Blog #16, The Great Debate, Shapley vs. Curtis

The Great Debate in astronomy was an argument between astronomers Harlow Shapley and Heber Curtis in 1920. The debate encompassed two major issues of the time including the size of our own milky way galaxy and whether there was more than 1 galaxy in the universe. Shapley argued that there was only one massive galaxy in the solar system with millions of stars. He argued that our sun was only part of this huge galaxy and by no means in the center of the galaxy. Curtis on the other hand believed that our Milky Way was one of just many galaxies, or Spiral Nebulae, in the universe but that our sun was at the center of the Milky Way. Shapley believed these other "galaxies" that Curtis observed were simply large clouds of gas within our own very large (Shapley believed the Milky Way to be 300,000 light years in diameter) galaxy. 

Shapley based his claims on the Magellanic Clouds and globular clusters. He believed that by finding the magnitude of a cepheid star, he could approximate the distance of all observable globular clusters and thus prove that they all fell within the supposed boundaries of the Milky Way. With the help of astronomer Adriaan van Maanen, Shapley was able to show that the galaxy M101 had an angular speed and orbital speed that would make it impossible to lie beyond the known size of the Milky Way, thusly "proving" his theories. 

Curtis agreed with Shapley on the idea that the globular clusters surrounded the core of the galaxy, however his estimate showed that the milky way was only 30,000 light years in diameter, only one tenth of Shapley's estimate. Using this claim, Curtis said the luminosity argument that Shapely brought up proved that the observed Galaxies must be "island" galaxies, completely separate and free from our Milky Way. 

The debate was settled in 1923 when Edwin Hubble was able to use a newly constructed 100 in.  telescope on Mt. Wilson to prove that the cluster of stars M31 was over 1.2 million light years away, much further than even Shapley's size of the Milky Way. This proved that Curtis was right about the existence of multiple (one hundred billion) galaxies. However, Shapley was correct about the Sun being on the outskirts of the Milky Way, and his luminosity-period relationship in Cepheid stars was actually correct. It turns out the van Maanen's calculations were simply wrong, leading to the faulty arguments on Shapley's part. 

One interesting take on The Great Debate was how the situation went on to push astronomers to be much more precise and careful with their claims moving forward. Both Shapley and Curtis made incorrect scientific statements with very little hard evidence to backup their claims. After the great debate, most astronomers tried to be much more thorough in their discoveries. 

Sources: 

http://apod.nasa.gov/htmltest/gifcity/cs_why.html

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

http://img.ezinemark.com/imagemanager2/files/30004252/2011/01/2011-01-20-22-58-37-1-whether-the-debates-result-went-both-scientist.jpeg

https://upload.wikimedia.org/wikipedia/commons/f/ff/Andromeda_galaxy.jpg