Henrietta Swan Leavitt (1868-1921)
A Scientist Spotlight by Laura Leader
A photograph of Henriette Swan Leavitt, from Nate Smith's Library of Congress blog post. https://blogs.loc.gov/inside_adams/2019/12/henriettaleavitt/
Henrietta Swan Leavitt was an American woman born on July 4, 1868. Her father, George Roswell Leavitt, graduated Radcliffe College and became a minister. Leavitt followed in his footsteps, attending the Society for the Collegiate Instruction of Women - a part of Radcliffe College (Smith, 2019). It was there that she discovered her love of astronomy.
Though she started as a computer at the Harvard College Observatory, she would make a discovery that has had lasting impacts on astronomy as a whole. Part of her job at the Harvard College observatory was to study glass photographic plates of variable stars - that is to say, stars whose luminosity (or brightness) varies over a period of time ("1912: Henrietta Swan Leavitt", 2014). It was in recording these varying luminosities that Leavitt made several significant discoveries.
Leavitt discovered the relationship between a star's absolute luminosity and the speed of its pulses of light (Lamb, 2005). Leavitt believed is may be possible to determine a star's inherent brightness by measuring the length of its period - that is, how long it took for the star to go from maximum brightness, to minimum, and back up to maximum. In order to define the relationship between period and luminosity, Leavitt would need to able to measure both of those values. This led to some complications, as there are many factors that impact a star's apparent brightness. Distance was the biggest factor, and one that could not be measured with the current technologies.
To get around this, Leavitt studied a specific type of variable stars: Cepheid variables. Specifically, she studied the Cepheid variables in the Small Magellanic Cloud. She did this because she believed it was likely that all of the stars were roughly the same age, and the same distance to earth. By taking distance out of the equation, she was able to compare the luminosity and period of Cepheid variables in this cloud.
After much observation and documentation, Leavitt was able to determine the relationship between period and luminosity. The longer the period, the brighter the star. An example of this relationship is demonstrated in the graph below.
A graph of the period and brightness of a Cepheid variable star in galaxy M31, from NASA/ESA https://esahubble.org/images/opo1115e/
This knowledge became invaluable in measuring distances outside of our galaxy. An astronomer could observe a Cepheid variable's period, and use that to determine the absolute luminosity of the star. Then, they could document the star's apparent brightness. Knowing both the absolute and apparent luminosity of a star allows the astronomer to discover the distance to this star.
Leavitt's contributions to science cannot be overstated - her work is a fundamental building block to the modern field of astrophysics. And yet, she was barely recognized for her work during her lifetime. And, in some ways, even less now. But, even if her name is not known by all, her influence is visible in every observatory, astrophysics textbook, and astronomy class.
Henrietta Swan Leavitt shall not be forgotten.
Works cited:
Carnegie Institution of Science. (2014, October 20). 1912: Henrietta Leavitt discovers the distance key. 1912: Henrietta Leavitt Discovers the Distance Key | Everyday Cosmology. Retrieved March 12, 2023, from https://cosmology.carnegiescience.edu/timeline/1912.html
Lamb, G. M. (2005, July 5). Before computers, there were these humans... The Christian Science Monitor. Retrieved March 11, 2023, from https://www.csmonitor.com/2005/0705/p15s01-bogn.html
Smith, N. (2019, December 12). Henrietta Leavitt, how she loved the 'clouds'. Library of Congress Blogs. Retrieved March 12, 2023, from https://blogs.loc.gov/inside_adams/2019/12/henriettaleavitt/
Blog post by Laura Leader
This work is licensed under a Creative Commons Attribution 4.0 International License.
