Harlow Shapley was an astronomer who performed groundbreaking research on stars. He broadened our view of the Milky Way and the sun’s position within it through his observation of Cepheid variable stars. Cepheid variable stars are low-mass stars that begin to pulsate and serve as important cosmological distance markers. Shapley used the period of pulsation to determine the distance of the star, using the inverse square law to show that the Milky Way was much larger than anticipated and that the sun was in a nondescript location within the galaxy. He also postulated the “Liquid Water Belt” theory, now known as the habitable zone, which defines the regions near stars that will allow for planetary masses to support Earth-like conditions. Shapely participated in the “Great Debate” at the 1920 annual meeting of the National Academy of Sciences. The topic of the debate––whether the Milky Way was the entire universe or the universe was made up of millions of galaxies that resemble the Milky Way––is one of the starting points for the field of extragalactic astronomy. Shapley’s argument, that the Milky Way comprised the entire universe, was later proved incorrect by Edwin Hubble.