The principal goal for extrasolar planet hunters, though, was finding evidence for “first generation” planets around stars like our Sun—planets that formed from the stellar nebula itself as a newborn star is created. That long-anticipated event at last occurred in 1994 when Geneva Observatory astronomers Michel Mayor and Didier Queloz, working from the Haute-Provence Observatory in southern France, discerned the presence of an object similar to Jupiter orbiting 51 Pegasi, a sunlike star forty-five light-years distant in the constellation Pegasus. They first revealed their discovery at a conference in Florence, Italy, and their fellow astronomers declared it a “spectacular detection.” Unlike our own solar system, this extrasolar planet is located a mere four and a half million miles (seven million kilometers) from its star (far closer than Mercury is to our Sun) and completes one orbit every four days. Planet hunters had assumed it would take years of collecting data before detecting the subtle and gradual stellar wobbles caused by a planet orbiting its parent star, but the small orbit of 51 Pegasi b enabled them to spot its variations quickly.
Other discoveries followed swiftly. Geoffrey Marcy and R. Paul Butler, then both at San Francisco State University and friendly competitors of the Geneva observers, had been gathering radial velocity data at Lick Observatory since 1987. Searching through their records, they found evidence for a planet similar to 51 Pegasi b, a body at least seven times the mass of Jupiter closely circling within 40 million miles (64 million kilometers) of the star 70 Virginis.
These finds challenged theorists, who had not imagined giant planets with eccentric orbits so close to their sun. These unusual planets, though, were quickly overshadowed by a simultaneous discovery by Marcy and Butler—a large planet orbiting 47 Ursae Majoris at a more distant 200 million miles (around 320 million kilometers). This companion of 47 Ursae Majoris thus gained special distinction for being more “reminiscent of solar system planets.” And by 1999, Butler, Marcy, and several colleagues found the first multiple planetary system, a trio of planets circling the star Upsilon Andromedae.
An illustration of the possible surface of TRAPPIST-1f, the fifth of the seven planets orbiting the host star.
(NASA/JPL-Caltech)
The floodgates were opened, and over the succeeding years thousands of exoplanets were (and continue to be) found. While at first only the biggest exoplanets were revealed (as it was easier to detect them), improved technologies and additional planet-hunting methods enabled the discovery of smaller exoplanets, including Earth-like planets like those in the TRAPPIST system. Space-based missions, such as the Kepler space telescope, were especially productive in spotting these extrasolar planetary systems. “We’ve gone from the early days of thinking maybe there are five or ten other planets out there, to realizing almost every star next to us might have a planet,” says astronomer Jennifer Burt at the Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology (MIT). Indeed, one team of astronomers in 2012 estimated that there might be one or more planets orbiting each and every Milky Way star. That means at least 200 billion potential homes for ET to call. “We conclude,” wrote the astronomers in their Nature report, “that stars are orbited by planets as a rule, rather than the exception.” Astronomers now know with assurance that the solar system is no longer the sole specimen of its species.
Realm of the Galaxies
By the late 1800s and into the next century, astronomers moved outward. They began to map the topography of our home galaxy, as well as trace how the galaxies themselves are uniquely arranged through the cosmos. And while Edwin Hubble opened our eyes to the existence of other galaxies, it was a woman, her skills at first overlooked, who began to show how those galaxies can evolve over time.
Astronomers also came to learn how starlight could be used to decipher the universe’s makeup, what elements resided in both stars and interstellar space. That hydrogen was the overwhelming prime element was discovered by a young woman graduate student in the 1920s. And another woman later provided the best evidence that regular matter—the stuff of stars, planets, and us—is not the major component of the universe. Instead, some unknown “dark matter” is five times more abundant.
Meanwhile, the theories of the great Einstein recast our vision of the universe. We no longer consider the universe as serene, but rather violent, often powered by Einsteinian objects with amazing energies.
All the while, new tools arrived over the past century to better explore the cosmos, instruments taking us beyond the visible-light spectrum. Astronomers now go underground to capture neutrinos emitted from distant events. They gather radio waves, cosmic rays, infrared radiation, X-rays, and gamma rays. They are even detecting the ripples generated in the very fabric of space-time as both black holes and neutron stars collide millions and billions of light-years away.
CHAPTER ELEVEN
Our Spiraling Home
The difficult task of mapping the Milky Way
IT’s an odd quirk of astronomy. Telescopes can peer billions of light-years outward, allowing us to observe a plethora of galaxies and to map their lacelike distribution through space and time with exquisite precision. Distant quasars have been thoroughly examined—by gathering their emissions from radio to gamma rays. And the European Space Agency’s Planck satellite provided one of the best baby pictures of the cosmos yet, an image depicting the universe when it was a mere 400,000 years old. It appears that the definitive cosmic atlas is within the grasp of astronomers.
And yet the topography of our local celestial landscape (that is, within only tens of thousands of light-years) remains frustratingly murky. What seems as if it should be the easiest structure to trace—that of the Milky Way, our home galaxy—is just the opposite. It’s like owning the best globe of the world, with your hometown missing. There’s a simple reason for this conundrum: our solar system is embedded inside the dusty plane of the Milky Way. Such a position makes viewing our galaxy’s exact configuration a difficult task. Try discerning the pattern on a piece of china with your eyes level to the edge of the plate. That’s what astronomers have long confronted when trying to map the Milky Way. How can you peer through that disk, full of dust and gas?
To find an answer, astronomers started with a hunch—a reasonable one at that. Since the disks of other galaxies displayed a beautiful spiraling architecture, they assumed that the Milky Way, too, has massive arms that wrap themselves around the galactic hub like coiled streamers.
By the 1930s, identifying the Milky Way’s spiral arms became a top item on astronomers’ agenda. At first they tried just counting stars, all the ones in sight, hoping denser concentrations in the tally would outline the arms. But, alas, they experienced little success. It took World War II, oddly enough, for astronomers to come across a new approach for solving this problem.
Because of the fear that the Japanese might attack the west coast of the United States, the Los Angeles area was blacked out nightly during the conflict. This war-imposed veil of darkness was heaven for one particular astronomer working at the nearby Mount Wilson Observatory, which operated the biggest telescope in its day: the Hooker telescope, with its 100-inch-wide (2.5-meter-wide) mirror. While many observatory staffers had temporarily left to carry out war work, German-born Walter Baade was designated an “enemy alien” and restricted to the Pasadena area. That meant he had almost unlimited time on the 100-inch, allowing him to get the best look ever at the Andromeda galaxy, the spiral galaxy closest to us, at a distance of 2.5 million light-years.
Pushing the telescope to its limits over months of observations, Baade came to recognize that highly luminous blue and blue-white supergiant stars, along with bright gaseous nebulae, tended to reside only in Andromeda’s spiral arms, acting much like the lights lining an airport runway. The reason spiral arms stand out is because they are regions where young, hot stars are forming.
The stars making up an arm are not permanently connected, as if part of a ropelike structure attached to a galaxy’s center. R
ather, that appearance reflects underlying density or shock waves that travel through the galaxy’s disk of gas and foment star formation. As the disk’s gas passes through this compression wave during its rotation around the galactic center, the material gets squeezed, huge clouds form, and within several million years big new stars turn on to illuminate the density wave’s spiraling structure. It’s like a cosmic traffic jam. The highly luminous stars are so short-lived, however, that they die off by the time they move out of the traffic tie-up. This mechanism behind a galaxy’s spiraling structure wasn’t identified until the 1960s, but nonetheless Baade had still found the perfect objects to delineate a spiral galaxy’s arms.
Soon after the war, others began applying this newfound knowledge to our own galaxy. Astronomer William W. Morgan at the Yerkes Observatory in Wisconsin got a head start on the problem, as he had already been carrying out a spectral study of the Milky Way’s brilliant supergiant stars. He first teamed up with Jason Nassau at the Warner and Swasey Observatory in Ohio, and together they pinpointed the positions of some 900 supergiants. Less than 6 percent of these stars had their distances reliably nailed down, yet this evidence, scanty as it was, suggested that a spiral arm might be running from the constellation Carina over to Cygnus in our local solar neighborhood. It was a start.
Soon after, Morgan joined forces with two student assistants, Stewart Sharpless and Donald Osterbrock, to push the survey even further. Along with tracking down blue and blue-white stars, they also plotted the distribution of luminous nebulae (notable for their energized hydrogen). To quickly spot the nebulae, the two students set up a special camera that was originally designed as a wide-angle projector for training aerial gunners during World War II. The dozens of photographic plates they took, revealing many new nebulae for Morgan to analyze, provided the breakthrough.
With the additional data, segments of two spiral arms could be reliably traced. One arm (labeled Orion) passed within 1,000 light-years of the Sun; the other (Perseus), located farther from the galactic center, was at its closest point to us some 6,500 light-years away. There was also the hint that a third spiral arm (Sagittarius) swept closer to the center of the Milky Way, about 5,000 light-years away from us.
The Yerkes team announced its findings at a 1951 meeting of the American Astronomical Society held in Cleveland, Ohio. Morgan presented a handmade model of the spiral arms, which used cotton balls to depict the positions of the bright nebulae. This map was far from complete, because it’s difficult for an optical telescope to peer much farther into the dust- and gas-filled plane of the Milky Way. But that didn’t dampen the reception Morgan’s work received at the astronomical conference.
“Astronomers are usually of a quiet and introspective disposition,” University of California astronomer Otto Struve later wrote. “They are not given to displays of emotion. . . . But in Cleveland, Morgan’s paper on galactic structure was greeted by an ovation such as I have never before witnessed. Clearly, he had in the course of a 15-minute paper presented so convincing an array of arguments that the audience for once threw caution to the wind and gave Morgan the recognition which he so fully deserved.”
There was clapping of hands and stomping of feet. And why not? The Yerkes astronomers were providing the first map (partial as it was) of our cosmic “hometown.” A problem that astronomers had struggled with for decades, astronomy historian Owen Gingerich has written, had “finally found its solution by a quite different avenue from the numerical star-counting procedures.”
And when it rains, it pours. Within two years, the spiraling segments were confirmed and extended with the use of a new instrument available to astronomers—the radio telescope, which could penetrate farther through the Milky Way’s dust and haze by tuning in to a radio frequency emitted by hydrogen gas.
The map is still incomplete, but some overall patterns are emerging. For one, there is now strong evidence that the Milky Way galaxy is a barred spiral, which means its center is extended like a bar rather than bulbous (as in the Andromeda galaxy). More than two-thirds of present-day spirals have a center bar, a structure that likely evolves as a spiral galaxy matures.
And from the 1950s into the 1990s, continuing surveys revealed further sections of our galaxy’s spiraling arms, piece by piece. By connecting the dots, astronomers came to believe that there were four gently curving arms, neatly arranged around the Milky Way’s center.
An illustration of the most up-to-date information on the Milky Way’s structure as a barred spiral. The Sun resides in the Orion spur, about two-thirds of the way from the galactic center.
(NASA/JPL-Caltech/R. Hurt [SSC/Caltech])
Infrared images taken by NASA’s Spitzer Space Telescope, released in 2008, changed that assumption, however. It now looks like our galaxy has two dominant arms. Each originates from an opposite end of the central bar and then bends outward, swirling nearly completely around our galaxy’s core. One of them, the Perseus arm, was partially seen by Morgan in 1951. The other is known as the Scutum-Centaurus arm.
What happened to Morgan’s other spiral-arm sightings? Much like the earliest maps of the New World, the cartography has been altered with better resolution. Morgan’s Sagittarius arm has been demoted to a more minor appendage, while the Orion arm (where the Sun resides) is now known to be a mere “spur.” Our view of the Milky Way arises within a smaller concentration of stars and nebulae that is positioned between the two major arms. We sit amid glory—but at a smaller table.
CHAPTER TWELVE
The Woman Who Chased Galaxies
The historic legacy of a “mere Dallas housewife”
BY the 1930s Edwin Hubble was well into his search for far-off galaxies. Using the great 100-inch telescope atop California’s Mount Wilson, he could see out to distances of a few hundreds of millions of light-years. But beyond that, the smudges on his photographic plates were dim, fuzzy, and next to impossible to identify. “There,” wrote Hubble in 1936, in his classic book The Realm of the Nebulae, “we measure shadows, and we search among ghostly errors of measurement for landmarks that are scarcely substantial.” Ever since, astronomers have struggled to trace the evolution of galaxies back through space-time—not just hundreds of millions of light-years outward, but billions.
Hubble himself saw no changes over the relatively shallow span he surveyed. Galaxies “are enormous systems, and it is reasonable to suppose that their evolution is correspondingly slow,” he concluded. And this became the prevailing view for the next three decades. Astronomers just assumed that all the galaxies—every spiraling pinwheel and bulbous elliptical—formed fairly quickly after the Big Bang and then coursed serenely through the cosmos, changing very little over the eons. They had no reason to doubt this. Given how far back astronomers could see at the time (which wasn’t very far), distant galaxies looked pretty much like the galaxies right by us.
Cosmologists in those early days depended on this axiom of constancy. Their prime motivation for tracking galaxies at all was their insatiable desire to learn the universe’s fate. They were little interested in the galaxies themselves; galaxies were simply markers, convenient spots in space to discern the rate of the universe’s expansion. By comparing the speeds of galaxies in earlier epochs with those of today, they hoped to judge whether galaxies were slowing down enough to someday stop in their tracks by the pull of gravitation and eventually fall back toward one another in a “Big Crunch.” On the other hand, maybe they were flying outward at an unstoppable speed, keeping the cosmos forever open.
Using galaxies for this cosmological measurement was a fine idea, as long as galaxies could be thought of as immutable objects that drifted on in tranquil isolation. By maintaining a uniform size and brightness over time, the galaxy could be used as a yardstick. An astronomer estimated a galaxy’s distance by measuring its luminosity and angular width on the sky. As observers peered deeper and deeper into space, viewing the universe as it was in the past, they assumed that ever more distant galaxies would appea
r dimmer and smaller in a systematic fashion.
But what if a galaxy gets either brighter or fainter with age? What if it changes its shape from eon to eon? Then all bets are off, and the universe’s destiny is far harder to determine in this way. Cosmologists learned this unhappy fact—galaxies change!—by the 1970s, and the person primarily responsible for establishing this new principle was Beatrice Tinsley.
Beatrice Tinsley.
(Astronomical Society of the Pacific, courtesy AIP Emilio Segrè
Visual Archives, Physics Today Collection)
Born in England in 1941 and raised in New Zealand, where her family moved after World War II, Tinsley did a master’s thesis in solid state physics. Soon after, in 1963, she moved to the United States when her husband, physicist Brian Tinsley, garnered a research job in Dallas. By then she had plans to pursue a doctorate, but now she wanted to specialize in her long-standing passion—cosmology, a choice that hadn’t been available to her in New Zealand.
Though some judged Tinsley as a mere Dallas housewife with no experience in astronomy, her top-notch academic record convinced the head of the astronomy department at the University of Texas, Austin, to take a chance on admitting her, even with the added burden of her commuting the two hundred miles (320 kilometers) from Dallas to Austin.
Initially Tinsley planned to take part in the long-standing cosmological pursuit of deciding whether the universe was open or closed. But as she examined all the observables in this line of work—the diameters of clusters of galaxies, galaxy magnitudes, galaxy sizes—one question kept diverting her: How were the galaxies changing over time? How were they evolving? That information was crucial to finding an answer to the universe’s fate.
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