This is Henrietta Leavitt. And she is one of astronomy’s great unsung heroes. Working at the Harvard College Observatory in 1893 as a humble assistant, she had been tasked with counting images on photographic plates. But while she was looking at these plates, she made a discovery which would later be pivotal to much of astronomy. Leavitt had discovered the period-luminosity relation of cepheid variable stars.
A cepheid variable is something of a stellar clock. These stars regularly brighten and dim, and one cycle of brightening and dimming is referred to as a period. While looking over the plates which recorded old observations, Leavitt noted that brighter stars had longer periods. At the time, this would become one of the most reliable ways to gauge the distance to stars. Simply, measuring the star’s period gave a means to work out it’s absolute luminosity – when compared with how bright a star appears (because fairly obviously, more distant stars appear dimmer) the distance can be estimated really rather well.
Back then, no one knew that there were galaxies outside the Milky Way. The Andromeda Galaxy was, at the time, known as “The Great Andromeda Nebula”, and it was one of several so-called “Spiral Nebulae”. This line of thought continued until Edwin Hubble discovered cepheids in the Andromeda Galaxy and, using Leavitt’s method, gauged the true distance to it. Thus, the discovery was made that the Universe was much larger than anyone had previously realised!
Hubble’s work would not have been possible if not for Henrietta Leavitt and her period-luminosity relation. Indeed, Hubble himself frequently said that Leavitt should be awarded the Nobel prize for her work. Unfortunately, it was a prize she would never receive. Though the paperwork had already begun for her nomination in 1924, Henrietta Leavitt had sadly passed away three years before, and the Nobel prize cannot be awarded posthumously.
Nonetheless, modern astronomy owes a lot to the legacy of Henrietta Swan Leavitt and her work. So the next time you look at a glamorous image of a galaxy, just remember that it was her who paved the way for us to know what we do today about other galaxies.