July 4, 2018 saw the 150th birthday of Henrietta Swan Leavitt (1868 – 1921), one of the most important astronomers of the 20th century. Born in Lancaster, Massachusetts, Leavitt graduated from Radfcliffe College, Harvard, in 1892. She then stayed on at the Harvard College Observatory as a volunteer research assistant. Whilst attempting a graduate degree in astronomy and travelling in Europe, she became ill with grave consequences for her hearing. In 1902, then director, Edward Pickering, invited Henrietta to join the permanent staff at Harvard, where she was assigned to study “variable” stars.
(Image credit: via wikimedia.org)
By comparing photographic plates taken of the same region of sky over successive nights, Leavitt would establish whether the brightness of individual stars varied over time, and by how much. Concentrating on the Magellanic Clouds, then thought to be rich starfields in the Milky Way, but now known to be companion galaxies, Leavitt discovered 1777 variable stars  and measured their periods and the shapes of their light curves. [footnote: there are now more than 4630 classical Cepheids known in the Small Magellanic Cloud]
As she studied these light curves, Leavitt stumbled on a remarkable discovery. By 1908, she realised that the periods ...