The Day We Found the Universe - Marcia Bartusiak [21]
And something more surprising developed as this work progressed. Each time Keeler took a photograph, he found even fainter nebulae loitering in the background of his image. At the start of his venture, when he first saw the seven nebulae on his plate of M51, he thought it “a rather remarkable number of nebulae to be found on a plate covering only about one square degree.” That's a segment of the sky the size of two full Moons. But he soon discovered that this celestial flock wasn't so remarkable after all. With each additional picture Keeler took, he detected more and more nebulae arrayed over the heavens. Throughout the fall of 1899, whenever the nighttime sky was clear and moonless, he made his way to the Crossley and kept adding to his count. He took a four-hour exposure of NGC 891, a spiral seen edge-on, and the plate revealed thirty-one new nebulae, scattered around the central spiral like background extras in a movie scene. On a photograph of NGC 7331 he saw twenty more and “there are nearly as many on several other plates,” he reported. “Besides these new nebulae…the plates contain a considerable number of objects which are probably nebulae so small that the resolving power of the telescope is insufficient to define them in their real form and to bring out their true character.”
Keeler was dumbfounded. Space was awash with tiny nebulae, and most of them displayed a conspicuous spiral form, though seen from assorted angles. “There are hundreds, if not thousands, of unrecorded nebulae within reach of our 36-inch reflector,” reported Keeler. By assuming that there were three new nebulae in each square degree (a number he admitted was far too conservative), he estimated that “the number of new nebulae in the whole sky would be about 120,000.” He was positive there were more. Before this, about nine thousand nebulae had been cataloged by astronomers but only seventy-nine were identified as spirals, less than 1 percent. The Yerkes Observatory, in Wisconsin, by then had opened to great fanfare with a bigger telescope, one with a lens forty inches in width, but it still could not compete with Keeler's reflector. Even Barnard conceded that his new home at Yerkes, situated at a more lowly thousand feet above sea level, was “a mirey climate for a great telescope and discoveries are few and far between.”
Keeler's 1899 image of NGC 891 with background nebulae marked
(Copyright UC Regents/Lick Observatory)
In an article in Astronomische Nachrichten, a highly respected German astronomical journal, Keeler drew attention to his baffling finds: “The spiral nebula has been regarded hitherto as a rara avis—a strange and unusual phenomenon among celestial objects, to be viewed by the observer with special interest, and marked in catalogues with exclamation points… But so many other nebulae also proved to be spirals that the classification…soon lost its significance… The same form occurs over and over again, on a smaller scale, among the fainter nebulae.” Spirals were now the norm, not the exception, in the celestial sky. Keeler figured they must be an important constituent of the universe, ranging in size “from the great nebula in Andromeda down to an object which is hardly distinguishable from a faint star disk.”
But what in blazes was a spiral nebula? No one knew for sure, solely because there was as yet no way to determine the distance, a recurrent problem for astronomers. If the spirals were nearby, part of the Milky Way, then they would be relatively small given their size in the sky, each possibly a new star forming. But if the spiraling patches were very far away, then they would have to be huge to appear as they did in telescopic photographs, as big as the