Henrietta Swan Leavitt - An Early Variable Star Astronomer
It may have been a sign that since Henrietta Swan Leavitt was born on Independence Day of 1868, that she was destined for great things. Her mother was Henrietta Swan (Kendrick) and her father, George Roswell Leavitt was a minister at the Congregational Church in Cambridge, Massachusetts. They we're direct descendants of Deacon John Leavitt, then spelled Levett, who had settled in the Massachusetts Bay Colony in the early 1600's.
Leavitt grew up with her brother, Roswell Leavitt and her sister, Mira Leavitt in Lancaster, Massachusetts. Her parents, though they were strict puritans did, in fact, support their daughter and encouraged Henrietta to use her intellect. They were mostly alone in their views, for at this period of history, most people did not support women being educated. Eventually, her entire family moved from their Lancaster home to the heartland of America, Cleveland, Ohio. Though her siblings died young, or perhaps because of it, Leavitt wanted to do great things for her family and the name Leavitt.
Despite societal standards and attitudes about the place of women, Leavitt was determined to attend college. Her first collegiate experience was at Oberlin College from 1885 to 1888. After her time at Oberlin, Leavitt transferred to the Society for the Collegiate Instruction of Women, later known as Radcliffe College of Harvard University. The well rounded education she got at Radcliff incorporated Classical Greece, fine arts, philosophy, differential calculus and analytical geometry.
By 1892 Leavitt had finished her B.A. degree, but, ultimately something more important happened during her time in college. In her senior year, she took a course in astronomy and she fell in love with the topic.
Leavitt followed up with her passion for astronomy after her graduation and pursued a course in it. Unfortunately, this was around the time she was struck by a debilitating illness which eventually rendered her completely deaf. She stayed home most of the time while she was fighting her illness, but, in time, she returned to Radcliffe when she took on a volunteering position at Harvard College Observatory.
Early Professional Life
When Leavitt took a position at Harvard College Observatory in 1893, she was assigned to the role of a research assistant. Harvard Observatory quickly became her life and she abandoned all frivolous pursuits, dividing her time between work, church, and family.
Her assignment during her time as a volunteer was to determine stars' magnitudes based on photographic evidence of the sky. Leavitt's devotion to her work impressed the full time staff, most notably, Edward Pickering. Pickering offered Leavitt a full time position at the observatory in 1902, which paid her 30 cents per hour. According to the American Association of Variable Star Observers' website, "She soon rose by her scientific ability and intense application."
Pickering hired other notable women, such as Annie Jump Cannon and Williamina Fleming, to the position of 'computers.' It was more financially sound for Pickering to hire women for these positions as men of the same education level would require larger salaries.
It was not long before Leavitt was promoted to the position of head of the photographic photometry department. This may sound like a fantastic opportunity, especially for a woman of the time, but it did have its drawbacks. Leavitt was assigned to work on projects that interested Pickering, and was left with little time to follow her own theoretical work.
Due to the way film registers light, adjustments to the magnitude scales which were being used by astronomers needed adjustment. These adjustments needed to be made to accurately gauge the brightness of stars.
One of the projects assigned to Leavitt, which became vital to her reputation as an astronomer, was to look through photographic plates in order to search for variable stars in the Magellanic Clouds. Though the Magellanic clouds are only visible from the southern hemisphere, they prove as potentially important since they are dwarf galaxies which may very well orbit our Milky Way galaxy.
The method Leavitt used to search for variable stars was simple, yet brilliant. She took a negative plate and laid it over a positive plate with a picture taken on a different night. Any of the stars that were different, did not line up, between the positive and the negative plates were flagged as potential variable stars. Using this technique, called superposition, Leavitt discovered more than 200 variable stars by 1904. The following year, the number grew to 840 variable stars.
It was because of the ferocity Leavitt displayed that Charles Young first coined the term "variable-star fiend" when describing her. Leavitt's work on the Magellanic Cloud Stars that Leavitt discovered a method to rank the brightness, or magnitude, of the stars that were on photographic plates. This ranking is now known as the Harvard Standard.
Video about Henrietta Leavitt
A specific kind of star, known as a Cepheid, is a variable star that brightens and dims in a regular cycle. These Cepheids were of particular interest to Leavitt. In 1904, Leavitt came across a profound discovery. She determined that the length of time it took for a Cepheid to complete a cycle was directly related to its absolute magnitude. Leavitt wanted to prove this, but relying solely on observation left the potential for the results to be skewed by countless factors. Not to be discouraged, Leavitt published her findings in 1908.
She worked on this research until the year 1912 when she found enough correlating evidence to corroborate her theory. Leavitt published a table of 25 Cepheid cycles which range in length from as little as 127 days to 1,253 days. The stars' brightness levels were also included. The same year that this research was published, Leavitt wrote "A straight line can be readily drawn among each of the two series points corresponding to maxima and minima, thus showing that there is a simple relation between the brightness of the variable and their periods."
Early in the 1900s Leavitt was asked to create a method to describe star brightness and to standardize magnitudes, this was later called the "North Polar Sequence." Since stars emit different colored light, the amount of light seen to either the naked eye or through a telescope is not reliable and can play tricks on the eyes. This led to inaccurate results.
Leavitt used 46 stars near the North Pole as the basis for her varying degree of star magnitudes. She compared and contrasted the stars using images from telescopes across the globe. She created a scale which gave stars a range from the 4th to the 21st magnitude. These findings were published in the Annals of Harvard College Observatory.
Leavitt admitted that the low degree of magnitude of some stars, combined with the small availability of photographs, existing at the time, made the correct measurement of celestial objects "unusually difficult." The International Committee on Photographic Magnitudes officially adopted Leavitt's system, the "North Polar Sequence" in 1913. She establishes sequences for 108 areas of the sky, when her boss, Edward Pickering, established 48 "Harvard standard regions," Leavitt created secondary brightness standards for them.
In addition to her method, which was eventually modified into the form used by astronomers today, Leavitt catalogued 2400 variable stars. During her time, this was half of all the known variable stars. She studied Algol-type eclipsing binary stars. These type of variable stars are those that coincide with the line of sight from earth. Leavitt discovered four novae and several asteroids as well. Had Leavitt not provided the key for determining the size of the universe, it is entirely possible that Edwin P. Hubble, most famous for the Hubble telescope, would have never been able to determine that the universe is expanding.
Levitt was a member of the first Phi Beta Kappa group. She was an active member of the American Association of University Women, The American Association for the Advancement of Science, the American Astronomical and Astrophysical Society and was an honorary member of the American Association of Variable Star Observers.
The impact of Leavitt may have been even more astounding had she had the freedom to explore the fields that truly interested her. But, Pickering was determined to not have his employees waste time following leads he did not believe would be beneficial. In this way Leavitt, and many of her female colleagues, to follow menial tasks.
Importance of the Period-Luminosity Releationship
The Period-Luminosity relationship for Cephid type variable stars allows astronomers a method to estimate distance to star clusters and other galaxies. The term Cepheid comes from the star Delta Cephei in constellation Cepheus. The variability of Delta Cephei was first discover by John Goodricke in 1784. A Cepheid variable star changes in brightness in a regular manner due to pulsations of the star. A star of this type varies between a larger, bright state and a smaller more dense, dimmer state. What Henrietta Leavitt pointed out was that the relationship between the period of pulsation and the intrinsic brightness of the star had a well defined relationship. By visually observing or by looking at photographic plates one could determine the period of light variation and as Ms. Leavitt discovered that the period was directly related to the absolute brightness. She arrived at this conclusion by observing several Cephid type variable stars in the Small and Large Magellanic Clouds. These are two small satellite galaxies that orbit the Milky Way galaxy. Since the linear size of these satellite galaxies is much smaller than the distance from the Milky Way, therefore, Ms. Leavitt concluded that all the stars in these galaxies were at the same distance. Leavitt reasoned that the more luminous Cepheids pulsated more slowly, which is the basis of the period-luminosity relation.
The period-luminosity was very useful but it only gave relative distances to stars, not the actual distance in light years. In 1923 Ejnar Hertzsprung took up this task and calibrated Leavitt's relation using Cepheids in our galaxy for which the distances could be determined. In the calibration process, Herzsprung put actual values to the luminosity part of the period-luminosity relation. With this calibrated period-luminosity relationship astronomers could now use Cepheid variables as standard candles to determine the distances to distant star clusters and galaxies.
Sidelined by health problems and family issues, the work that Henrietta Leavitt did with Harvard was sporadic throughout her life. Already struck completely deaf during her college years, Leavitt was susceptible to illnesses.
Leavitt was appointed head of stellar photometry, in 1921 when Harlow Shapely took over as director of the observatory. Her promotion was short lived however, as the battle she had been fighting with stomach cancer was ultimately lost. She died December 12th, 1921 at the age of 53.
Her unexpected and fairly early death, sent a shockwave to her colleagues. It was not simply her scientific mind and contributions that were mourned by the community. One of her closest colleagues, Solon I. Bailey wrote 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."
Hubble was the first to suggest that Leavitt deserved a Nobel Prize. She was nominated by a member of the Swedish Academy of Sciences, Gösta Mittag-Leffle, in 1924. When the Academy learned of Henrietta Leavitt's death, which occurred three years prior, they withdrew their nomination instead of awarding the Nobel Prize posthumously.
She currently rests next to other members of her family at Cambridge Cemetery in Cambridge, Massachusetts. According to George Johnson in the biography he authored about Leavitt, she sits on top of a gentle hill. The spot where she is buried is marked by a tall hexagonal monument which is topped by a globe on a draped marble pedestal.
Though the gravesites of some of her male relatives are more frequently visited than Henrietta's, the marble sculpture marking her grave is fitting for someone who made so many contributions to her field. Not only was she an astronomer, she was a female astronomer at a time when women were under appreciated in the sciences.
Johnson, George. Miss Leavitt's Stars: The Untold Story of the Woman Who Discovered How to Measure the Universe (Great Discoveries). W. W. Norton & Company; Reprint edition. 2006.