Astronomer Edwin Hubble made revolutionary contributions to our understanding of the universe. His observation and measurement of a pulsating star in the constellation Andromeda indicated that it was roughly a million light-years away and therefore could not be located in the Milky Way. This discovery overturned the widely held idea that the Milky Way was the entire universe, demonstrating that it was one of many stellar systems (galaxies) in a larger universe. Hubble’s observation of the stellar spectra of many of these newly identified galaxies led him to show that the recessional velocities of galaxies were increasing in proportion to their distances from Earth. This discovery, known as Hubble’s Law, indicated that the universe was expanding. Hubble’s work led to the widespread acceptance of the idea of an expanding universe that began with an event known as the Big Bang.