First in a two-part series about stars and how we classify them. Variables are a very specific kind of star that have a regular variation in brightness, like a heartbeat. They were first categorized and analyzed by Henrietta Swan Leavitt at the turn of the 20th century at Harvard University, along with other women computers at the time. Leavitt noticed variable stars in the Magellanic Clouds and came up with her luminosity law, where the pulse rates of Cepheid variable stars are proportional to their luminosity—the brighter they are, the greater their period is. This law helped estimate interstellar and intergalactic distances. Cepheids and other kinds of variable stars have helped astronomers map out the size of our galaxy, the spaces between celestial objects, and the distance to the outer reaches of our universe.