History of Discovery
Almost all stars are located in galaxies. When galaxies interact with each other, a very small number of stars may get lost and end up outside of a galaxy. However, because star formation requires relatively dense clouds of gas, which are only found in galaxies, the vast majority of stars are found in galaxies.
Until about 100 years ago, the Milky Way galaxy was thought to be only a few thousand light years across, and most thought it was the entire universe. Other galaxies had been discovered, but they were thought to be smaller objects within our galaxy.
The first galaxies were identified in the 17th Century by the French astronomer Charles Messier, although at the time he did not know what they were. Messier, who was a keen observer of comets, spotted a number of other fuzzy objects in the sky which he knew were not comets. Worried that other comet hunters might be similarly confused, he compiled a list to prevent their misidentification. Messier's list (where objects are identified by M for Messier, followed by a number, e.g. M51) contained information on 110 star clusters and “spiral nebulae” (galaxies) but it was almost 300 years before astronomers figured out what the fuzzy “spiral nebulae” actually were.
Some people argued that these nebulae were “island universes” - objects like our Milky Way galaxy, but external to it. Others disagreed, and thought that these spiral objects were clouds of gas within the Milky Way. The argument went on until the 1920s, when the American astronomer Edwin Hubble finally measured the distance to one of these spiral nebulae.
