The Day We Found the Universe - Marcia Bartusiak [89]
But Curtis's warning was not heeded. With hindsight, it now seems easy to dismiss van Maanen's measurements. But at the time it was extremely difficult to assess. Van Maanen was no slouch at telescopic measurements and his finding a spiral rotating appeared quite reasonable. One of the era's leading theorists, the Britisher James Jeans, was especially eager to jump on van Maanen's bandwagon. Upon hearing the Dutch astronomer's results, Jeans speedily sent off a letter to the journal Observatory, saying they were “entirely in agreement with some speculations in which I have recently been indulging.” In calculating the behavior of a blob of gas, rotating and condensing, he had determined that tidal forces would lead to the formation of spiraling arms. And now Van Maanen was providing the observational evidence to back him up. Jeans eventually wrote up his ideas in the book Problems of Cosmogony and Stellar Dynamics, which exerted a tremendous influence on astronomers at the time. Moreover, both van Maanen and Jeans began to calculate higher masses for the spirals. So, instead of a single solar system in the making, they began to think of a spiral nebula as the start of a dense (but still small) cluster of stars.
As more plates became available, van Maanen expanded his study to include other spirals. He measured a rotational period of 160,000 years for M33 (Triangulum), 45,000 years for M51 (Whirlpool), and 58,000 years for M81, a handsome spiral in the Ursa Major constellation. Other nebulae followed. All were rotating in such a way that the spirals appeared to be unwinding their arms, spreading them farther outward. He figured the spirals were no more than several hundred light-years wide and ranged in distance from one hundred to a few thousand light-years away.
Soon van Maanen was running out of spiral nebulae to measure, as few had been regularly photographed for comparison over the years. As a double check on his dexterity with the Blink, he measured a simple globular star cluster, M13, which was known not to rotate. If there were any instrumental error, he should have mistakenly measured a motion, but he didn't, which seemed to imply his methods were valid. A British astronomer independently checked his methods as well and concluded that no one “would be so bold as to question the authenticity of the internal motions…. In fact, the more one studies [van Maanen's] measures, the greater is the admiration which they evoke.”
Adriaan van Maanen's markings on a photo of M33 indicating the
rotation he measured (From Astrophysical Journal 57 [1923]:
264-78, Plate XIX, courtesy of the American Astronomical Society)
“I finished…my measures of M51,” van Maanen wrote Shapley in the spring of 1921. “The results look more convincing than M101… Motion outwards along the spirals + some motion away from the center…. By this time Curtis and [Swedish astronomer Knut] Lundmark must be the only strong? defenders of the island-universe theory.”
“Congratulations on the nebulous results!” responded Shapley. “Between us we have put a crimp in the island universes, it seems,—you by bringing the spirals in and I by pushing the Galaxy out. We are indeed clever, we are.” Shapley reported on his friend's latest results at that summer's American Astronomical Society meeting in Connecticut. “I think that your nebular motions are taken seriously now,” he told van Maanen afterward, “and nobody…dared raise his head after I explained how dead the island universes are if your measures are accepted.