Blog #16: The Great Debate

The Great Debate Between Shapley and Curtis

In 1920, Heber Curtis and Harlow Shapley, two notable astronomers at the time, debated the size and the constitution of the universe. Curtis argued that the universe contained many galaxies, whereas Shapley believed that the universe only contained one galaxy. They each based parts of their arguments on what were then called "spiral nebulae."

Consistent with his views of the universe's size, Curtis thought the spiral nebulae were located outside of our own Galaxy. He thought the Galaxy was small (compared to Shapley's estimate) and that the nebulae, which he believed to be about the same size as the Galaxy, were thus located closer to us than Shapely thought. He also believed that the nebulae were similar in composition to the Milky Way, based on their appearance and their optical spectra, which were the same as that of our Galaxy.

Shapley, on the other hand, argued that they were simply large collections of dust within our Galaxy. Shapley based this idea on a rough calculation of the distance to the Magellanic Clouds, which yielded a distance smaller than the diameter of the Galaxy, and on measured rotational speeds of other nearby objects that would have to be rotating faster than the speed of light if they were outside of the Galaxy (this measured rotation speed turned out to be incorrect).

Neither of the two can really be considered as the "winner" of the debate, since each was right on some points and wrong on other points. Curtis was correct in that the nebulae are external to the Milky Way, but Shapley was right in determining that our Galaxy is larger than previously assumed and that our Sun is not at its center.

Soon after the debate, Cepheids were identified in the Andromeda Galaxy, revealing that it was a galaxy similar to our own, and far enough away to be well outside of the Milky Way. The next decade, the understanding of interstellar absorption of light allowed for better understanding of the sheer scale of the Milky Way and the position of the Sun. We now have better technology, and our understanding of galactic and extragalactic distances, as well as our knowledge of other galaxies far from our own, has improved vastly.

Sources: http://www.nasonline.org/about-nas/history/archives/milestones-in-NAS-history/the-great-debate-of-1920.html

https://en.wikipedia.org/wiki/Great_Debate_(astronomy)

http://astronomy.nmsu.edu/geas/lectures/lecture27/slide01.html

http://apod.nasa.gov/diamond_jubilee/1920/cs_why.html

http://encyclopedia2.thefreedictionary.com/Interstellar+Absorption