The Life and Discoveries of Astronomer Henrietta Swan Leavitt

Leavitt Lit a "Standard Candle" to Measure the Cosmic Darkness

The Andromeda Galaxy is the closest spiral galaxy to the Milky Way. Its distance was determined first in the 1920s, using a discovery made by astronomer Henrietta Swan Leavitt. Adam Evans/Wikimedia Commons.

Henrietta Swan Leavitt (1868-1921) was a U.S. astronomer whose work guided the field to understand distances in the universe. At a time when women's contributions were undervalued, attributed to male scientists, or ignored, Leavitt's findings were seminal to astronomy as we understand it today.

Leavitt's careful work measuring the brightness of variable stars, forms the basis of astronomical understanding of such topics as distances in the universe and the evolution of stars. Such luminaries as astronomer Edwin P. Hubble praised her, stating that his own discoveries rested largely on her accomplishments. 

Early Life and Career

Henrietta Swan Leavitt
Henrietta Swan Leavitt at work on cataloging stars while at Harvard Observatory. Harvard College Observatory

Henrietta Swan Leavitt was born on July 4, 1869, in Massachusetts to George Roswell Leavitt and Henrietta Swan. Little is known about her private life. As a college student, she studied a number of subjects, falling in love with astronomy during her years at what later became Radcliffe College. She spent some years traveling around the world before settling back in the Boston area to pursue further studies and work in astronomy.

Leavitt never married and was considered a serious, church-going woman with little time to waste on more frivolous aspects of life. Her co-workers described her as pleasant and friendly, and very focused on the importance of the work she was doing. She began to lose her hearing as a young woman due to a condition that only worsened with time.

In 1893 she began working at Harvard College Observatory under the direction of astronomer E.C. Pickering. He directed a group of women, dubbed merely as "computers". These "computers" conducted important astronomy research by studying photograph plates of the sky and cataloging characteristics of stars. The women were not allowed to operate telescopes, which limited their ability to conduct their own research. 

The project involved careful comparisons of stars by looking at photographs of star fields taken several weeks apart to look for variable stars. Leavitt used an instrument called a "blink comparator" which allowed her to measure brightness changes of stars. It's the same instrument that Clyde Tombaugh used in the 1930s to discover Pluto

At first, Leavitt took on the project for no pay (since she had her own income), but eventually, she was hired at a rate of thirty cents an hour.

Pickering took credit for much of Leavitt's work, building his own reputation on it.

The Mystery of Variable Stars

A cepheid variable.
A typical Cepheid variable star called RS Puppis. This image was made by data taken by Hubble Space Telescope. NASA/STSCI

Leavitt's main focus was a certain type of star called a Cepheid variable. These are stars that have very steady and regular variations in their brightnesses. She discovered a number of them in the photographic plates and carefully cataloged their luminosities and the period of time between their minimum and maximum brightnesses.

After charting a number of these stars, she noticed a curious fact: that the period of time it took for a star to go from bright to dim and back again was related to its absolute magnitude (the brightness of the star as it would appear from a distance of 10 parsecs (32.6 light-years).

During the course of her work, Leavitt discovered and cataloged 1,777 variables. She also worked on refining standards for photographic measurements of stars called the Harvard Standard. Her analysis led to a way to catalog star luminosities across seventeen different magnitude levels and is still used today, along with other methods to determine a star's temperature and brightness.

For astronomers, her discovery of the "period-luminosity relationship" was huge. It meant they could accurately calculate distances to nearby stars by measuring their changing brightnesses. A number of astronomers began using her work to do just that, including the famous Ejnar Hertzsprung (who devised a classification diagram for stars called the "Hertzsprung-Russell diagram"), and measured several Cepheids in the Milky Way.

Leavitt's work provided the "standard candle" in the cosmic darkness they could use to find out just how far away things were. Today, astronomers routinely use such "candles" even as they still seek to understand why these stars vary in their brightness over time.

The Expanding Universe

The Cepheid variable in Andromeda that Hubble observed.
This Hubble image shows the Andromeda Galaxy and the variable star that Edwin P. Hubble used to determine the distance to Andromeda. His work was based on Henrietta Leavitt's work on the period-luminosity relationship. The upper right image is a closeup of the starfield. The lower right image shows his chart and notes upon discovery. NASA/ESA/STScI

It was one thing to use the variability of Cepheids to determine distances in the Milky Way—essentially in our cosmic "back yard"—but quite another to apply Leavitt's period-luminosity law to objects beyond it. For one thing, until the mid 1920s, astronomers largely thought that the Milky Way was the entirety of the universe. There was much debate about the mysterious "spiral nebulae" that they saw through telescopes and in photographs. Some astronomers insisted they were part of the Milky Way. Others argued they were not. However, it was difficult to prove what they were without accurate ways of measuring stellar distances.

Henrietta Leavitt's work changed that. It allowed astronomer Edwin P. Hubble to use a Cepheid variable in the nearby Andromeda Galaxy to calculate the distance to it. What he found was astonishing: the galaxy was outside our own. That meant the universe was much larger than astronomers understood at the time. With measurements of other Cepheids in other galaxies, astronomers came to understand distances in the cosmos.

Without Leavitt's important work, astronomers wouldn't have been able to calculate cosmic distances. Even today, the period-luminosity relationship is an important part of the astronomer's toolbox. Henrietta Leavitt's persistence and attention to detail led to the discovery of how to measure the ​size of the universe.

Henrietta Leavitt's Legacy

variable star
The study of variable stars by Henrietta Leavitt is her legacy to astronomy. NASA

Henrietta Leavitt continued her research until just before her death, always thinking of herself as an astronomer, despite her start as a nameless "computer" in Pickering's department. While Leavitt was not officially recognized during her life for her seminal work, Harlow Shapley, the astronomer who took over as the Harvard Observatory director, did recognize her worth and made her Head of Stellar Photometry in 1921.

By that time, Leavitt was already suffering from cancer, and she died the same year. This prevented her from being nominated for a Nobel Prize for her contributions. In the years since her death, she has been honored by having her name placed on a lunar crater, and asteroid 5383 Leavitt carries her name. At least one book has been published about her and her name is usually cited as part of the history of astronomical contributions.

Henrietta Swan Leavitt is buried in Cambridge, Massachusetts. At the time of her death, she was a member of Phi Beta Kappa, the American Association of University Women, the American Association for the Advancement of Science. She was honored by the American Association of Variable Star Observers, and her publications and observations are archived at AAVSO and Harvard.

Henrietta Swan Leavitt Fast Facts

Born: July 4, 1869

Died: December 12, 1921

Parents: George Roswell Leavitt and Henrietta Swan

Birthplace: Lancaster, Massachusetts

Education: Oberlin College (1886-88), Society for the Collegiate Instruction of  Women (to become Radcliffe College) graduated 1892. Permanent staff appointment to Harvard Observatory: 1902 and became head of stellar photometry. 

Legacy: Discovery of period-luminosity relation in variables (1912), led to a law that allowed astronomers to calculate cosmic distance; the discovery of more than 2,400 variable stars; developed a standard for photographic measurements of stars, later named the Harvard Standard.

Sources and Further Reading

For more information about Henrietta Leavitt and her contributions to astronomy, see: 

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Petersen, Carolyn Collins. "The Life and Discoveries of Astronomer Henrietta Swan Leavitt." ThoughtCo, Mar. 27, 2018, Petersen, Carolyn Collins. (2018, March 27). The Life and Discoveries of Astronomer Henrietta Swan Leavitt. Retrieved from Petersen, Carolyn Collins. "The Life and Discoveries of Astronomer Henrietta Swan Leavitt." ThoughtCo. (accessed May 24, 2018).