In some ways, the 20th century could be described accurately as the century of invisible astronomy. Previous astronomers had concerned themselves with the luminous objects in the heavens, such as stars and bright nebulae. But such objects represent only a small fraction of al the matter in the universe. It’s ironic that the exploration of the work of an astronomer who was widely regarded as having the keenest vision of any then alive, Edward Emerson Barnard.
Bernard grew up in poverty in Nashville, Tennessee, just after the Civil War, but at the age of nine he was apprenticed at a photographic gallery. Soon afterward he became interested in astronomy, later making a name for himself as a successful discoverer of comets. He went on to a career as a professional astronomer at Lick and Yerkes observatories. Above all, he was able to apply the skills he had acquired in his many years at the photographic gallery to his passionate pursuit of abstronomy.
Photography proved one of the most important legacies of the old century for the new. Indeed, the great researches of modern astronomy would have been impossible without it. In the early days of wet-process plates, astronomical photography had been useless, but by the 1880s dry plates were coming into general use. In 1882, both the transit of Venus and a spectacular sun-grazing comet were recorded for all time on photographic plates.
At Lick Observatory in the 1890s, Barnard used an antique portrait lens to pioneer wide-angle photography of the Milky Way and comets. He puzzled over the dark markings in the Milky Way that appeared on his plates, were they actual holes in the heavens, as William and John Herschel had believed, or clouds of dark matter obscuring the distant starts? Haunted by the question, in January 1905 he took a specially designed photographic telescope to California’s Mount Wilson and spent seven months feverishly photographing the Milky Way. Barnard’s hours of work would have horrified any medical man. One of his colleagues later recalled that Barnard considered sleep a sheer waste of time, and for long intervals he would forget it altogether.
Barnard was the Ansel Adams of the Milky Way; in some ways, his photographic plates have never been surpassed. They splendidly reveal the large dark regions in Ophiuchus and Scorpius that appear almost entrirely free of stars. After a long struggle with himself, he concluded that these were indeed places where the light of the distant stars was being absorbed by obscuring clouds of dark matter.
Robert J. Trumpler of the Lick Observatory proved decisively in 1930 that these were, in fact, dust clouds along the plane of the Milky Way. Barnard, however, will always be remembered as the dark matter pioneer. Even today, a grat deal of dark matter rermains to be accounted of. Not all of it is the obscuring dust that Barnard discovered, and it seems increasingly likely that much of it will prove to be exotic matter different from the kind that forms stars. Earth, and even ourselves. In the entire universe, probably less than ten percent of the matter consists of that which we can actually see.