Henrietta Swan Leavitt was a pioneering astronomer who discovered the period-luminosity relation for Cepheid variable stars, a breakthrough that allowed astronomers to measure distances in space. Working at the Harvard College Observatory, Leavitt found that the brightness of Cepheid variables was directly related to their pulsation period. This "cosmic ruler" enabled later astronomers, including Edwin Hubble, to determine that the universe is expanding, leading to the Big Bang theory. Despite facing significant barriers as a woman in early 20th-century science, Leavitt's work remains essential to modern astrophysics and serves as an enduring inspiration.