The Great Debate: Blog 16

Like many discoveries in science, astronomy discoveries have not come into modern cannon uncontested. Many of our now-accepted theories began out of scientists trying to understand nebulous (get it?) data collected with limited technology. The question of the scale of the universe, which is still debated today, was a hot topic in 1920. 

"The Great Debate" as it has been named, took place between two astronomers on April 26 1920. The "contestants" were Harlow Shapley, a young upcoming astronomer at Mt. Wilson Solar Observatory, and Heber D. Curtis, an older, more established professor at the Lick Observatory. Shapely argued in favor of the Milky Way being the entire scale of the universe while Curtis argues in favor of the universe being composed of multiple, separate galaxies. The debate centered around "nebulae" in the Milky Way, such as Andromeda, and whether on not they were a part of our galaxy or separate entities.

Shapley and Curtis - Source

Both scientists had previously published their findings on the scale and composition of the universe in different journals. Curtis's research focussed on star count analysis and spectral distance estimates. He concluded that the Milky Way is about 10 kpc in diameter and lens shaped. He found the nebulae observed had a similar number of novae to the rest of the galaxy, and therefore must be their own galaxies. On the other hand, Shapley argued for a model where one massive Milky Way (100 kpc across) composed the entire universe. He looked at Cepheid variables in globular clusters to make distance determinations. Shapely used the already known distance to the M31 cluster to estimate the relative distances to other clusters and determined them to be at the edge of the Milky Way. Additionally, the data of Adriaan van Maanen, a friend of Shapley's at Mt. Wilson Observatory, found that the Pinwheel galaxy was rotating so quickly that you could observe one rotation in a matter of years. If this galaxy lay beyond the Milky Way, this would mean the galaxy was rotating so fast that it violated the speed of light! This provided further evidence for Shapley's case.

Source

On April 26, 1920 both scientists gave separate talks proposing their ideas during the day and came together for a discussion at night, where they could provide counter arguments to each other's points. Who "won" is unclear. There is a common misconception that Shapely won, as he became more famous, though many scientists present agreed with Curtis's argument.

Today we know that both scientists were right in some ways and wrong in others. Shapely got the order of magnitude of the universe correct and our relative placement in it. Curtis correctly concluded that the "nebulae" were other galaxies. This was confirmed when Edwin Hubble studied Cepheid variables in Andromeda and concluded it lies outside of the radius of the Milky Way. Alas, Maanen's measurements were proven to be incorrect, the rotation of the Pinwheel galaxy cannot be measured in years and the high recessional velocities observed are actually evidence for the expanding universe, a debate that occurred  a few years later.


Sources:
http://cosmos.phy.tufts.edu/~zirbel/ast21/handouts/Curtis-Shapley.pdf
http://apod.nasa.gov/diamond_jubilee/papers/trimble.htmlhttp://astronomy.nmsu.edu/geas/lectures/lecture27/slide01.html
http://astronomy.nmsu.edu/geas/lectures/lecture27/slide01.html