The Day We Found the Universe - Marcia Bartusiak [50]
“It has for a long time been suggested that the spiral nebulae are stellar systems seen at great distances,” said Slipher at the April conference in Philadelphia. “This is the so-called ‘island universe’ theory, which regards our stellar system and the Milky Way as a great spiral nebula which we see from within. This theory, it seems to me, gains favor in the present observations.” With all but four of his twenty-five spiral nebulae racing outward, Slipher speculated at one point that the spirals might be “scattering” in some way, a precocious intimation of the cosmic expansion that took many more years to fully recognize.
Though other astronomers were confirming a few of Slipher's results, the Lowell Observatory astronomer was the absolute ruler in this new celestial realm. He dominated the field for years. By 1925, forty-five spiral nebulae velocities were pegged with assurance, and it was Slipher who had measured nearly all of them. As early as 1915, researchers in Germany, Canada, the United States, and the Netherlands began to look for a pattern in Slipher's growing mound of data. It was an extremely difficult task, though, as the speeds measured for the spiral nebulae were entangled with other velocities, such as Earth's orbital travels and the Sun's journey through the galaxy. It was like trying to determine the exact speed of a train off in the distance, while you yourself are in a car racing down a highway.
The investigators began by subtracting out the extra factors—first the Earth's motion, then the Sun's—to see how fast the spiral nebulae were truly moving. Once these secondary velocities were removed, the astronomers saw that the nebular speeds continued to be enormous, far higher than the average velocity of a star within our galaxy. More important, they confirmed that the mistlike disks were indeed generally headed away from us. A few nebulae, such as Andromeda, were exceptions (they didn't yet know that Andromeda and the Milky Way were gravitationally bound together and so wouldn't be flying away from each other), but all in all the spiral nebulae were primarily moving outward into space in all directions. The German astronomer Carl Wirtz went even further in 1922 by looking at a nebula's size and luminosity to roughly judge which of the nebulae were closer to us and which were farther out. By making this assumption, he noticed a particular progression to the stampede outward: The more distant the nebula, the faster it was receding. That was intriguing.
But perhaps this relationship between speed and distance was a false impression. Maybe the effect would disappear as the velocities of more and more nebulae, especially those found in the southern celestial sky, were measured. It could all average out: half of the nebulae moving toward us, the others away. Astronomers began to worry that what looked like an overall recession might turn out to be a temporary illusion. To take care of this, they began to insert a special component into their equations, a term they labeled K, which kept track of the trend. Maybe this term would eventually fade away, but maybe not.
Despite these loose ends, by the time of the 1917 American Philosophical Society meeting, the island-universe theory was rousing from its slumber. Heber Curtis had begun to publish his findings on the spiral nebulae in the major journals, and his cogent arguments in support of distant galaxies were already convincing the top astronomers who counted, such luminaries as Eddington at Cambridge University, in England, Campbell at Lick, and Hertzsprung, then at the Potsdam Observatory, in Germany. The swift velocities that Slipher was finding only strengthened the idea that the spirals were indeed situated far beyond the Milky Way's borders. But success could not be fully grasped until astronomers figured out a method for determining how far away Andromeda and its sister spirals truly were. Nothing could ever