Henrietta Swan Leavitt

Astronomer

• Born: July 4, 1868
• Birthplace: Lancaster, Massachusetts
• Died: December 12, 1921
• Place of death: Cambridge, Massachusetts

American astronomer

While studying Cepheid variable stars, American astronomer Henrietta Swan Leavitt discovered the period-luminosity law, which is the relationship between a star’s brightness and its distance. Leavitt also established the Harvard Standard for the photographic measurement of stars.

Born: July 4, 1868; Lancaster, Massachusetts

Died: December 12, 1921; Cambridge, Massachusetts

Primary field: Astronomy

Specialty: Observational astronomy

Early Life

Henrietta Swan Leavitt was born on July 4, 1868, in Lancaster, Massachusetts. She was one of seven children born into an upper middle-class family. Her father, George Roswell Leavitt, was a minister. As a child, she was taught the importance of patience and a strong work ethic, which became useful in her later work as an amateur scientist. Leavitt entered Oberlin College in Ohio in 1885, but she later transferred to Radcliffe College, then known as the Society for Collegiate Instruction of Women, in Cambridge, Massachusetts. She became interested in astronomy during her senior year and received her bachelor’s degree in 1892.

After graduation, Leavitt took another course in astronomy, but she then became very ill. Although she did recover, her illness made her deaf, and she was unable to continue her studies. In 1895, Leavitt began working as a volunteer research assistant at the Harvard College Observatory in Cambridge. The scientists at the observatory were trying to determine the magnitude, or brightness, of as many stars as possible.

In 1902, Professor Charles Pickering, the director of the observatory, hired Leavitt, along with several other women, full time. He hired women because they could be paid less than men; they were eventually nicknamed the “Harvard Computers” because their job was to process astronomical data. Leavitt was hired to perform routine tasks, one of which was to measure the magnitudes of variable stars. Unlike that of other stars, the brightness, or luminosity, of variable stars changes over time. Measuring the luminosity of a star gives astronomers a way to determine the amount of energy that is being released by the star. To measure, Leavitt would compare a picture of the variable star at one point in time to a picture of the same star taken at another time.

Life’s Work

For more than twenty years, Leavitt measured the brightness of stars using glass photographic plates. The process depended solely on the human eye being able to pick up tiny differences between a light-colored stain and an even lighter-colored one on the plates, indicating that the star’s magnitude had changed. The photographic plates were black, white, and shades of gray. Like traditional photographic negatives, the light and dark were reversed. This made distinguishing the differences between stars’ magnitudes difficult.

The measurement of stars’ brightness using photographs was not standardized, since the work depended on who was looking at the pictures to determine what types of changes had occurred. Leavitt spent a lot of time developing a uniform scale, or standard, that all viewers of the plates could apply. She wanted evaluations of magnitude to be more consistent and less prone to subjectivity. To accomplish her goal of standardization, Leavitt used nearly three hundred photographic plates, more than a dozen different telescopes, and several complex mathematic equations. She organized the plates in order of brightness and eventually created a scale with seventeen levels of magnitude.

Leavitt published the results of her research in 1908. In 1913, the International Committee on Photographic Magnitudes accepted and adopted Leavitt’s standard, naming it the Harvard Standard. Leavitt continued working on the standard to refine it and include information from additional photographic plates. Using the Harvard Standard, she measured the magnitude of 2,400 variable stars. This was about half the number of known stars at the time.

In the course of Leavitt’s study of photographic plates leading up to the development of the Harvard Standard, she discovered nearly 1,800 variable stars in the Small Magellanic Cloud, a dwarf galaxy readily visible from Earth. These stars, called Cepheid variables, became the other focus of Leavitt’s research. Cepheid variables are giant, pulsing stars that are sometimes found in young clusters of stars.

Over time, Leavitt determined that this pulsing luminosity has a regular rhythm to it. In addition, she discovered that the rhythm is directly proportional to the magnitude of light being emitted. In general, Leavitt found that a star’s brightness would range from a few percent to a factor of two or three, and that this phenomenon would reoccur on a regular schedule that varied from star to star. Leavitt published her findings in 1912, in the Harvard College Observatory Circular. The relationship between a star’s rhythmic pulsing and its luminosity later became known as the period-luminosity relation, or the period-luminosity law.

During her career as an astronomer, Leavitt was a member of several academic, scientific, and professional organizations, including: Phi Beta Kappa, the American Astronomical and Astrophysical Society (later the American Astronomical Society), the American Association of University Women, and the American Association for the Advancement of Science. She was also made an honorary member of the American Association of Variable Star Observers. In 1921, Leavitt was appointed head of the photographic photometry at the observatory. She died of cancer shortly thereafter, on December 12, 1921, at the age of fifty-three.

Impact

Leavitt’s Harvard Standard continued to be used for several years after her death. Since then, modern technology has helped to more accurately measure the intensity of light emitted by stars. The Cepheid variables Leavitt studied became “standard candles,” or class of astronomical objects with a known brightness, for measuring stellar distances. Her discovery of the period-luminosity relation enabled later astronomers to measure the distance between Earth and various stars, as well as distances between other galaxies and our own. Building on Leavitt’s work, astronomers were able to create the first reliable estimates of the size and shape of the Milky Way.

Immediate applications of Leavitt’s work appeared in the studies of American astronomers Harlow Shapley and Edwin Hubble. From RR Lyrae stars, Cepheid-like pulsating stars located within the globular clusters that surround our galaxy, Shapley produced an approximate distance map for the entire Milky Way Galaxy. He demonstrated that the Milky Way is a flattened spiral disk with a thickened center. He also measured the approximate diameter of the Milky Way and estimated that the sun is about fifty thousand light-years from the galactic center. This was later corrected to about thirty thousand light-years.

Hubble used the work of Leavitt and Shapley to measure the distances to RR Lyrae stars located within the Andromeda Galaxy, obtaining an approximate intergalactic distance of 750,000 light-years—later recalibrated to one million light-years—showing Andromeda to be one of the closest galaxies to Earth. Hubble applied this technique to other galaxies as well. This work contributed to his later studies of galactic redshift velocities, which led to his discovery that the universe is expanding.

Bibliography

Bartusiak, Marcia. The Day We Found the Universe. New York: Vintage, 2009. Print. A history about discovering the universe, centered on the early 1900s. Includes Leavitt’s research. Illustrations, bibliography, index.

Johnson, George. Miss Leavitt’s Stars: The Untold Story of the Woman Who Discovered How to Measure the Universe. New York: Norton, 2005. Print. Biography of Leavitt, detailing her work at Harvard and emphasizing its significance. Illustrations, bibliography, index.

Percy, John R. Understanding Variable Stars. New York: Cambridge UP, 2011. Print. Discusses the variety, structure, properties, and evolution of variable stars, including Cepheids. Illustrations, bibliography, index.