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.
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 of some variables in the Small Magellanic Cloud appeared to be proportional to their apparent brightness , and if the stars all lie roughly at the same distance, then their periods are directly linked to their true brightnesses . Secondly, if stars with the same period have the same true brightness, the difference in apparent brightness is only due to their different distance.
The stars that Leavitt observed are known as Cepheid variables. The first Cepheids (eta Aquilae and delta Cephei) had been discovered 120 years earlier by two English friends, John Goodricke and Edward Pigott, with periods of 7.2 and 5.4 days, respectively. Leavitt’s Cepheids with preiods of 5.3 days appear more than ten thousand times fainter than delta Cephei, and so must be more than 100 times distant.
Leavitt’s discovery would shake the world of science and set 20th century astronomy on an extraordinary path. Using Leavitt’s period-luminosity relation, as it came to be known, you could measure the distance to any star cluster or galaxy, so long as you could identify some Cepheid variables. All of a sudden, the size of the universe could be measured. It would be shown that the Sun does not lie at the center of the Milky Way and that the Magellanic Clouds and the Great Nebulae lie at vast distances beyond. Ultimately, it would pave the way for Edwin Hubble to establish the distance red-shift relation for galaxies and demonstrate the expansion of the Universe.
Leavitt’s discovery lies at the core of much contemporary research. A Hubble Space Telescope key project was to measure Cepheid periods in galaxies in the Virgo cluster. The spacecraft Gaia is measuring direct distances to millions of stars in the Milky Way — including the Cepheids used as standard candles to measure the universe. Cepheids are extrarodinary stars in their own right; their pulsations pose questions to physics, and provide clues to a star’s internal structure. Such work continues today at the Armagh Observatory and Planetarium.
Increasingly sidelined by ill health and family obligations, Leavitt continued to work sporadically at Harvard until 1921, when the new director Harlow Shapley appointed her director of stellar photometry. Sadly she died of cancer in the same year. She did not receive the recognition in her lifetime that she deserved, though Hubble would later remark that she deserved a Nobel prize. Today, she stands as one of the giants of 20th century astronomy. A recent tribute ends “The maps of galaxies go on and on expanding. She’s watching from a soundless place, light years ahead.” 
1. Leavitt, H. S. 1908. “1777 Variables in the Magellanic Clouds,” Annals of Harvard College Observatory 60: 4 (87–108)
2. Leavitt, H. S. & Pickering, E. C. 1912. “Periods of 25 Variable Stars in the Small Magellanic Cloud,” Harvard College Observatory Circular 173 (1-3)
3. ‘The brightest star’: Elizabeth Burns: http://www.poetrypf.co.uk/elizabethburnspoems.shtml
Article written by Professor C. Simon Jeffery
Research Astronomer, Armagh Observatory and Planetarium
Exploring the Cosmos since 1790 // arm.ac.uk/~csj/
Adjunct Professor, School of Physics, Trinity College Dublin
President, IAU Commission G4 – Pulsating Stars