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About Henrietta Swan Leavitt
Henrietta Swan Leavitt (July 4, 1868 – December 12, 1921) was an American astronomer. A graduate of Radcliffe College, in 1893 Leavitt started working at the Harvard College Observatory as a "computer", tasked with examining photographic plates in order to measure and catalog the brightness of stars. Leavitt discovered the relation between the luminosity and the period of Cepheid variable stars. Though she received little recognition in her lifetime, it was her discovery that first allowed astronomers to measure the distance between the Earth and faraway galaxies. After Leavitt's death, Edwin Hubble used the luminosity-period relation for Cepheids to determine that the Universe is expanding (see Hubble's law).
Early years and education
Henrietta Swan Leavitt, the daughter of Congregational church minister George Roswell Leavitt and his wife Henrietta Swan (Kendrick), was born in Lancaster, Massachusetts, a descendant of Deacon John Leavitt, an English Puritan tailor, who settled in the Massachusetts Bay Colony in the early seventeenth century. (The family name was spelled Levett in early Massachusetts records.) She attended Oberlin College, and graduated from Radcliffe College, then called the Society for the Collegiate Instruction for Women, with a bachelor's degree in 1892. It wasn't until her fourth year of college that Leavitt took a course in astronomy, in which she earned an A–.
Leavitt began work in 1893 at the Harvard College Observatory as one of the women human ‘computers’ brought in by Edward Charles Pickering to measure and catalog the brightness of stars in the observatory's photographic plate collection. (In the early 1900s, women were not allowed to operate telescopes). She noted thousands of variable stars in images of the Magellanic Clouds. In 1908 she published her results in the Annals of the Astronomical Observatory of Harvard College, noting that a few of the variables showed a pattern: brighter ones appeared to have longer periods. After further study, she confirmed in 1912 that the variable stars of greater intrinsic luminosity – actually Cepheid variables – did indeed have longer periods, and the relationship was quite close and predictable.
Leavitt used the simplifying assumption that all of the Cepheids within each Magellanic Cloud were at approximately the same distances from the earth, so that their intrinsic brightness could be deduced from their apparent brightness (as measured from the photographic plates) and from the distance to each of the clouds. "Since the variables are probably at nearly the same distance from the Earth, their periods are apparently associated with their actual emission of light, as determined by their mass, density, and surface brightness."
Her discovery is known as the "period-luminosity relationship": The logarithm of the period is linearly related to the star's average, intrinsic luminosity (which is defined as a logarithm of the amount of power radiated by the star in the visible spectrum). In Leavitt's words, taken from her study of 1,777 variable stars recorded on Harvard's photographic plates, "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 the [Cepheid] variable and their periods".
The period-luminosity relationship for Cepheids made them the first "standard candle" in astronomy, allowing scientists to compute the distances to galaxies too remote for stellar parallax observations to be useful. One year after Leavitt reported her results, Ejnar Hertzsprung determined the distance of several Cepheids in the Milky Way, and with this calibration the distance to any Cepheid could be accurately determined.
At the time, it was not clear that there were millions of nebulae that were actually galaxies outside of our Milky Way. Their distances were too great to be measured using parallax and the Cepheid period-luminosity relationship provided the key to estimating those distances. Cepheids were soon detected in other galaxies such as the Andromeda Galaxy (notably by Edwin Hubble in 1923–24). Cepheids were an important part of the evidence that galaxies are far outside of the Milky Way and were key to settling the Great Debate as to the nature of spiral nebulae and whether the Universe was larger than the Milky Way. Our picture of the universe was changed forever, largely because of Leavitt's discovery.
The accomplishments of Edwin Hubble, the American astronomer, were made possible by Leavitt's groundbreaking research and Leavitt's Law. "If Henrietta Leavitt had provided the key to determine the size of the cosmos, then it was Edwin Powell Hubble who inserted it in the lock and provided the observations that allowed it to be turned," wrote David H. and Matthew D.H. Clark in their book Measuring the Cosmos.To his credit, Hubble himself often said that Leavitt deserved the Nobel for her work. Gösta Mittag-Leffler of the Swedish Academy of Sciences had begun paperwork on her nomination in 1924, only to learn that she had died of cancer three years earlier (the Nobel prize cannot be awarded posthumously).
Illness and death
Leavitt worked sporadically during her time at Harvard, often sidelined by health problems and family obligations. An illness contracted after her graduation from Radcliffe College rendered her increasingly deaf. By 1921, when Harlow Shapley took over as director of the observatory, Leavitt was made head of stellar photometry. By the end of that year she had succumbed to cancer, and was buried in the Leavitt family plot at Cambridge Cemetery in Cambridge, 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 (cradled on a draped marble pedestal) sits a globe. Her uncle Erasmus Darwin Leavitt and his family are also buried there, along with other Leavitts. A plaque memorializing Henrietta and her two siblings who died so young, Mira and Roswell, is mounted directly below the continent of Australia. Off to one side, and more often visited, are the graves of Henry and William James."
Leavitt was a member of Phi Beta Kappa, the American Association of University Women, the American Astronomical and Astrophysical Society, the American Association for the Advancement of Science, and an honorary member of the American Association of Variable Star Observers. Her early passing was seen as a tragedy by her colleagues for reasons that went beyond her scientific achievements.
In an obituary her colleague, Solon I. Bailey, noted that "she had the happy 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."
Awards and honors
The asteroid 5383 Leavitt and the crater Leavitt on the Moon are named in her honor.
Unaware of her death four years prior, the Swedish mathematician Gösta Mittag-Leffler considered nominating her for the 1926 Nobel Prize in Physics, and wrote to Shapley requesting more information on her work on Cepheid variables, offering to send her his monograph on Sofia Kovalevskaya. Shapley replied, let Mittag-Leffler know that Leavitt had died, and suggested that the true credit belonged to his (Shapley's) interpretation of her findings. She was never nominated, because the Nobel Prize is not awarded posthumously.