Messier 33 (AKA the Triangulum Galaxy) is 2.72 million light years away. It was one of the first "spiral nebulae" discovered by Lord Rosse in 1850. For decades, there was debate as to whether these objects were nebulae in our own galaxy, or were independent of it. In 1922-23, variable stars were discovered in the nebula, and in 1926, Edwin Hubble found that a number of these were Cepheid variables. Cepheids are "standard candles" in astronomy--their true brightness is a function of the periodicity of their variability. Cepheids can therefore allow us to estimate distances. Hubble was able to demonstrate that M 33 was not nebulosity in our own galaxy, but was a separate galaxy (or "island universe" as they were then called) of stars, gas, and dust.
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