She worked hard, but was not expected to find anything, instead just hired to do work that the men didn't want. In fact, Leavitt would make a discovery that would change astronomy. In one study published in the Annals of the Astronomical Observatory of Harvard College, Leavitt noted that variable stars with greater luminosity had longer periods. She confirmed this by studying the Cepheid variables intensely, and realized that the relationship between period and luminosity was very concrete. The discovery is known as the "period-luminosity relationship" which says that the logarithm of the period s linearly related to the star's intrinsic luminosity. This finally allowed scientists to measure the distance to galaxies which were too far away to be computed through stellar parallax. Thanks to Leavitt's discovery, the distances to any Cepheid in the Milky Way could be determined with great accuracy. Later, Edwin Hubble used Leavitt's discovery to measure distances to Cepheids in the Andromeda Galaxy. Thus, Leavitt's discovery led to the realization that the universe is much larger than our Milky Way, and other spiral galaxies exist independently outside of our Milky Way. This discovery was one of the most revolutionary in history.
In 1921, Leavitt was made director of stellar photometry at Harvard. However, by the end of that year, illnesses which plagued her since graduating from college finally took their toll, causing Leavitt to die from cancer. She was buried in the Cambridge Cemetery. Other than making the most influential discovery in astronomy, she received many honors. She was a member of Phi Beta Kappa, the American Association of University Women, the American Astronomical and Astrophysical Society, the American Association for the Advancement of Science, and an honorary member of the American Association of Variable Star Observers.
