Henrietta Swan Leavitt (/ˈlɛvɪt/; July 4, 1868 – December 12, 1921) was an Americanastronomer.[2][1][3] Her discovery of how to effectively measure vast astronomical distances led to a shift in the understanding of the scale and nature of the universe.[4][5][6]
Before Leavitt discovered theperiod-luminosity relationship forCepheid variables (sometimes referred to as Leavitt's Law), the only techniques available to astronomers for measuring the distance to a star were based onstellar parallax. Such techniques can only be used for measuring distances out to several hundredlight years. Leavitt's great insight was that while no one knew the distance to theSmall Magellanic Cloud, all its stars must be roughly the same distance from Earth. Therefore, a relationship she discovered in it, between the period of certain variable stars (Cepheids) and their apparent brightness, reflected a relationship in their absolute brightness. Once calibrated by measuring the distance to a nearby star of the same type via parallax, her discovery became a measuring stick with vastly greater reach.[9]
After Leavitt's death,Edwin Hubble found Cepheids in several nebulae, including theAndromeda Nebula, and, using Leavitt's Law, calculated that their distance was far too great to be part of the Milky Way and were separate galaxies in their own right. This settled astronomy'sGreat Debate over the size of the universe. Hubble later used Leavitt's Law, together with galacticredshifts, to establish that the universe is expanding (seeHubble's law)[citation needed].
Henrietta Swan Leavitt was born inLancaster, Massachusetts, the daughter of Henrietta Swan Kendrick andCongregational church minister George Roswell Leavitt.[10] She was a descendant of DeaconJohn Leavitt, an EnglishPuritan tailor, who settled in theMassachusetts Bay Colony in the early seventeenth century.[11] (In the early Massachusetts records, the family name was spelled "Levett".) Henrietta Leavitt remained deeply religious and committed to her church throughout her life.[12]
Henrietta Leavitt (third from left) is shown working among "The Harvard Computers" who assisted astronomer Edward Charles Pickering at the Harvard College Observatory. The assistants also includedAnnie Jump Cannon,Williamina Fleming, andAntonia Maury.
Leavitt also began working as volunteer assistant, one of the"computers" at theHarvard College Observatory.[13] In 1902, she was hired by the director of the observatory,Edward Charles Pickering, to measure and catalog the brightness of stars as they appeared in the observatory'sphotographic plate collection. (In the early 1900s, women were not allowed to operate telescopes,[14] but the scientific data were on the photographic plates.)[15][dubious –discuss]
In 1893, Leavitt obtained credits toward a graduate degree in astronomy for her work at the Harvard College Observatory, but due to chronic illness, she never completed that degree.[16] In 1898, she became a member of the Harvard staff. Leavitt left the observatory to make two trips to Europe and completed a stint as an art assistant atBeloit College in Wisconsin. At this time, she contracted an illness that led to progressivehearing loss.[17][18] Leavitt gradually became deaf.[6]: 28, 31, 44
Leavitt working at her desk in the Harvard College Observatory[18]
Leavitt returned to the Harvard College Observatory in 1903. Because Leavitt was financially independent, Pickering initially did not have to pay her. Later, she received$0.30 (equivalent to $10.5 in 2024) an hour for her work,[6]: 32 and only$10.50 (equivalent to $367.46 in 2024) per week. She was reportedly "hard-working, serious-minded …, little given to frivolous pursuits and selflessly devoted to her family, her church, and her career."[4] At the Harvard Observatory, Leavitt worked alongsideAnnie Jump Cannon, who also was deaf.[19]
Pickering assigned Leavitt to studyvariable stars of theSmall andLarge Magellanic Clouds, as recorded on photographic plates taken with the Bruce Astrograph of theBoyden Station of the Harvard Observatory inArequipa,Peru. She identified 1,777 variable stars. In 1908, Leavitt published the results of her studies in theAnnals of the Astronomical Observatory of Harvard College, noting that the brighter variables had the longer period.[7]
Plot from a paper prepared by Leavitt in 1912 - The horizontal axis is the logarithm of the period of the corresponding Cepheid, and the vertical axis is itsmagnitude. The lines drawn connect points corresponding respectively to the stars' minimum and maximum brightness.[8][20]
In a 1912 paper, Leavitt examined the relationship between the periods and the brightness of a sample of 25 of the Cepheid variables in theSmall Magellanic Cloud. The paper was communicated and signed by Edward Pickering, but the first sentence indicates that it was "prepared by Miss Leavitt".[8] Leavitt made a graph of magnitude versus logarithm of period and determined that, in her own words,
A straight line can be readily drawn among each of the two series of points corresponding to maxima and minima, thus showing that there is a simple relation between the brightness of theCepheid variables and their periods.[8]
Leavitt then used the simplifying assumption that all of the Cepheids within the Small Magellanic Cloud were at approximately the same distance, so that theirintrinsic brightness could be deduced from theirapparent brightness as registered in the photographic plates, up to a scale factor, since the distance to the Magellanic Clouds was as yet unknown. She expressed the hope thatparallaxes to some Cepheids would be measured. This soon occurred,[9] allowing her period-luminosity scale to be calibrated.[8] This reasoning allowed Leavitt to establish that thelogarithm of theperiod is linearly related to the logarithm of the star's average intrinsic opticalluminosity (the amount of power radiated by the star in thevisible spectrum).[21]
Henrietta found thatDelta Cepheus was the "standard candle" that had long been sought by astronomers. A similar five-daycepheid variable in the SmallMagellanic cloud she found to be about one ten-thousandth as bright as our five-day Delta Cepheus. Using theinverse-square law, she calculated that the Small Magellanic Cloud was 100 times as far away as Delta Cepheus, thus having discovered a way to calculate the distance to another galaxy.
Leavitt also developed, and continued to refine, the Harvard Standard for photographic measurements, a logarithmic scale that orders stars by brightness greater than 17 magnitudes. She initially analyzed 299 plates from 13 telescopes to construct her scale, which was accepted by theInternational Committee of Photographic Magnitudes in 1913.[22]
In 1913, Leavitt discoveredT Pyxidis, arecurrent nova in the constellationPyxis, and one of the most frequent recurrent novae in the sky, with eruptions observed in 1890, 1902, 1920, 1944, 1967, and 2011.[23]
According to science writerJeremy Bernstein, "variable stars had been of interest for years, but when she was studying those plates, I doubt Pickering thought she would make a significant discovery—one that would eventually change astronomy."[24] The period–luminosity relationship for Cepheids, now known as "Leavitt's law", made the stars the first "standard candle" in astronomy, allowing scientists to compute thedistances to stars too remote forstellar parallax observations to be useful. One year after Leavitt reported her results,Ejnar Hertzsprung determined the distance of several Cepheids in theMilky Way; with this calibration, the distance to any Cepheid could be determined accurately.[9]
Cepheids were soon detected in other galaxies, such asAndromeda (notably by Edwin Hubble in 1923–24), and they became an important part of the evidence that "spiral nebulae" are independent galaxies located far outside of theMilky Way. Thus, Leavitt's discovery would forever change humanity's picture of the universe, as it promptedHarlow Shapley to move the Sun from the center of the galaxy in the "Great Debate" andEdwin Hubble to move the Milky Way galaxy from the center of the universe.
Leavitt's discovery of an accurate way to measure distances on an intergalactic scale paved the way for modern astronomy's understanding of the structure and scale of the universe.[4] The accomplishments ofEdwin Hubble, the American astronomer who established that the universe is expanding, were also made possible by Leavitt's groundbreaking research.
Hubble often said that Leavitt deserved the Nobel Prize for her work.[25] MathematicianGösta Mittag-Leffler, a member of theSwedish Academy of Sciences, tried to nominate her for that prize in 1924, only to learn that she had died of cancer three years earlier.[5][6]: 118 (The Nobel Prize is not awarded posthumously.)
Cepheid variables allow astronomers to measure distances up to about 60 million light-years. Even greater distances can now be measured by using the theoretical maximum mass ofwhite dwarfs calculated bySubrahmanyan Chandrasekhar.[26][27]
Leavitt's scientific work at Harvard was frequently interrupted by illness and family obligations. Her early death at the age of 53, from stomach cancer,[28] was seen as a tragedy by her colleagues for reasons that went beyond her scientific achievements. Her colleagueSolon I. Bailey wrote in his obituary for Leavitt that "she had the happy, joyful, faculty of appreciating all that was worthy and lovable in others, and was possessed of a nature so full of sunshine that, to her, all of life became beautiful and full of meaning."[6]: 28
She was buried in the Leavitt family plot atCambridge Cemetery inCambridge, Massachusetts."Sitting at the top of a gentle hill", writes George Johnson in his biography of Leavitt, "the spot is marked by a tall hexagonal monument, on top of which sits a globe cradled on a draped marble pedestal. Her uncleErasmus Darwin Leavitt and his family also are buried there, along with other Leavitts." A plaque memorializing Henrietta and her two siblings, Mira and Roswell, is mounted on one side of the monument. Nearby are the graves ofHenry andWilliam James.[6]: 90
Anna Von Mertens designed a book-based work of art,Attention Is Discovery: The Life and Legacy of Henrietta Leavitt. The pages weave Von Merton's artistic interpretations of Leavitt's work with photos and descriptions of the work of Leavitt and her fellow Harvard Computers.[32]
George Johnson wrote a 2005 biography,Miss Leavitt's Stars, which showcases the triumphs of women's progress in science through the story of Leavitt.[33]
Robert Burleigh wrote the 2013 biographyLook Up!: Henrietta Leavitt, Pioneering Woman Astronomer for a younger audience. It is written for four- to eight-year-olds.[34]
Lauren Gunderson wrote a 2015 play,Silent Sky, which followed Leavitt's journey from her acceptance at Harvard to her death.[35]
Theo Strassell wrote a play,The Troubling Things We Do, anabsurdist piece that details the life of Henrietta Leavitt, among other scientists from her era.[36]
Dava Sobel's bookThe Glass Universe chronicles the work of the women analyzing images taken of the stars at the Harvard College Observatory.[37]
The BBC included Leavitt in theirMissed Genius series designed to celebrate individuals from diverse backgrounds who have had a profound effect on our world.[14]
^Shearer, Benjamin F.; Shearer, Barbara Smith (1997).Notable women in the physical sciences: a biographical dictionary. Westport (Conn.): Greenwood press.ISBN978-0-313-29303-0.
^Sobel, Dava (2016).The Glass Universe. Viking. p. 113.ISBN9780698148697.
^Malatesta, Kerri (April 13, 2010)."Delta Cephei". American Association of Variable Star Observers. RetrievedJanuary 1, 2020.
^Madore, Barry F.; Rigby, Jane; Freedman, Wendy L.; Persson, S. E.; Sturch, Laura; Mager, Violet (2009). "The Cepheid Period–Luminosity Relation (The Leavitt Law) at Mid-Infrared Wavelengths. III. Cepheids in NGC 6822".The Astrophysical Journal.693 (1):936–939.arXiv:0812.0186.Bibcode:2009ApJ...693..936M.doi:10.1088/0004-637X/693/1/936.S2CID6028335.
