The Day We Found the Universe (15 page)

He returned to the problem on December 3 and 4, when the Moon no longer rose at night to interfere with his observation of the dim nebula. This time, Slipher scribbled in his workbook that the transparency of the air was “
very good
,” underlining it for emphasis. Over the two nights he was able to gather his sparse photons for a total of thirteen and a half hours. The only problem that arose was a troublesome clock drive that took fifteen minutes to fix.

When carrying out these observations, the interior of the wooden dome at times resembled the movie version of a mad scientist's laboratory, with high-voltage induction coils sparking and sputtering by the side of the telescope. A row of old-fashioned Leyden jars provided the ignition. It was a wonder that Slipher didn't electrocute himself. This Rube Goldbergian contraption vaporized samples of iron and vanadium, whose light then served as a calibration for Slipher's measurement. The spectrum of these elements, at rest within the dome, could be compared to the spectrum of the nebula rushing around in space; the difference between the spectra determined the nebula's speed.

Since each spectrum that Slipher produced from Andromeda was so tiny, he needed a microscope to measure how much the spectral lines had shifted, compared to their positions on the calibrated standard. The more the shift, the higher the velocity of the nebula. The microscope had been with Lowell in Boston temporarily, and Slipher didn't get it back until mid-December. But once the scope 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 Andromeda would be moving
toward
Earth. “I congratulate you on this fine bit of work,” Lowell wrote back.

Vesto Slipher using the spectrograph mounted on Lowell Observatory's
24-inch refracting telescope
(Lowell Observatory Archives)

But Slipher felt he needed to acquire an even better spectrum to peg the exact speed. It was an endeavor, he told Lowell, that “would doubtless impress all these observers as a quite hopeless undertaking, and maybe it is, but I want [to] make an attempt.”

He started the final measurement on December 29 at 7:35 p.m. and stayed with it until some clouds rolled in near midnight. On a scale from 1 to 10—1 being the worst, 10 the best—Lowell Observatory 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 hours. Perhaps pressing his luck, he went into a third night, New Year's Eve. This time the weather was poor, and he had to finish up just before 1913 rang in. Yet, the additional attempt allowed him to squeeze one more hour of data onto his photographic plate.

Slipher had no time as yet to accurately measure this last plate, but he did a speedy check and right away knew that something was up. “I feel safe to say here that the velocity bids fair to come out unusually large,” he wrote Lowell right away. For Slipher to make such an impetuous claim at such an early stage was downright radical for a man normally so cautious. He must have been thrilled at what he had found.

Throughout January he focused on measuring all four of his plates more carefully, in order to gauge the velocity of Andromeda precisely. He did this by placing the plate of the nebula's spectrum in a “spectrocomparator,” which measured it against the standard spectrum—the rest frame. By turning a screw, he shifted one plate relative to the other. When the spectral lines at last matched, he recorded how much he had to shift the nebula plate to get it in line with the standard. The amount of shift established the velocity of the nebula. His calculations to convert the measured shift into a velocity filled page after loose page, with his figures neatly recorded in pencil. He started on January 7 and ended on the twenty-fourth.

The final result astonished Slipher. The Andromeda nebula was rushing toward Earth at the ridiculous speed of 300 kilometers per second (or around a million kilometers per hour), about ten times faster than Slipher had been expecting, given the average speed of a star in the Milky Way. Nebulae weren't supposed to act like this. Astrophysicists at the time generally believed that nebulae were rather slow cosmic creatures, plodding along at speeds far lower than stars. Instead, spiral nebulae seemed to be in a special class all to themselves. Andromeda was setting a cosmic speed record. In present-day terms, it's nearly forty times faster than a space shuttle in orbit.

Slipher, prudent as always, remeasured the plates he had just taken to make sure there was no error. He also sent a print of the spectrum to Edward Fath to obtain an independent check that the shift was real. In 1908, when Fath had taken his own spectrum of the Andromeda nebula at the Lick Observatory, he too had discovered a shift in its spectral lines. But at the time he simply wrote off the unexpected change as a likely malfunction of his spectrograph. It was the accepted wisdom that celestial objects simply did not move that swiftly. He heedlessly decided to brush aside the anomaly because, as he reported, “the shift has no direct bearing on the question for which an answer was sought.” Again, the hapless Fath missed his chance at making astronomical history. One can imagine his chagrin at receiving Slipher's print. He had seen the same spectral message as Slipher four years earlier, only to ignore it and not follow up.

By February Slipher came to trust both his instrument and his expertise (which in hindsight was truly incredible; today, with far better equipment, astronomers measure Andromeda approaching us at 301 kilometers per second, a difference from Slipher's rate of less than a third of a percent). Slipher informed Lowell that the plates “agree as closely as could be expected and I can not doubt the reality of the displacement.” Andromeda had to be moving at an astounding clip. Instead of announcing the result in a major astronomical journal, though, Slipher chose to publish his brief account—just nine paragraphs—in the
Lowell Observatory Bulletin
. True to form, Slipher held off on any grander statement until he had secured some confirmation.

Yet even one spiral nebula velocity was an exceptional accomplishment. Many were thrilled for Slipher. “It looks to me as though you have found a gold mine,” wrote Miller, “and that, by working carefully, you can make a contribution that is as significant as the one that Kepler made, but in an entirely different way.”

Max Wolf at the Königstuhl Observatory in Heidelberg admired the spectrum's “beauty.” Edwin Frost, then editor of the
Astrophysical Journal
, wrote his sincere congratulations at the revelation of such an “incredible” velocity. “It is hard to attribute it to anything but Doppler shift,” he said. “Your success on this object indicates the value of elevation above the sea…. It is a pity that someone cannot try other objects of this sort at elevations of 12,000 to 15,000 ft.” Astronomers would, but only decades later.

Then there were others, such as Campbell at Lick (predictably), who were highly skeptical. “Your high velocity for [the] Andromeda Nebula is surprising in the extreme. I suppose…the error of [your] radial velocity measurement may be pretty large. I hope you have more than one result for velocity.”

To be fair to Campbell, an extraordinary finding like this needed extraordinary proof, and Slipher knew that as well. He had already put out the call for others to try to confirm it. Within a year, Wolf was able to follow up. His spectrum was cruder but still in fair agreement. Soon after, even persnickety Lick Observatory came to confirm Andromeda's fleetness. Lick astronomer William H. Wright obtained a velocity that nearly matched Slipher's. “I had planned to get at this work years ago when Fath got his big displacement… but you seem to have beaten me to it,” Wright told Slipher.

Lowell was enormously pleased. “It looks as if you had made a great discovery,” he wrote, right after Slipher's initial finding. And then the director added, “Try some more spiral nebulae for confirmation.” Slipher took up the challenge with great enterprise, for he was better at following directions than initiating his own scientific pursuits.

Working on Andromeda, though, was a holiday compared to gathering the spectral light from other spirals. Though its center is barely discernible to the naked eye, Andromeda is still the biggest and brightest spiral in the nighttime sky. The others only get progressively smaller and dimmer, which made it even harder for Slipher to obtain their velocity. “Spectrograms of spiral nebulae are becoming more laborious now because the additional objects observed are increasingly more faint and require extremely long exposures that are often difficult to arrange and carry through owing to Moon, clouds and pressing demands on the instrument for other work,” he noted in his work papers. The job for him was “heavy and the accumulation of results slow.”

Slipher's first target after Andromeda was M81, a spiral that is brighter than most, and then he looked at a peculiar nebula situated in the Virgo constellation known as NGC 4594. In his notes, he described it a “telescopic object of great beauty.” It's now popularly known as the Sombrero galaxy for its distinctive resemblance to a Mexican hat viewed from the side. Slipher eventually saw that NGC 4594 was moving at a speed “no less than three times that of the great Andromeda Nebula.” This time, however, the nebula was not traveling
toward
Earth but instead was whisking
away
at some 1,000 kilometers per second. Slipher was greatly relieved. Finding a nebula that was racing outward rather than approaching removed any lingering doubts that the velocities might not be real. “When I got the velocity of the Andr. N. I went slow for fear it might be some unheard-of physical phenomenon,” he wrote his mentor Miller. Now, by the spring of 1913, he was reassured that the spectral shifts on his plates reliably meant
movement
.

At this stage, with just a few measurements in hand, Slipher began to think of the nebulae as drifting by the Milky Way—coming toward us on one side of the galaxy, and wandering away on the other side. He was reluctant to speculate publicly on what the spiral nebulae might be, but he did share some of his pet theories in private correspondence with his astronomer friends. At first he thought they might be dust clouds illuminated by reflected starlight, much the way he had already proven, to great acclaim, how the famous Pleiades star cluster shines. Or maybe, he went on to muse, the spirals were very old stars “undergoing a strange disintegration, brought about possibly by their swift flight through stellar space.” But, even then, he was beginning to have reservations about such interpretations. If the spirals were indeed single stars surrounded by fine matter, Slipher posed in one 1913 letter, why are spirals not “more numerous in, rather than outside, the Galaxy?” That was the very same question Curtis was starting to ask over at the Lick Observatory.

Throughout the succeeding months Slipher kept expanding his list, one spiral at a time. His accomplishment was all the more amazing, considering the relative crudeness of his instrument. Lowell Observatory's 24-inch telescope had only manual controls, ones that weren't yet sophisticated enough for fine guiding. Yet he had to hold the tiny image of each spiral nebula on the slit of the spectrograph with utmost care and steadiness for hours on end as the heavens progressively rotated above him. When asked years later how he was able to do this, Slipher replied dryly, “I leaned against it.” Given the faintness of his targets, his exposures often ran twenty to forty hours, which meant they extended over several nights, even weeks if there was unfavorable weather. And nothing could be done when the Moon was brightly shining. “With such prolonged exposures the accumulation of plates is not very rapid,” he informed Lowell, “but the results are worth while and encouraging,” so much so that Slipher was beginning to feel uncharacteristically possessive of his findings. “It is our problem now and I hope we can keep it,” he told his boss.

Slipher need not have worried. No one else could catch up to him. By the summer of 1914 he had the velocities of fourteen spiral nebulae in hand. And with this bounty of data, an undeniable trend was at last emerging: While a few nebulae, such as Andromeda, were approaching us, the majority were rapidly moving away.

For island-universe devotees this was great news. “My harty
[sic]
congratulations to your beautiful discovery of the great radial velocity of some spiral nebulae,” wrote Danish astronomer Ejnar Hertzsprung. “It seems to me, that with this discovery the great question, if the spirals belong to the system of the milky way or not, is answered with great certainty to the end, that they do not.” The speeds were simply too great for them to stay put within our home galaxy. But Slipher at this stage was still on the fence. “It is a question in my mind to what extent the spirals are distant galaxies,” he responded.