The Day We Found the Universe - Marcia Bartusiak [48]
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.
For most of his career Slipher published few detailed papers of his work, outside of his observatory's in-house bulletin. He either sat on his data until he was absolutely sure of the results or generously sent his findings to others to use in their analyses. Part of this might have been a reaction to the rumpus the observatory faced whenever Lowell defended his more sensational findings. Slipher inwardly feared that the unwelcome