Henrietta Swan Leavitt
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Henrietta Swan Leavitt
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| Born | July 4, 1868
Lancaster, Massachusetts, U.S.
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| Died | December 12, 1921 (aged 53)
Cambridge, Massachusetts, U.S.
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| Residence | Cambridge, Massachusetts |
| Nationality | American |
| Citizenship | United States |
| Alma mater | Radcliffe College, Oberlin College |
| Known for | Leavitt's law: the period–luminosity relationship for Cepheid variables |
| Scientific career | |
| Fields | Astronomy |
| Institutions | Harvard University |
Henrietta Swan Leavitt was an American astronomer. A graduate of Radcliffe College, she worked at the Harvard College Observatory as a "computer", tasked with examining photographic plates in order to measure and catalog the brightness of stars. This work led her to discover the relation between the luminosity and the period of Cepheid variables. Though she received little recognition in her lifetime, Leavitt's discovery provided astronomers with the first "standard candle" with which to measure the distance to faraway galaxies. After her death, Edwin Hubble used Leavitt's luminosity–period relation, together with the galactic spectral shifts first measured by Vesto Slipher at Lowell Observatory, in order to establish that the universe is expanding.
Life
Early years and education
Henrietta Swan Leavitt was born in Lancaster, Massachusetts, the daughter of Congregational church minister George Roswell Leavitt and his wife Henrietta Swan Kendrick. She was a descendant of Deacon John Leavitt, an English Puritan tailor, who settled in the Massachusetts Bay Colony in the early seventeenth century. (In the early Massachusetts records the family name was spelled "Levett".)
Leavitt attended Oberlin College before transferring to Harvard University's Society for the Collegiate Instruction of Women (later known as Radcliffe College), receiving a bachelor's degree in 1892.
At Oberlin and Harvard, Leavitt studied a broad curriculum that
included classical Greek, fine arts, philosophy, analytic geometry, and
calculus. It wasn't until her fourth year of college that Leavitt took a course in astronomy, in which she earned an A–. Leavitt also began working as one of the women human "computers" at the Harvard College Observatory, hired by its director Edward Charles Pickering to measure and catalog the brightness of stars as they appeared in the observatory's photographic plate collection. (In the early 1900s, women were not allowed to operate telescopes.)
In 1893, Leavitt obtained credits toward a graduate degree in
astronomy for her work at the Harvard College Observatory, but she never
completed that degree. Leavitt left the observatory to make two trips to Europe and complete a stint as an art assistant at Beloit College in Wisconsin. At this time she contracted an illness that caused her increasingly to lose her hearing.
Astronomical career
Henrietta Swan Leavitt working at her desk in the Harvard College Observatory
Leavitt returned to the Harvard College Observatory in 1903. Because
Leavitt had independent means, Pickering initially did not have to pay
her. Later, she received $0.30 an hour for her work,
being paid only $10.50 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". One of the women that Leavitt worked with in the Harvard Observatory was Annie Jump Cannon, who shared the experience of being deaf.
Pickering assigned Leavitt to the study of variable stars of the Small and Large Magellanic Clouds, as recorded on photographic plates taken with the Bruce Astrograph of the Boyden Station of the Harvard Observatory in Arequipa, Peru. She identified 1777 variable stars. In 1908 she published her results in the Annals of the Astronomical Observatory of Harvard College, noting that the brighter variables had the longer period.
In another paper published in 1912, Leavitt looked carefully at
the relation between the periods and the brightness of a sample of 25 of
the Cepheids variables in the Small Magellanic Cloud. This paper was
communicated and signed by Edward Pickering, but the first sentence
indicates that it was "prepared by Miss Leavitt". 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 the Cepheid variables and their periods.
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 its magnitude. The lines drawn connect points corresponding to the stars' minimum and maximum brightness, respectively.
She then used the simplifying assumption that all of the Cepheids
within the Small Magellanic Cloud were at approximately the same
distance, so that their intrinsic brightness
could be deduced from their apparent brightness as registered in the
photographic plates, up to a scale factor since the distance to the
Magellanic Clouds were as yet unknown. She expressed the hope that
parallaxes to some Cepheids would be measured, which eventually happened
thereby allowing her period-luminosity scale to be calibrated. This reasoning allowed Leavitt to establish that the logarithm of the period is linearly related to the logarithm of the star's average intrinsic optical luminosity (which is the amount of power radiated by the star in the visible spectrum).
Leavitt also developed, and continued to refine, the Harvard
Standard for photographic measurements, a logarithmic scale that orders
stars by brightness over 17 magnitudes. She initially analyzed 299
plates from 13 telescopes to construct her scale, which was accepted by
the International Committee of Photographic Magnitudes in 1913.
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. In 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.
Illness and death
Leavitt family monument in Cambridge Cemetery
Leavitt's scientific work at Harvard was frequently interrupted by
illness and family obligations. Her early death, at the age of 53, was
seen as a tragedy by her colleagues for reasons that went beyond her
scientific achievements. Her colleague Solon I. Bailey
wrote in her obituary 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."
"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 uncle Erasmus 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 of Henry and William James. There is no epitaph at the grave site memorializing Henrietta Leavitt's achievements in astronomy.
Scientific impact
According to science writer Jeremy 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." 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 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 that, with this calibration, the distance to any Cepheid could be accurately determined.
Cepheids were soon detected in other galaxies, such as Andromeda
(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 our own Milky Way. Thus, Leavitt's discovery would forever change our picture of the universe, as it prompted Harlow Shapley to move our Sun from the center of the galaxy in the "Great Debate" and Edwin Hubble to move our galaxy from the center of the universe.
Leavitt's discovery of a way to accurately measure distances on
an inter-galactic scale, paved the way for modern astronomy's
understanding of the structure and scale of the universe. The accomplishments of Edwin Hubble,
the American astronomer who established that the universe is expanding,
also were made possible by Leavitt's groundbreaking research. Hubble
often said that Leavitt deserved the Nobel Prize for her work. Mathematician Gösta Mittag-Leffler, a member of the Swedish Academy of Sciences, tried to nominate her for that prize in 1924, only to learn that she had died of cancer three years earlier. (The Nobel Prize is not awarded posthumously.)
Posthumous honors
- The asteroid 5383 Leavitt and the crater Leavitt on the Moon are named after her to honor deaf men and women who have worked as astronomers.
- One of ASAS-SN telescopes, located in the McDonald Observatory in Texas, is named in her honor.
Books and plays
Lauren Gunderson wrote a play, Silent Sky, which followed Leavitt's journey from her acceptance at Harvard to her death.
George Johnson wrote a biography, Miss Leavitt's Stars, which showcases the triumphs of women's progress in science through the story of Leavitt.
Robert Burleigh wrote the biography Look Up!: Henrietta Leavitt, Pioneering Woman Astronomer for a younger audience. It is written for four to eight year olds.
