Henrietta Swan Leavitt
Henrietta Swan Leavitt (1868–1921) was an astronomer whose research led to two of the most important discoveries in the field. Through her analysis of pulsating stars called Cepheid variables, she determined that the time between pulses relates to the Cepheid’s brightness. This allowed astronomers to estimate the distances to stars and galaxies and thus measure the size of the observable universe. After her death, Edwin Hubble used Leavitt’s research to determine that the universe is expanding, and in the 1990s astronomers built on this work to determine that the expansion is accelerating.