At the turn of the 20th century, astronomers were struggling with the question of how far away the ‘island universes,’
or other galaxies, are from us here in the Milky Way. It took the work of astronomer Henrietta Leavitt at Harvard College Observatory to tell us.
Ms. Leavitt was tasked with discovering the set of stars that change their brightness with time, the so-called ‘variable stars.’ She discovered nearly 1800 variable stars and noticed that some of them dimmed and then brightened again with predictable regularity. She measured the length of time it took for each variable star to undergo a full cycle of dimming and then brightening up again in a book published in the year 1908. At the end of book she noted that the brighter variable stars had longer such measured periods.
In between suffering from various illnesses and an unfortunate loss of hearing, she returned to this finding to establish an important relation regarding the set of variable stars now called “Cepheids.” The discovery was that there is a tight correlation between the total intrinsic brightness of a Cepheid variable star and its period, in the sense that the longer the period, the brighter the Cepheid star.
She measured the periods of Cepheids in one of the closest galaxies to us, the Large Magellanic Cloud. Based on the periods, she used her own relation of to determine what the luminosity is, which in turn is dependent on its distance from us by the inverse square law for light.
Voila! The distance to the Large Magellanic Cloud was found to be about 160,000 light years across, not far off from that of the diameter of the Milky Way. This discovery enabled astronomers to measure the distances to other nearby galaxies too. In Part Two we will look more at the work of this extraordinary person.