Henrietta Swan Leavitt

American astronomer (1868–1921)

Henrietta Leavitt was born the daughter of a Congregational minister in Lancaster, Massachusetts. Her interest in astronomy was aroused while she was at Radcliffe College (then the Society for the Collegiate Instruction of Women), from which she graduated in 1892. In 1895 she became a volunteer research assistant at the Harvard College Observatory, receiving a permanent post in 1902. She was soon head of the department of photographic photometry. Like her colleague Annie Cannon, she was extremely deaf.

Leavitt's work involved the determination of the photographic magnitudes of stars, i.e., their brightness as recorded on a photographic plate. The photographic magnitude of a star differs somewhat from its visual magnitude since a photographic emulsion is more sensitive to blue light than the eye. The accurate measurement of visual magnitudes had been part of the program of the Harvard College Observatory since the 1870s. In 1907 the director of the observatory, Edward Pickering, announced plans to redetermine stellar magnitudes by photographic techniques. The photographic magnitudes of a group of stars near the north celestial pole were to act as standards of reference for other stars. Leavitt was selected to measure these magnitudes, known as the ‘north polar sequence’, and the results were published in 1917 in the Annals of Harvard College Observatory (vol. 71 no. 3). She also spent many years measuring secondary stellar magnitudes, based on the north polar sequence, which was adopted as an international standard until superseded by photometric measurements of magnitude.

Leavitt also did much work on variable stars, discovering about 2400 – roughly one half of those known in her time. She is best known, however, for her studies of Cepheid variables. At Harvard Observatory's field station at Arequipo, Peru, a series of photographic plates had been taken of the Magellanic Clouds (now known to be small neighboring galaxies). From her analysis of the plates, Leavitt detected nearly 1800 variable stars, some of which belonged to a class known as Cepheid variables. The variation in brightness of Cepheids is extremely regular and in 1908 Leavitt noted that the brighter Cepheids had the longer periods. By 1912 she was able to show that the apparent magnitude, i.e., observed brightness, of Cepheids decreased linearly with the logarithm of the period.

It was this seemingly simple discovery that led to an invaluable means for determining very great distances; previous to this only distances out to a hundred light-years could be estimated. Leavitt's work on the light variation of Cepheids was extended first by Ejnar Hertzsprung and Harlow Shapley and then by Walter Baade to give the period–luminosity relation of Cepheids. Using this relation the luminosity, or intrinsic brightness, of a Cepheid can be determined directly from a measure of its period and this in turn allows the distance – of the Cepheid and its surroundings – to be calculated. Distances of galaxies up to ten million light-years away can be determined this way.

Henrietta Swan Leavitt (1868-1921) was an American astronomer of the first magnitude. Her research resulted in numerous advances within the field, the effects of which extended well beyond her lifetime. She discovered a means to rank stars's magnitudes using photographic plates, which became a standard in the field. Leavitt also discovered a means by which astronomers became better able to accurately measure extra galactic distances known as the period-luminosity relation. She also discovered more variable stars than any other astronomer in her time.

Parents Supported Her Education

Henrietta Swan Leavitt was born in Lancaster, Massachusetts, on July 4, 1868, where she was one of seven children. Her parents were Henrietta Swan Kendrick and George Roswell Leavitt, a Congregationalist minister whose parish was in Cambridge, Massachusetts. Her parents, who were said to have been strict Puritans, did encourage Leavitt to use her intellect. The majority of people in that period did not support education for women. The Leavitt family eventually relocated to Cleveland, Ohio.

Leavitt studied at Oberlin College in Ohio between 1885 and 1888. She transferred to the Society for the Collegiate Instruction of Women (which would later be known as Radcliffe College of Harvard University) in Cambridge, Massachusetts, where she finished her A.B. degree in 1892. It was while in her senior year of college that Leavitt first became interested in astronomy.

After graduation she took another astronomy course, but then suffered a debilitating illness. It left her profoundly deaf and she stayed home for several years.

From Volunteer to Harvard Researcher

Leavitt received an appointment as a research assistant at the Harvard College Observatory in Cambridge in 1895. This was a voluntary post; her assignment was to determine stars' magnitudes by consulting photographs of the heavens. Her work impressed the staff. "She soon rose 'by her scientific ability and intense application,' " according to her biography on the Amercian Association of Variable Star Observers (AAVSO) website.

She was given a permanent position in 1902 by Edward Pickering, a noted astronomer who was head of the Harvard College Observatory. Her salary was 30 cents per hour. Leavitt was one of a group of women working at the observatory who were known as computers. It has been said that Pickering hired women in order to save money because he would have had to pay men with the same education greater salaries. Other women in this group also became well-respected astronomers, including Annie Jump Cannon and Williamina Fleming.

Leavitt was soon promoted as head of the photographic photometry department. Photometry, as its name implies, is the science of measuring stars' brightness. Employing photography in astronomy necessitated adjusting astronomers' magnitude scale to compensate for the way film registers light. This post did not give Leavitt time to indulge in theoretical work. Actually, she was given no latitude in her choice of research. Pickering would typically assign work to Leavitt on topics that interested him.

Developed Reputation as Variable Star "Fiend"

In her role in the photometry department, Leavitt was assigned to search photographic plates for variable stars in the Magellanic Clouds regions. According to Astronomy, "The technique for variable hunting was strikingly uncomplicated. Leavitt would simply overlay the positive plate of a region of sky on the negative plate taken on a different night. If the positive and negative star images didn't match up, she would flag a potential variable." This technique was known as superposition. By 1904 she discovered more than 200 variable stars using this method. The following year that number grew to more than 840 stars.

"What a variable-star 'fiend' Miss Leavitt is," wrote Charles Young of Princeton in a letter to Pickering. "One can't keep up with the roll of the new discoveries." It was while working at the observatory that Leavitt discovered a means to rank the magnitudes or brightness of stars on photographic plates. This ranking would become a standard used by astronomers, known as the Harvard Standard.

Worked on Several Major Research Projects

Leavitt was interested in Cepheids. These variable stars become brighter, then dim in a regular cycle. The Cepheids were named after the first star of this type to be discovered, Delta Cephei. Leavitt first made the observation in 1904 that there was a relationship between how long a Cepheid took to complete one of these cycles and its ultimate magnitude. The difficulty was that this could not be confirmed by observing these stars in this galaxy. There were too many factors that could skew the possible results. She eventually published her discovery in 1908. Leavitt continued to work on this research for four more years.

Beginning in 1907, Leavitt was asked to develop a "North Polar Sequence" by which star brightness could be described and which would serve as a standard in the field. She used 46 stars near the North Pole to represent each of the varying degrees of star magnitudes. Magnitude could not be reliably determined from photographic images nor from telescopic images. Stars typically emit light in various colors, which can confuse or trick the eye. Using this data as a basis for determining magnitude gave varied and often inaccurate results. What Leavitt did was compare and contrast the stars with each other using various images from many different telescopes. This scale she created assigned brightnesses for the stars in a range from the 4th to the 21st magnitude. Her findings were published in the Annals of Harvard College Observatory.

While studying photographic images of the estimated 1800 Cepheid variable stars in a system known as the Magellanic Clouds taken from Harvard's observatory in Peru in 1912, Leavitt found a direct correlation between the brightness of a Cepheid variable star and the period of its variability to confirm her theory. The Magellanic Clouds are star systems that are located just outside the Milky Way. The theoretical relationship she posited is commonly known as the period-luminosity theory. Leavitt had thought since the stars in this system are approximately the same distance from Earth, then there might be a relationship between these two factors. Her hypothesis was correct.

Expressed arithmetically, she determined the Cepheid's apparent magnitude increases linearly with the logarithm of the period. In 1912, Leavitt published her results and a table of the 25 Cepheid periods. These periods ranged in length from 1.253 days to 127 days, with an average period duration of five days. These stars' apparent brightnesses were also included in the table.

"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 variable and their periods," wrote Leavitt in 1912, as quoted on the AAVSO website. She noted that "since the variables are probably 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."

Since the stars in this system are approximately the same distance from Earth, if the range of a single Cepheid could be calculated, then that data could be used to calculate the distances to the Magellanic Clouds. The data could also then be further used to calculate the distance to even more distant Cepheids.

"The measurement and discussion of these objects present problems of unusual difficulty, on account of the large area covered by the two regions, the extremely crowded distribution of the stars contained in them, the faintness of the variables, and the shortness of their periods," according to a document published by the observatory in 1912 and quoted on the Bloomfield Science Museum website. "As many of them never become brighter than the fifteenth magnitude, while very few exceed the thirteenth magnitude at maximum, long exposures are necessary, and the number of available photographs is small."

In 1913 Leavitt's system for describing magnitudes or "North Polar Sequence" was adopted by the International Committee on Photographic Magnitudes. She established these sequences for 108 areas in the heavens. When her supervisor, Pickering, established 48 "Harvard standard regions," Leavitt calculated secondary brightness standards for each of them. These international standards were used until the methodology improved.

Using Leavitt's period-luminosity theory, a Danish astronomer was able in 1913 to calculate some star's distances only using their period. Astronomers typically used a measurement method known as the parallax method to determine distances between stars. This worked well for distances up to 100-light-years, but making these measurements was difficult. Additional work by other astronomers further refined how the relationship between absolute brightness and period could be accurately used to calculate distances greater than 10 million light-years, as well as to determine the actual distance between the Earth and a given star. This also gave astronomers a better idea as to the vastness of the heavens. It enabled Harlow Shapley to measure the size of the Milky Way galaxy.

Leavitt also discovered more variable stars during her career than had any other astronomer. Leavitt catalogued about 2400 variable stars while working at Harvard. At the time, this was about half the known variable stars. She also had discovered four novae and studied Algol-type eclipsing binary stars and asteroids as well.

Consequences of Consignment to Menial Tasks Mulled by Colleagues

As previously mentioned, Pickering dictated the work Leavitt and the other computers were to do. "If Leavitt had been free to choose her own research projects, she might have investigated the consequences of the period-luminosity relationship she had discovered," according to an excerpt from Women of Science: Righting the Record on a UCLA physics website. "Pickering hired people to do a specific job and didn't want them wasting their time doing anything else." That Leavitt was not given free reign to explore her passion for variable stars most likely impeded progress in the field. Cecilia Payne-Gaposchkin, one of these women who knew Leavitt, wrote that for Pickering to relegate her solely to photometry "was a harsh decision, which condemned a brilliant scientist to uncongenial work, and probably set back the study of variable stars for several decades."

Among the professional organizations of which she was a member were the American Association for the Advancement of Science and the Astronomical and Astrophysical Society of America. Leavitt was also an honorary member of the American Association of Variable Star Observers as well as a member of Phi Beta Kappa and The American Association of University Women.

"Miss Leavitt inherited, in a somewhat chastened form, the stern virtues of her puritan ancestors," Solon I. Bailey, a Harvard professor wrote of Leavitt in 1922, quoted on the AAVSO website. "She took life seriously. Her sense of duty, justice and loyalty was strong. For light amusements she appeared to care little. She was a devoted member of her intimate family circle, unselfishly considerate in her friendships, steadfastly loyal to her principles, and deeply conscientious and sincere in her attachment to her religion and church. 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."

Honored by Nobel Committee

Leavitt worked at Harvard until her death. She died of cancer December 21, 1921, in Cambridge, Massachusetts. Her colleagues mourned her passing, in particular the void her death created. Some of her colleagues thought her to have been the brightest among them. She had made an irreplaceable contribution to the field.

Books

A to Z of Women in Science and Math, Facts on File, 1999.

American Science Leaders, ABC-CLIO, 2001.

American Women in Science: A Biographical Dictionary, ABC-CLIO, 1994.

Notable Scientists: From 1900 to the Present, Gale Group, 2001.

World of Scientific Discovery, 2nd ed. Gale Group, 1999.

Periodicals

Astronomy, July 2002.

Online

American Decades CD-ROM, Gale Research, 1998.

"Henrietta Swan Leavitt," CWP at UCLA, http://www.physics.ucla.edu/∼cwp/Phase2/Leavitt,_Henrietta_Swan@871234567.html (March 3, 2003).

"Henrietta Swan Leavitt," Famous Science Innovators, Bloomfield Science Museum Jerusalem, http://www.mada.org.il/website/html/eng/2_1-1-31.htm (March 3, 2003).

"Henrietta Swan Leavitt," Hands On Astronomy, Amercian Association of Variable Star Observers website, http://hoa.aavso.org/posterswan.htm (February 28, 2003).

Henrietta Swan Leavitt
Henrietta Swan Leavitt
Born	July 4, 1868
Died	December 12, 1921 (aged 53)
Nationality	United States
Fields	Astronomy
Known for	period-luminosity relationship of Cepheid stars

Henrietta Swan Leavitt (July 4, 1868 – December 12, 1921) was an American astronomer. A graduate of Radcliffe College, Leavitt went to work in 1893 at the Harvard College Observatory in a menial capacity as a "computer", assigned to count images on photographic plates. Study of the plates led Leavitt to propound a groundbreaking theory, worked out while she labored as a $10.50-a-week assistant, that was the basis for the pivotal work of astronomer Edwin Hubble. Leavitt's discovery of the period-luminosity relation of Cepheid variables radically changed the theory of modern astronomy, an accomplishment for which she received almost no recognition during her lifetime.

Contents 1 Early years and education 2 Career 3 Awards and honors 4 Notes 5 References 6 External links

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 bachelors degree in 1892.

Career

Leavitt began work in 1893 at 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.^[3] (In the early 1900s, women were not allowed to operate telescopes).^[4] 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,^[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 variable stars of greater intrinsic luminosity – actually Cepheid variables – did indeed have longer periods,^[6] and the relationship was quite close and predictable.

Leavitt's discovery is known as the 'period-luminosity relationship'. "A straight line can be readily drawn among each of the two series of points corresponding to maxima and minima," Leavitt wrote of her study of 1,777 variable stars recorded on Harvard's photographic plates, "thus showing that there is a simple relation between the brightness of the variable and their periods."^[7]

This relationship provided an important yardstick for measuring distances in the Universe, if it could be calibrated. 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 determined.

At the time, it was not clear that millions of (what we now call) galaxies were actually outside of the Milky Way galaxy. Their distance could not be measured until a tool existed. 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. Our picture of the universe was changed forever, largely because of Leavitt's discovery.

The accomplishments of Edwin Hubble, renowned 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," write David H. and Matthew D.H. Clark in their book Measuring the Cosmos.^[8] Hubble himself often said that Leavitt deserved the Nobel for her work.^{[citation needed]}

Henrietta Swan Leavitt – one of several women working in "Pickering's Harem" who made fundamental contributions to astronomy^[9]

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.^[9] 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 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. As Solon I. Bailey noted:

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.^[10]

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.^[11][10]

Notes

1. ^ Gregory M. Lamb (July 5, 2005). "Before computers, there were these humans...". Christian Science Monitor. http://www.csmonitor.com/2005/0705/p15s01-bogn.html. Retrieved 2007-05-18. 
2. ^ Out of Shadows: Contributions of Twentieth-century Women to Physics, Nina Byers, Gary Williams, Cambridge University Press, 2006, ISBN 0521821975, 9780521821971
3. ^ Leavitt began working for Pickering as a volunteer in 1893. Because she had 'independent means', Pickering did not have to pay her. Later, as a "computer", she was paid $10.50 a week for her work studying photographic plates and deciphering what they meant. Pickering assigned Leavitt to study 'variable stars', whose luminosity varies over time. "Variable stars had been of interest for years," writes noted science author Jeremy Bernstein in The Los Angeles Times, "but when she was studying those plates, I doubt Pickering thought she would make a significant discovery – one that would eventually change astronomy." Story.
4. ^ Exploratorium note
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. ^ Henrietta Swan Leavitt: She Showed How to Calculate the Distances to Galaxies, Hands-On Astrophysics, American Associaton of Variable Star Observers
8. ^ Measuring the Cosmos: How Scientists Discovered the Dimensions of the Universe, David H. Clark, Matthew D.H. Clark, Rutgers University Press, 2004
9. ^ ^{a b} Hamblin, Jacob Darwin (2005). Science in the early twentieth century: an encyclopedia. ABC-CLIO. pp. 181-184. ISBN 1851096655. http://books.google.com/books?id=mpiZRAiE0JwC&pg=PA181&lpg=PA181&dq=Leavitt+california+benjamin+leavitt&source=web&ots=qbhrQbSbE2&sig=lx35eM7pNSpM41l2ltmMLrU3R2g&hl=en&sa=X&oi=book_result&resnum=10&ct=result#PPA181,M1. 
10. ^ ^{a b} ACS bio
11. ^ Johnson, pp 118–119

References

• Johnson, George (2005). Miss Leavitt's Stars: The Untold Story of the Woman Who Discovered How to Measure the Universe. New York: W.W. Norton & Company. ISBN 0-393-05128-5. 
• Korneck, Helena: "Frauen in der Astronomie", Sterne und Weltraum, Oct. 1982 412–414
• Lorenzen, Michael (1997). "Henrietta Swan Leavitt", in Notable Women in the Physical Sciences: A Biographical Dictionary. Edited by Barbara and Benjamin Shearer. Westport, CT: Greenwood Press, 233–237. ISBN 0-313-29303-1.

External links

• Bibliography from the Astronomical Society of the Pacific
• Periods Of 25 Variable Stars In The Small Magellanic Cloud. Edward C. Pickering, March 3, 1912; credits Leavitt.
• Henrietta Swan Leavitt: a Star of the Brightest Magnitude ACS Biography with several links
• Henrietta Swan Leavitt at Find a Grave

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