gamow supernova explosions. and Ralph Alpher showed how all 1929 Welsh astronomer All but a few elements of the hydrogen, most of the helium, Robert Atkinson and Dutch can be created in stars by and tiny amounts of lithium could physicist Fritz Houtermans eight distinct processes. have been made in the “Big Bang.” figure out how, at the However, the origin of other temperatures inside stars, Stars are factories elements was a mystery. nuclei of light elements could for the chemical join up, while at the same Building to iron time releasing energy. elements. The discovery of their origin was made largely thanks to the work AFTER of British astronomer Fred Hoyle. 1979 Scientists discover that Starting from chance conversations almost all nuclei of the light with leading astronomers in the elements lithium, beryllium, US during an academic tour in and boron in the universe are 1944, he developed an idea that made by the impact of cosmic most chemical elements might be rays (highly energetic particles) created step-by-step by nuclear on other nuclei in space, and reactions in stars—a process called not in stars. nucleosynthesis. Hans Bethe had already shown in 1939 that hydrogen could combine to make helium in star cores, but Bethe offered no suggestions for how