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.

Globular Clusters

Compared to open clusters globular clusters are old and regularly shaped We find them outside the galactic plane in something called the galactic halo.

First of all, we know they are old because they lack metals. They also are typically red meaning that they are not newly formed as blue stars are typically young. They also are large in size and contain hundred of thousands of star in a sphere hundreds or thousands of light years in diameter.

They are spherical because of the multitude of stars which allow gravity to pull them into a sphere. Unlike open clusters with relatively few stars, which do not have a define shape. Globular clusters also formed near the time when our galaxy formed.

They were first discovered by Charles Messier in the 1700's while he was searching for comets. Many are found in the Messier catalogue along with open clusters and nebulae. All of the globular clusters are outside the plane of the Milky Way. They have also been found orbiting other galaxies similar to ours.

M13 (Messier Catalog, 13th item), also known as the Hercules Cluster (in the constellation Hercules)
Image Credit:
Martin Pugh

M72

Image Credit:

NASA, ESA, Hubble, HPOW

Globular clusters were also important in determining the size of the Milky Way. Back in the 1920's, there was a Great Debate on how big the Milky Way was The determining factor was to look at how the globular clusters orbited. the center of the galaxy, and astronomers could use that to figure out out that our galaxy was about 100,000 light-years in size.