Blog Post 16, The Great Debate

The Shapley-Curtis Debate, also known as the "Great Debate," occurred in 1920 between Harlow Shapley and Heber D. Curtis, both American astronomers attempting to understand our place in the universe.

Shapley (left) and Curtis (right)

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

Curtis's model of the universe argued that the universe is composed of many galaxies, of which the Milky Way is only one of many. Within the Milky Way, the Sun is relatively near the center of the galaxy. His main arguments included:

• The Galaxy is relatively small (compared to Shapley's model), with a diameter of only 30,000 light years. 
• Spiral-shaped areas of light that had been observed by telescopes, known as "spiral nebulae," are galaxies themselves, located outside of the Milky Way. 

Curtis's model were based on observations of the optical spectrum of the "spiral nebulae" - he observed that they had very similar spectra to that of the Milky Way, and thus must actually be galaxies themselves.

Shapley's model argued that the entire universe is composed of only our galaxy, the Milky Way. Within this galaxy, the Sun was very far from the center. His main arguments included:

• The entire Galaxy/Universe is approximately 300,000 light years in diameter. 
• Sun is far from the center of the galaxy. 
• "Spiral nebulae" are actually gas clouds within the Milky Way. 

Shapley's ideas were based on observations made of globular clusters. He used the known distance to a particular globular cluster (M13) to estimate the distance to other globular clusters based on each cluster's apparent size, on the assumption that each cluster is roughly the same size.

Using the discovery made by Henrietta Swan Leavitt that the luminosity and period of Cepheid variable stars - stars in which their luminosity periodically brightens and then dims - positively correlate (as the luminosity of the star brightens, the longer the period), Shapley estimated the distance to the Small and Large Magellanic Clouds. He determined that because they were at a distance much within his estimated diameter of the Milky Way, they must be contained within the Milky  Way.

In reality, neither astronomer was entirely correct, nor entirely wrong. In 1923, Edwin Hubble used the 100-inch Hooker Telescope on Mt. Wilson to locate Cepheid variable stars within M31, the Andromeda Galaxy. Hubble determined that the distance to Andromeda was actually around 1.2 million light years - and thus definitely outside of the Milky Way. Curtis was thus correct in arguing that the universe is composed of many galaxies. However, as we know now, Curtis was much more correct than Shapley in determining the location of the Sun within the Milky Way - the Sun is considerably far from the center of the galaxy, at a distance of approximately 8 kpc.