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he erected the “Great Lick Refractor,” a thirty-six-inch piece of glass mounted at the end of a jaw-droppingly long tube that remains to this day one of the most beautiful astronomical instruments ever constructed, a true concours d’elegance of science. But this telescope—one of the last of the great refractors ever built—was mainly employed in the study of planets and stars, which had come to consume astronomical careers. So when the observatory hired a young upstart astronomer who specialized in spectroscopy named James Keeler, he was sent across the valley to another peak where a secondary dome housed an inelegant workaday reflector telescope with a thirty-six-inch mirror and skeletal struts instead of a tube.

The transition between the old and the new—between the refracting lens and the reflecting mirror—was more than symbolic. (See figure 19.) The size of a lens is restricted by its weight because it can only be supported around the edge. Over time it may begin to sag and distort. A mirror, however, can be fully buttressed from beneath, and so a reflecting telescope can be made large enough to gather those precious few photons of light arriving from the far reaches of the universe. The Crossley, so named for the wealthy textile manufacturer who bought it in 1885 and then donated it to the Lick Observatory, had another advantage for the spectroscopist: glass lenses discriminately absorb some wavelengths more than others, limiting the scope and quality of spectroscopic analysis, whereas a mirror reflects all wavelengths equally, providing a truer portrait of the contents of the mysterious nebulae.14

One of the first long-term exposures Keeler made with the Crossley was of the controversial M51 Whirlpool galaxy, which stunned even the most conservative of astronomers with its obvious spiral shape implying motion, along with internal structure in the form of distinct arms. As an added bonus, the four-hour exposure revealed seven other previously unknown nebulae, hinting that there were many more out there than anyone had previously imagined. Over time the Messier catalog had been vastly surpassed by the New General Catalog (NGC), which featured thousands of nebulae. As Keeler wheeled the Crossley around the sky snapping long exposures of this and that NGC object, he began to see a pattern of flattened discs of spiraling arms swirling around a bright center. In the background were countless more not-yet-cataloged tiny splotches of light. It was what we would today call a fractal pattern: with each increase of magnification for a particular patch of sky, a similar pattern emerged of scattered nebulae behind the primary target of the viewfinder. Extrapolating from his data set, an average of three nebulae per square degree in the sky, Keeler estimated that there were at least 120,000 of these celestial sphinxes, but he privately suspected that there were many more than this, perhaps an order of magnitude more.

Figure 19. Lick Observatory’s Telescope and the Mysterious Nebulae It Revealed

a. The Crossley telescope at the Lick Observatory contains a thirty-six-inch mirror at the bottom and a secondary mirror at the top of its tube, that together reflect the focused light into an eyepiece or spectroscope on the side of the tube. Through this instrument James Keeler was able to image thousands of nebulae. PHOTOGRAPH BY THE AUTHOR.

b. One such nebula was NGC 891 (the 891st object in the New General Catalog of deep space objects), which, when examined more closely, was discovered to include many other nebulae, from which Keeler concluded that they are separate “island universes” outside of the Milky Way galaxy. The close-up image with individual nebulae identified with arrows and the three bright stars corresponds to the upper right corner of the wide-angle photograph of galaxy NGC 891. COURTESY OF THE LICK OBSERVATORY.

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Again, with hindsight we wonder how Keeler and his colleagues could not have immediately inferred spiral arms of countless stars at great distance, but the prevailing theory of star formation