Blog #16: The Great Debate (Shapley-Curtis Debate)

Harlow Shapley (left) and Heber Curtis (right)
Source: http://education.ezinemark.com/top-10-science-debates-in-history-773690137c70.html

On April 26, 1920, in the halls of the Smithsonian Museum of Natural History stirred one of the greatest debates in astronomy occurred. On the one hand, we had Harlow Shapley, who argued that the Milky Way galaxy encompassed the entirety of the Universe, and all other observed galaxies were simply "nebulae" within the Milky Way. Opposing Shapley, Heber Curtis argued that the Milky Way was not the entirety of the Universe, and that the Andromeda galaxy, the Pinwheel Galaxy, and all other observed galaxies were separate entities, sort of like "mini-universes" in their own right.

At the heart of this debate was the question of the true scale of the Universe. Shapley asserted that Andromeda could not have been a separate galaxy because otherwise, by its apparent size and magnitude, it would have to be $10^8$ light years away. That distance seemed outlandish because it was further away than any other object in the Universe (based on the understanding at the time). Furthermore, astronomer Adriaan vas Maanen claimed that he measured what is now known as the Pinwheel Galaxy to be rotating within a timescale of years. Extrapolating the rate of rotation in the circumstance that the Pinwheel Galaxy was a separate galaxy would require the orbital velocity to be faster than the speed of light, thus defying the laws of physics. Finally, Shapley argued that he had observed a nova in the Andromeda "nebula" that outshone the entire nebula. Considering its brightness relative to all of Andromeda, if Andromeda was a separate galaxy, the amounts of energy release in the nova would be unimaginable.

Heber Curtis, however argued that it was strange that there were more "novas" inside the region marked as Andromeda, than at any other part of the Milky Way. This would suggest that Andromeda must be it's own galaxy far away. Curtis also cited dust clouds found in other galaxies similar to those in the Milky Way. Finally, he observed massive doppler shifts in objects he thought were galaxies, thus giving these galaxies a signature age that was different from that of the Milky Way.

However, the argument presented by Adriaan vas Maanen regarding the observation of rotating pinwheels in the Pinwheel Galaxy seemed to be the arbiter of the debate, since Curtis agreed that if the Pinwheel Galaxy was indeed rotating within an observational scale of years, then it would suggest that the orbital velocity of the galaxy was faster than the speed of light, and therefore his hypothesis would be wrong. It was later observed that, in fact, the rotation of the Pinwheel Galaxy could not be observed within the lifespan of humans, thus giving credibility to Curtis' argument.

Curtis' arguments were reconsidered when Edwin Hubble discovered Cepheid Variables in Andromeda and other "nebulae", which were calculated to be much further away than the Milky Way, suggesting that the Universe was, in fact, comprised of multiple galaxies, not just the Milky Way. Shapley's argument regarding the massively bright nova in Andromeda as having too much energy to be feasible was debunked when the existence of supernova was proven. Supernova do outside their respective galaxy, and are high-energy events measured in scales unimaginable in the early days of astronomy.

This debate provided the platform to discuss the "Scale of the Universe" to get a better insight of the immensity of the Universe we reside in. This debate showed how different points of view in astronomy were formulated based on the best information available at the time. Now we know that the Universe is filled with hundreds of billions of galaxies, and the Milky Way is just one of them.

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