Henrietta Swan Levitt

Henrietta Swan Leavitt (; July 4, 1868 – December 12, 1921) was an American astronomer. 
After she graduated from Radcliffe College, she worked at the Harvard College Observatory.
She examined photographic plates to measure and catalog the brightness of stars. She discovered the relation between the luminosity and the period of Cepheid variables.
This gave astronomers with the first "standard candle" to measure the distance to faraway galaxies.
She discovered the period-luminosity relationship for Cepheid variables. Her work allowed astronomers to measure distances up to about 20 million light years. As a result of this, it is now known that our own galaxy, the Milky Way, has a diameter of about 100,000 light years. 
After Leavitt's death, Edwin Hubble established that the universe is expanding (see Hubble's law). He used Leavitt's period-luminosity relation, and the galactic spectral shifts first measured by Vesto Slipher at Lowell Observatory.