During his time at the Yerkes as a postgrad, Hubble began studying faint, hazy patches of light in the sky called nebulae (from the Latin word for cloud). The word "nebula" is today used to describe clouds of dust and gas within galaxies, but in Hubble's time, astronomers hadn't been able to distinguish nebulae from distant galaxies, which also appeared as cloudy patches. Edwin became interested in two particular nebulae: the Large Magellanic Cloud and the Small Magellanic Cloud.
Working in 1912, American astronomer Henrietta Leavitt used the degree of brightness of Cepheid starsstars whose brightness vary at regular intervalsin the Magellanic Clouds to measure their distance from the Earth. The longer the time a Cepheid star takes to undergo a complete cycle, the higher the star's average brightness or average "absolute magnitude." Leavitt was able to determine the distance from Earth to the nebula by comparing the brightness of the star as seen from Earth with the star's actual brightness, which was estimated using the length of the star's cycle. With this work, Leavitt and her colleagues demonstrated that the Magellanic Clouds existed beyond the Milky Way's boundaries.