Dispatches from Planet 3

by Marcia Bartusiak

  The universe was completely refashioned over the past century. Shortly after a proper cosmic yardstick was devised and we learned that the Milky Way was accompanied by billions of other galaxies in the universe, we were further astounded to find out that space-time was expanding, with galaxies surfing outward on the wave. And it wasn’t just Edwin Hubble who provided the evidence for this expansive behavior; he was helped by a former farm boy from the Midwest who is little known today, along with a Belgian priest.

  Once the cosmic expansion was accepted, it didn’t take long for scientists to imagine that ballooning in reverse, leading to the conception of the Big Bang. How to prove that our universe began with a mighty explosion, however, took time. What to look for was first revealed in 1948, but that prediction was not firmly proven for nearly two decades.

  Since then, cosmologists have added new details to the story of our cosmic creation. For example, our universe may have begun with a brief moment of superaccelerated expansion, called inflation. The “bang” came at the end, when inflation’s latent energy transformed into all the particles and radiation that surround us today. More than that, it’s possible that parallel universes were generated in a similar way, meaning we reside within a “multiverse,” side by side with other universes. But to prove that, theorists must first wrestle with the nature of time.

  CHAPTER TWENTY-FOUR

  Finding a Cosmic Yardstick

  Henrietta Swan Leavitt’s painstaking observations

  inspired a new way to determine the distances to

  far-off galaxies

  FIRST-TIME travelers to the Southern Hemisphere might mistake the deep-space nebular clouds visible there for high cirrus formations, somehow made luminous in the dark of night. Yet the Large and Small Magellanic Clouds are each a chaotic collection of stars, richly diffused with glowing gas. Such novel and fascinating sights were a compelling reason for early European and American astronomers to set up observatories in the Southern Hemisphere.

  In the early 1890s, the Harvard College Observatory established a southern station in the highlands of Peru. For more than a decade, Harvard had been cataloging every star in the northern sky and accurately gauging its color and brightness. With a sizable endowment for a program in spectroscopy, observatory director Edward C. Pickering resolved to further classify the brightest stars by their chemical spectra. The Peruvian observatory allowed Harvard to extend all those endeavors to the southern sky. In doing this, Pickering was helping astronomy move beyond just tracking the motions of stars across the sky to figuring out their basic properties.

  With a huge number of glass photographic plates of the northern and southern skies stacking up, Pickering shrewdly recognized the value of smart young women yearning to contribute in an era that generally denied them full access to scientific institutions. These woman “computers,” as they were called, some with college degrees in science, could be hired for less than half the pay of a man. Stationed at the observatory’s headquarters in Cambridge, Massachusetts, they peered at plates all day through magnifying glasses, swiftly and accurately numbering each star, determining its exact position, and assigning it either a spectral class or a photographic magnitude.

  Henrietta Leavitt working at the Harvard College Observatory.

  (AIP Emilio Segrè Visual Archives, Physics Today Collection)

  One of Pickering’s most brilliant hires was Henrietta Swan Leavitt, who began work as a volunteer soon after graduating, in 1892, from what later became Radcliffe College. She proved herself an expert in stellar photometry, gauging the magnitude of a star by assessing the size of the spot it imprinted upon a photographic plate. As she worked, she was also instructed to keep an eye out for variable stars, those that regularly increase and decrease in brightness.

  Leavitt left Harvard for a time in 1896, first traveling through Europe for two years and then moving to Wisconsin to be with her father. But by 1902, she returned to Harvard as a paid employee, and within two years, variable stars came back into her life in full force.

  Looking through a magnifying eyepiece at two plates of the Small Magellanic Cloud, taken at different times, she noticed that several stars had changed in brightness, as if they were undergoing a slow-motion twinkle. Over the following year, she looked at additional images of the cloud and found dozens more variable stars. Soon in her tally she included old plates going back to 1893, and then started examining the Large Magellanic Cloud as well. By 1907 she had found a record-setting total of 1,777 new variable stars within these prominent, mistlike clouds.

  Leavitt dutifully reported her findings in the 1908 Annals of the Astronomical Observatory of Harvard College, paying particular attention to a special group of sixteen variable stars in the Small Magellanic Cloud. They were later identified as Cepheid variables, stars thousands of times more luminous than our Sun. One sentence in Leavitt’s report would become her most venerated statement. “It is worthy of notice,” she wrote, “that . . . the brighter variables have the longer periods.” Because all her Cepheids were situated in the Small Magellanic Cloud, Leavitt could assume they were all roughly the same distance from Earth. Their periods, therefore, were directly associated not only with their apparent brightness as seen from Earth, but with the actual emission of light. Leavitt’s discovery would lead to a new cosmic yardstick, one that would allow later astronomers to determine the distances to far-off celestial objects, which had never been measurable before.

  The Small and Large Magellanic Clouds (top left, bottom left) as seen from Cerro-Tololo Inter-American Observatory in Chile. The Milky Way is on the right.

  (Roger Smith/NOAO/AURA/NSF/WIYN)

  Leavitt was on track to discover the celestial equivalents of lighthouses on Earth. A sailor at sea who knows the intensity of light emitted by a lighthouse can estimate how far away it is by how bright the beacon appears. Similarly, if an astronomer could know the absolute brightness of a Cepheid—how luminous it would appear up close—he could estimate how far away it must be to appear as the faint point of light seen from Earth. But, just as some lighthouses shine with brighter lights than others, so do Cepheids. Only their relative intensities can be measured from afar. The promise of Leavitt’s discovery was this: if the absolute brightness of just one Cepheid could be known, the absolute brightness of the others could be figured out based on the differences in their periods. In this way, each Cepheid could become an invaluable “standard candle” (as astronomers call it) for gauging distances deep into space.

  In 1908, however, Leavitt was wary that her initial sample of sixteen Cepheids was too small to secure a firm and predictable “period-luminosity” law. She needed more, but chronic illnesses, one of which had earlier left her deaf, and the death of her father delayed her a few years. Moreover, Cepheids, though very bright, are also very rare. Not until 1912 was Leavitt able to add nine more Small Magellanic Cepheids to her list. With twenty-five in hand, all at roughly the same distance from Earth, she could at last establish a distinct mathematical relationship between the rate of a Cepheid’s blinking and its perceived brightness. In a logarithmic-scale graph of her data, the visible brightness of her Cepheids rises in a sure, straight diagonal line as the stars’ periods get longer and longer. She had found her law.

  Cepheids stood ready to be the perfect standard candles, but first Leavitt needed to know the true brightness of at least one. From that one, her graph could be calibrated such that an astronomer could pick out a far-off Cepheid anywhere in the sky, measure its period, and infer its actual luminosity. Knowing that, the star’s distance could be calculated from its much fainter apparent brightness. First, however, Leavitt required the reverse: knowing the distance to one bona fide Cepheid was the only way to calculate its true brightness!

  But Leavitt’s going to a telescope to pursue an answer was out of the question, not only because women were denied access to the best telescopes at the time, but because of her frail condition. She had been advised by her doctor to avoid the chil
ly night air habitually braved by observers. If she had the know-how, she could have carried out a calculation from her desk, using stellar data from previously published work, but Pickering held the strong conviction that his observatory’s prime function was to collect and classify data, rather than apply it to solve problems. He had other things for her to do. At his behest, Leavitt dedicated herself for several years to a separate project on stellar magnitudes. Ultimately, her work served as the basis for an internationally accepted system that is still in use, though now revised.

  In the meantime, recognizing the value in Leavitt’s truncated research, the Danish astronomer Ejnar Hertzsprung picked up where she left off. In 1913, he devised a statistical model using known Cepheids in the Milky Way to calibrate Leavitt’s period-luminosity graph. From that, he calculated the first intergalactic distance, to the Small Magellanic Cloud, thereby fulfilling the momentous promise of her work.

  Yet Leavitt’s desire to pursue further research on the variables never left her. Soon after Pickering’s death in 1919, she at last divulged her interest to the observatory’s soon-to-be director, Harlow Shapley. But just as she was on the verge of completing her prolonged stellar-magnitude project—when she might have at last returned to her work on variables—Henrietta Leavitt passed away, at the age of fifty-three. She had endured a grueling struggle with stomach cancer. By the time of her death, on December 12, 1921, she had discovered some 2,400 variable stars, about half the number then known to exist.

  Unaware of Leavitt’s passing, a member of the Royal Swedish Academy of Sciences four years after her death contacted the Harvard College Observatory to inquire about her discovery, intending to use the information to nominate her for a Nobel Prize in Physics. By the rules of the award, however, the names of deceased individuals could not be submitted.

  Leavitt’s work certainly deserved the prize. By the time the Swede’s message had reached the Harvard College Observatory, her period-luminosity law had led to two momentous astronomical discoveries. It allowed Shapley in 1918 to demonstrate that our Milky Way was far larger than originally thought, with the Sun relocated away from the galactic center. And by 1923, Edwin Hubble spotted a Cepheid in the Andromeda nebula, which turned out to reside far beyond the borders of our galaxy. Leavitt’s law helped prove that the Milky Way is not alone in the universe but just one of many galaxies.

  CHAPTER TWENTY-FIVE

  The Cosmologist Left Behind

  Edwin Hubble usually gets the credit, but

  Vesto Slipher was the first to see the signs

  that the universe is expanding

  AT the end of the nineteenth century, the wealthy Bostonian Percival Lowell—the black sheep of one of New England’s leading families—built a private observatory atop a pine-forested mesa in Flagstaff, Arizona, to study Mars, its supposed canals, and its presumed inhabitants. There, some 1.4 miles above sea level, Lowell installed a 24-inch (61-centimeter) Alvan Clark refractor—not a very large telescope even for the time, but one perched higher than the 36-inch (91-centimeter) refractor at the venerable Lick Observatory in California.

  This pleased Lowell immensely, for he sought to outdo his California competitor at every turn. In 1900 he ordered a custom-built spectrograph that was an improved version of the one at Lick. To operate this new instrument, Lowell hired a recent graduate of the Indiana University astronomy program: an Indiana farm boy named Vesto Melvin Slipher.

  Lowell chose well. Slipher took a spectrograph intended for planetary work and with great skill eventually extended the observatory’s work far beyond the solar system. Instead of discerning new features on the Red Planet, the observatory’s raison d’être, Slipher found himself confronting a surprising aspect of the wider cosmos, previously unknown. He detected the very first hint—the earliest glimmers of data—that the universe is expanding. But it took more than a decade for astronomers to fully recognize what he had done.

  Vesto Slipher.

  (From the Lowell Observatory Archives)

  A century ago, when one-third of Americans lived on rural farms lit by only candle or kerosene, the nighttime sky was breathtaking. The Milky Way arched across the celestial sphere like an army of ghosts. This sublime stellar landscape must have been a powerful inspiration, for many of America’s greatest astronomers a century ago were born on Midwest farms, like Slipher.

  “V. M.,” as he was known to friends, must have had qualms upon arriving at Flagstaff in the summer of 1901. The biggest telescope he had ever operated was a 4.5-inch (11-centimeter) reflector. The young man struggled for a year to handle the spectrograph with ease. He even initially confused the red and blue ends of the spectrum on its black-and-white photographic plates, a scientific faux pas of the first magnitude. In distress, Slipher asked Lowell if he could go to Lick for instruction, but his boss firmly said no. Given the rivalry between the two observatories, Lowell didn’t want Lick knowing that one of his staff needed help.

  Slipher and Lowell were an intriguing mesh of personalities, like a harmony created from two different notes. Flamboyant and aggressive, Lowell hated to share the spotlight. Slipher was, fortunately, Lowell’s opposite in character. A modest and reserved man, he knew it wasn’t wise to steal Lowell’s thunder. More than that, he didn’t want to.

  Slipher made progress on the spectrograph, eventually becoming a virtuoso at its operation. By 1909 he was able to confirm that thin gas existed in the seemingly empty space between the stars; it left spectral lines in starlight that were narrower and at slightly different Doppler shifts than the spectral lines arising in the stars’ atmospheres. This triumph won him praise from astronomers around the world. In 1912 he determined that the faint Merope Nebula in the Pleiades had the same spectrum as the Pleiades stars themselves, the first proof of a reflection nebula made of interstellar dust (“pulverulent matter,” he called it). In due course these pursuits led Slipher to his greatest discovery of all.

  It began innocently enough. On February 8, 1909, Lowell in Boston sent a typed letter to Slipher with concise instructions: “Dear Mr. Slipher, I would like to have you take with your red sensitive plates the spectrum of a white nebula—preferably one that has marked centres of condensation.” By “white nebula” Lowell meant what we now call a spiral galaxy. At the time, however, many astronomers assumed that these spiraling nebulae were nearby planetary systems under construction. Lowell stressed that he wanted “its outer parts.” He longed to see if the chemical elements at a spiral nebula’s edge, as revealed by their spectral lines, matched the composition of the giant planets in our outer solar system. A connection would mean the spirals could indeed be the precursors of planetary systems.

  Slipher balked at first. “I do not see much hope of our getting the spectrum,” he told Lowell. Photographic emulsions in 1909 had extremely slow speeds. Slipher knew that it would take at least a thirty-hour exposure to take just an ordinary photograph of the nebula with the long-focus refractor. To acquire a spectrum—what with light being lost in the spectrograph and the remaining light being spread out into a strip—seemed impossible.

  Although Slipher considered the task hopeless, he persevered and by December 1910 was able to wrench some feeble data from the Great Nebula in Andromeda (M31). “This plate of mine,” he informed Lowell by letter, “seems to me to show faintly peculiarities not commented upon.” He was convinced that he had captured something on the spectrum previously unseen by other spectroscopists.

  By trial and error, Slipher made improvements to the spectrograph. Instead of using a set of three prisms, which separated spectral lines widely, he decided to use just one, which reduced the light loss and also spread out the light less on the plate. He also understood that increasing the speed of the system was vital; he bought a very fast, commercially available photographic lens to go ahead of the plate.

  Planet studies and reports on the return of Halley’s Comet kept Slipher from getting back to the Andromeda Nebula until the fall of 1912. But by then his refas
hioned spectrograph was operating two hundred times faster than its original specifications, allowing him to slash his long exposure times. He could at last try for the spectrum he had so long sought.

  Slipher made his first exposure with the new system on September 17. It took a total of six hours and fifty minutes for Andromeda’s faint light to fully register. “It is not really very good and I am of the opinion that we can do much better,” he relayed to Lowell. He soon acquired two more spectra. When carrying out these observations, the interior of the wooden dome at times could resemble the movie version of a mad scientist’s laboratory, with a high-voltage induction coil sparking and sputtering by the side of the telescope. A row of old-fashioned Leyden jars served as capacitors to juice up the sparks. This contraption served to vaporize traces of iron and vanadium inside the spectrograph; the light of the sparks passed through the spectrograph and onto the photographic plate. The known emission spectra of the vaporized elements provided the calibration lines needed to measure the exact wavelengths of the absorption lines in the nebula’s spectrum.

  Each spectrum that Slipher produced was tiny: just a centimeter long and a millimeter wide. He needed a microscope to measure how much the spectral lines might have been Doppler-shifted from their rest wavelengths. The microscope was with Lowell in Boston temporarily, and Slipher didn’t get it back until mid-December. But once it arrived, he couldn’t resist taking a quick peek at the Andromeda plates he had so far. There were “encouraging results or (I should say) indications,” Slipher reported to Lowell, “as there appears to be an appreciable displacement of the nebular lines toward the violet.” A shift of the lines toward the blue-violet end of the spectrum meant that Andromeda was moving toward Earth.

  But Slipher felt he needed a better spectrum to measure the speed accurately. He started the final exposure on December 29 and stayed with it until clouds rolled in near midnight. On a seeing-quality scale from 1 to 10—1 being the worst, 10 the best—Lowell astronomers often joked that at 10 you can see the Moon, at 5 you can still see the telescope, and at 1 you can only feel the telescope. Fortunately, the sky was clear the following night, and he was able to collect additional light for nearly seven more hours. Perhaps pressing his luck, he went into a third night, New Year’s Eve.