The Size of the Milky Way

For centuries, everything we thought about the universe was contained in a tiny package that centered on the Earth, and then the Sun after the heliocentric model was accepted. We only knew about what we could see and assumed that since it was observable, it had to be relatively close. If only we knew.

But first, we should see how the evolution of determining the size of the Milky Way galaxy itself took place.

The first actual attempt to determine the size of the Milky Way galaxy was done by William Herschel. He did a simple thing: he just decided to count all the stars he could see, and assuming that the Sun was the center of the galaxy (galactocentricism - yes, there is a word for it), was able to predict, correctly, that our galaxy was a disk. Of course, he was wrong about where the Sun is located with respect to the disk of the Milky Way.

The shape of our Galaxy as deduced from star counts by William Herschel in 1785; the solar system was assumed near center. (NOTE: The image shown is flipped 180 degrees on the horizontal axis from the original, as first published in the Philosophical Transactions of the Royal Society in 1785; the bifurcated arms of the illustration should be on the left.)

"Herschel-Galaxy" by Caroline Herschel. Licensed under Public Domain via Wikimedia Commons

It wasn't until the 1900s that a more extensive survey of the Milky Way took place to determine the size of the Milky Way. Jacobus Kapteyn (cap-tine) deduced that the Milky Way was similar to Herschel's drawing in that the Milky Way is disk-like, heliocentric, and about 20,000 parsecs in diameter.

Kapteyn's Model of a Heliocentric galaxy

Harlow Shapley immediately published a rebuttal stating that the Sun not at the center, but about 2/3 of the way out into the disk and the Milky Way was about 120,000 parsecs in size. Shapley used the distribution of globular clusters and found them more concentrated around the center of the disk of the Milky Way, instead of near where the Sun is located.

Shapley's Model to Determine the Size of the Milky Way

Who was correct? Actually, both were correct, but also, both were wrong. Kapteyn was around the correct size, but Shapley had the Sun in the correct relative location. This actually led to a debate between Harlow Shapley and Heber Curtis, referred to by astronomers as the Great Debate (or the Shapley-Curtis debate).