Henrietta Leavitt was born in Lancaster, Massachusetts. She discovered the law of Cepheid variables: their absolute magnitude is proportional to the period of their variability. This provided a method for estimating the distances of galaxies, which allowed her to discover, for example, that the small spiral nebulae seen in the sky were not within our galaxy (the Milky Way) but far outside it, and were therefore much larger than we thought: they were themselves galaxies like our own, or even larger. The volume of the known universe would suddenly become billions of times larger.
