The Great Debate 

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During the early part of the twentieth century, there was great debate as to the nature of the spiral nebulae. Some astronomers believed they were nearby objects, within our own Galaxy. Others believed they were galaxies in their own right, very large and very distant. The controversy culminated in the Great Debate in 1920 at the National Academy of Science in Washington, DC.

 
 
 

Harlow Shapley: Spiral nebulae are nearby members of our Galaxy.

• Novae
Novae had been seen in the Andromeda Nebula. If the Andromeda Nebula was as large as the Milky Way, its distance would be immense and the novae which had been observed would have luminosities much greater than those in the Milky Way.
 
• Rotation
Adrian van Maanen had observed proper motion of objects in the spiral nebula M101. If M101 was as big as the Milky Way, its outer edge would be moving at a fantastic velocity. van Maanen's data indicated that M101 rotated once every 65 million years. If so, the outer edge of M101 would be rotating at nearly 5,000 km/s, much faster than the Milky Way's rotation speed of 220 km/s.

• Novae
If the Andromeda nebula were within our own Galaxy, its novae would have luminosities much less than the other novae in our Galaxy.
 
• Radial velocities
Most of the spiral nebulae have very large radial velocities (>1000 km/s). How then could they remain part of the Milky Way -- they should escape.

Also, if they were nearby and moving that fast, we ought to see proper motions. We don't.

We now know, of course, that the nebulae are distant galaxies. So what was wrong with Shapley's arguments?

 1923: Debate Resolved!

Edwin Hubble detects Cepheid variables in the Andromeda nebula using the new 100" telescope on Mt Wilson. By measuring the period of the Cepheids, he calculated their absolute magnitude. From their observed apparent magnitude, he could then solve for the distance:

d=285 kpc, well outside the Milky Way!

(As it turns out, Hubble actually got it wrong. He was observing classical Cepheid variables, but had calibrated their absolute magnitude using Type II Cepheids, which are systematically less luminous than classical Cepheids. The actual distance to Andromeda is more like 750 kpc...)