Hubble and the Birth of Modern Cosmology Continued
The debate centered around things called nebulae---fuzzy structures in the sky that could not be seen clearly with the telescopes then available. It was long thought by astronomers that the nebulae were dust clouds or large collections of stars, but no one was sure whether they were in the Milky Way or outside it. Because his new telescope allowed him to see individual Cepheid variable stars in some nebulae (something no one had been able to do before), Hubble was able to measure the distance to them. It turned out that the Cepheid variable stars were extremely faint, so the distance to the nearest one, located in the Andromeda nebula, was some 2 million light years, far outside the bounds of the Milky Way. Thus, with a single observation, Hubble established one the most important facts about the universe we live in: It is made up of billion of galaxies, of which the Milky Way is but one.
