Henrietta Swan Leavitt

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Henrietta Swan Leavitt
upper-body & face of Henrietta Swan Leavitt
Henrietta Swan Leavitt
BornJuly 4, 1868
Lancaster, Massachusetts
DiedDecember 12, 1921(1921-12-12) (aged 53)
Cambridge, Massachusetts
NationalityAmerican
Alma materRadcliffe College, Oberlin College
Known forperiod-luminosity relationship of Cepheid stars
Scientific career
FieldsAstronomy
InstitutionsHarvard University

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," 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[1] 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.[2] (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–.

Career

Early photo of ‘Pickering's Harem’, as the group of women computers assembled by Harvard astronomer Edward Charles Pickering was dubbed. The group included Leavitt, Annie Jump Cannon, Williamina Fleming and Antonia Maury

In 1893, Leavitt began working at the Harvard College Observatory as one of the women human "computers" hired by 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).[3] Because Leavitt had independent means, Pickering initially did not have to pay her. Later, she received $10.50 a week for her work.

Pickering assigned Leavitt to study "variable stars," whose luminosity varies over time. 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."[4] Leavitt 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,[5] 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 Cepheid variables with greater intrinsic luminosity did have longer periods,[6] and that 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."[7]

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] variables and their periods."[8][9]

Influence

Title page of Leavitt's 1777 Variables in the Magellanic Clouds, Annals of the Harvard College Observatory, 1908

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.

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 actually independent galaxies located far outside of our own Milky Way. Thus, Leavitt's discovery helped to settle the "Great Debate" on whether the Universe was larger than the Milky Way, and thus changed our picture of the Universe forever.

The accomplishments of the American astronomer Edwin Hubble, who established that the Universe is expanding, were also made possible by Leavitt's groundbreaking research. "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.[10] To his credit, Hubble himself often said that Leavitt deserved the Nobel Prize for her work.[11] Gösta Mittag-Leffler 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[12] (the Nobel Prize is not awarded posthumously).

Illness and death

Woman sitting at desk writing, with short hair, long-sleeved white blouse and vest
Henrietta Swan Leavitt working at her desk in the Harvard College Observatory[13]

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.[13] 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.

"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."[14]

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."[15]

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.[16]

See also

Notes

  1. ^ Gregory M. Lamb (July 5, 2005). "Before computers, there were these humans..." Christian Science Monitor. Retrieved 2007-05-18.
  2. ^ Out of Shadows: Contributions of Twentieth-century Women to Physics, Nina Byers, Gary Williams, Cambridge University Press, 2006, ISBN 0-521-82197-5, ISBN 978-0-521-82197-1
  3. ^ Exploratorium note
  4. ^ Jeremy Bernstein, "Review: George Johnson's Miss Leavitt's Stars", Los Angeles Times, July 17, 2005
  5. ^ Leavitt, Henrietta S. "1777 Variables in the Magellanic Clouds". Annals of Harvard College Observatory. LX(IV) (1908) 87-110
  6. ^ Miss Leavitt in Pickering, Edward C. "Periods of 25 Variable Stars in the Small Magellanic Cloud" Harvard College Observatory Circular 173 (1912) 1-3.
  7. ^ Periods Of 25 Variable Stars In The Small Magellanic Cloud, Harvard College Observatory Circular 173, 1912, Edward C. Pickering citing Henrietta Leavitt
  8. ^ Miss Leavitt in Pickering, Edward C. "Periods of 25 Variable Stars in the Small Magellanic Cloud" Harvard College Observatory Circular 173 (1912) 2. Retrieved 2013-03-14
  9. ^ Kerri Malatesta (16 July 2010). "Delta Cephei". American Association of Variable Star Observers.
  10. ^ David H. Clark; Matthew D.H. Clark (2004). Measuring the Cosmos: How Scientists Discovered the Dimensions of the Universe. Rutgers University Press. ISBN 0-8135-3404-6.
  11. ^ Ventrudo, (2009)
  12. ^ Singh, Simon (2005). Big Bang: The Origin of the Universe. HarperCollins. ISBN 0-00-716221-9.
  13. ^ a b Hamblin, Jacob Darwin (2005). Science in the early twentieth century: an encyclopedia. ABC-CLIO. pp. 181–184. ISBN 1-85109-665-5.
  14. ^ Johnson, p 90
  15. ^ Johnson, p 28
  16. ^ Johnson, pp 118–119

References

External links

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