The Day We Found the Universe - Marcia Bartusiak [37]
This was at a time when Yerkes astronomer E. E. Barnard was also acquainting astronomers with myriad “dark nebulae” within the Milky Way. Barnard was gathering exquisite photographic evidence that the coal-black regions within the Milky Way that appeared to be devoid of stars (“holes in the heavens,” Herschel called them) were actually clouds of cosmic gas and dust—colossal streams of inky darkness without the hint of a glow. Curtis immediately connected this finding to his work: The dark lanes he was sighting in the spirals had to be “due to the same general cause that produces certain occulting effects in our own galaxy….” The dark bands were almost certainly matter—but matter that wasn't glowing.
This also explained why no spiral was ever seen in certain areas of the celestial sky, aptly named the “zone of avoidance.” Spiral nebulae were very exclusive objects; they tended to huddle around the north and south galactic poles, as if shunning the long white swath of the Milky Way. Astronomers had long scratched their heads over this peculiar distribution. If spirals were truly the birthplaces of new stars, why weren't they found in the richest star fields? Why were the spirals found in only those sectors of the sky where stars were scarce? Not one spiral had ever been spotted in the thick of the Milky Way. Curtis cleverly deduced that this cosmic quarantine was only an illusion: If his dark-banded spirals were truly distant galaxies, then the Milky Way, too, must have its own dark band. All the dark gaseous clouds within the Milky Way were collectively acting like an opaque wall, making it impossible to see the spirals that resided beyond this obstruction, keeping the spirals hidden. “[The] great band of occulting matter in the plane of our galaxy … serves to cut off from our view the distant spirals lying near the projection of our galactic plane in space,” explained Curtis. And that couldn't happen unless the spirals were very far off.
To Curtis this argument made perfect sense, but he was presenting the idea at a time when most astronomers still thought of the vast expanses between the stars as a pristine emptiness and the Milky Way as transparent as a glass window. His reasoning wasn't as readily accepted as he had hoped.
Curtis spent much of the 1910s on this fight—gathering data, giving lectures, coming up with fresh new arguments. He gathered clues as if he were a cosmic sleuth. “Were the Great Nebula in Andromeda situated five hundred times as far away as at present,” reasoned Curtis, “it would appear as a structureless oval…with [a] very bright center, and not to be distinguished from the thousands of very small, round or oval nebulae found wherever the spirals are found. There is an unbroken progression from such minute objects up to the Great Nebula in Andromeda itself; I see no reason to believe that these very small nebulae are of a different type from their larger neighbors.” But his mounting certainty that the spirals he photographed, both large and small, were all distant galaxies strewn through space was based solely on circumstantial evidence. He had convinced his colleagues at Lick, which came to be identified as a stronghold of island-universe supporters, but the majority of astronomers still preferred to think of all the stars and nebulae as inhabiting one great system, the Milky Way. Curtis was absolutely right, but convincing the wider community of astronomers was an entirely different matter.
And then something interesting…and very unusual…happened.
On July 19, 1917, some three hundred miles southeast of Mount Hamilton, George Ritchey was taking a routine photograph of a spiral nebula with the 60-inch reflector at the Mount Wilson Observatory. It was the fourth in a series of long-exposure photos he had been taking of NGC 6946 over the previous seven years. This