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By Root 429 0

recently taken a picture of the spiral nebula M101, the Pinwheel, which he had also photographed in 1910. With both images in hand, he asked van Maanen to put the plates into his trusty stereocomparator and see if any changes could be detected in the nebula over those intervening years. Van Maanen at first measured no variation but got permission from Ritchey to keep the plates to study them further. Adapting methods he had used previously in other work, he chose thirty-two stars, equally bright and positioned uniformly around the nebula on each plate, and measured how dozens of points within the spiral nebula may have shifted in comparison to those stars. Extending his study, he borrowed additional plates of M101 from the Lick Observatory, photographs taken in 1899, 1908, and 1914. He didn't rush. Van Maanen was so meticulous that he even made sure the temperature in the room where he was making his measurements was tightly controlled. He wanted to take care that any thermal expansion, in either the glass photographic plate or the measuring machine, would be negligible.

In the end, he decided that the nebular material within M101 was moving after all, although exactly how was not immediately obvious. “If the results…could be taken at their face value, they would certainly seem to indicate a motion of rotation or possibly motion along the arms of the spiral,” he reported. If turning at his measured rate, M101 was completing one full rotation every eighty-five thousand years. As noted earlier, that meant if the Pinwheel were truly the size of the Milky Way and located way off in distant space, the nebula's edge had to be traveling faster than the speed of light, an impossibility given Einstein's special theory of relativity, which said that no bit of matter can move fast enough to overtake a beam of light.

Given what was at stake, van Maanen followed all the precautions: He switched the plates in the holders to eliminate machine error, and he got a colleague to redo the measurements with a different machine, to make sure there wasn't an instrumental error or personal bias. He came to believe that the matter within the spiral was drifting away from the center—outward along the arms—and noted in his report that this agreed with the Chamberlin-Moulton model on the origin of spiral nebulae, which involved a collision between a star and a nebula. Thomas Chamberlin was elated to hear the news from Hale. “While the recent revival of the notion that spiral nebulae are mere distant constellations has not seemed to me to have any substantial basis, it is a satisfaction to feel that definite evidence is about to give it a quietus,” he responded.

Van Maanen was aware that his work “might indicate that these bodies are not as distant as is usually supposed to be the case,” but he kept that speculation out of his early reports. That's partly because in 1917 he measured a rotation for the Andromeda nebula with error bars larger than his result. “So that we do not know yet if this is an island universe!” he told Hale.

But that was the exception. Van Maanen primarily got the answer that many expected: Spiral nebulae exhibited internal motions and so must be relatively nearby. Moreover, the announcement was being made by a widely respected astronomer working at the world's premier observatory, whose expertise in stellar measurements was lauded. “His wide experience in astrometric work,” Walter Adams later recalled, “gave his conclusions a high standing among astronomers.” Other observers even confirmed that the spirals were changing; concurring reports came out of Mount Wilson, the Lowell Observatory, and observatories in both Russia and the Netherlands. It became the conventional wisdom among astronomers. And why not? It fit the general opinion of the time.

Only a few, such as Heber Curtis, openly disagreed. Curtis, with his wealth of spiral nebulae photographs at Lick, had earlier attempted to measure a change in the spirals over the years but could only conclude that “a much greater time interval will probably be necessary before nebular