More about M31
More Information about M31 (the Andromeda Galaxy)
Credit: Public Domain
The Andromeda galaxy can be seen with the naked eye in the constellation of Andromeda. It looks like a bright cookie in the sky several degrees across. In the sense that one does not need an optical aid to see it, one could say that the earliest people to look up at the night sky "discovered" it.
In terms of when it was realized that it is a galaxy, and in fact that there are such things as galaxies, each "island universes" in and of themselves, that was only about 50 to 60 yrs ago. Up until that time, telescopes were not powerful enough to be able to discern individual stars in galaxies outside our own, so the objects which we now know to be galaxies were referred to as "nebulae" - or bright gas clouds. In the early 1920s there was a debate between Curtis and Shapley - two famous astronomers of the day - as to the nature of the "nebulae". Curtis argued for the island universe interpretation whereby each nebula is a galaxy consisting of many individual stars; Shapley argued the traditional view - namely that all that exists in the Universe is within a few thousand light years of us, and our galaxy IS the Universe. Although Shapley was generally acknowledged to be the "winner" of the debate by virtue of having more correct and supportable arguments, Curtis turned out to be right in the end.
The Andromeda galaxy is about 2 million light years from Earth.
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