The Great Debate

The Great Debate in Astronomy was an argument in 1920 between Harlow Shapley and Heber Curtis over the size of the Milky Way, and in turn, the size of the Universe.


Harlow Shapley believed that all the visible universe was contained in the Milky Way which he thought was 120,000 parsecs in diameter, with the Sun 2/3 of the way from the center to the edge of the Milky Way. To him, all nebulae seen in the sky, were in the disk of the Milky Way and there was nothing beyond the Milky Way.


Heber Curtis (who was director at the Allegheny Observatory) thought that nebulae and stars could exist beyond the Milky Way and that the Universe encompassed more galaxies than just the Milky Way.


Edwin Hubble gave the proof of the size of the Universe being larger than the Milky Way when he found a Cepheid variable star in the Andromeda galaxy. By observing the star, and comparing its brightness to known Cepheids within the Milky Way, he was able to determine that the Andromeda galaxy was about 778,000 parsecs away, much larger a distance than even Shapley thought the Milky Way was.


We now know that the Milky Way is just one of billions of galaxies in the visible universe and is only about 35,000 parsecs in diameter.