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.
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.
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.
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.
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.
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."
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.
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.
Astronomy, July 2002.
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).