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.

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).

Spiral Galaxies

Spiral galaxies are just want they sound like. They look like whirlpools, pinwheels, anything that has arms rotating around a central body. In fact, there are two galaxies named the Whirlpool Galaxy and Pinwheel Galaxy, and surprise, surprise, they are spiral galaxies.

Whirlpool Galaxy

Image Credit:

NASA/Hubble

Pinwheel Galaxy

Image Credit:

  NASA, ESA, CFHT, NOAO

Acknowledgement - K.Kuntz (GSFC), F.Bresolin (U.Hawaii), J.Trauger (JPL), J.Mould (NOAO), Y.-H.Chu (U. Illinois)

What constitutes a spiral galaxy? The spiral galaxy contains three main parts:

• Galactic Bulge
• Galactic Halo
• Galactic Disk

The galactic disk is the portion of the galaxy where the arms are. The arms wrap in the direction of rotation, i.e. in both images above, the direction of rotation is counterclockwise (anti-clockwise for those not used to American usage). The faster the rotation, the tighter the arms (see post on angular velocity why this happens). The disk is also where a lot of star formation occurs because of the abundance of gas and dust. As see in both images, the arms/disk are bluer than the rest of the galaxy. This implies that the stars in the disk tend to be younger than the rest of galaxy. Also, we know that generally the stars have a higher metal content because of their age. This is the portion of the galaxy where we expect to have habitable planets and possibly life. Consider the disk as the suburbs of the galaxy. Lots of stars but not too crowded.

The galactic halo has been mentioned before. This is the area where we find the globular clusters. The stars in the halo tend to be older and therefore, redder than the disk stars. There is very little gas and dust, so not much star formation occurs in the halo. We find little metals and therefore, probably few (if any) planets. This would be like the country. Lots of space, not heavily populated.

The galactic bulge is downtown. Very crowd with stars, gas, and dust. The portion of the bulge nearest the disk is bluer, while transiting towards the halo, the stars get redder. The center of the bulge most likely contains a supermassive black hole around which everything orbits.

Spiral galaxies actually have two types: normal spiral galaxies or barred spirals. Barred spiral galaxies have a bar of stars, gas, and dust crossing through the center of the bulge with the arms emanating from the ends of the bar.

Barred Spiral Galaxy NGC 1300

Image Credit:

 Hubble Heritage Team, ESA, NASA

Spiral galaxies can be differentiated by the tightness of their spirals and the size of their bulges.

• Sa and SBa: Very large bulge with tightly wound arms
• Sb and SBb: Medium-sized bulge with less tightly wound arms
• Sc and SBc: Small bulge with loose arms
• Sd and SBd: Very small bulge with almost open arms