History: Henrietta Swan Leavitt – The Woman Who Changed Astronomy

Currently, I’m reading the book „The End of Everything“ (by Katie Mack), there I found out about Henrietta Swan Leavitt and her contribution to astronomy. It inspired me to make this blog entry about Leavitt; here I lay down a short biography with her main work emphasized on and a honourable mention for the ‚harvard computers‘.

Timeline

1868, July 4
Born in Lancaster, Massachusetts

1886-1888
Attended the Oberlin College in Oberlin, Ohio, then transferred to the Society for the Collegiate Instruction of Women (1879-1894, in 1894 it was renamed to Radcliffe College and became fully integrated into Harvard in 1999).

1892
Graduated from the Society for the Collegiate Instruction of Women.

1895
An interest of hers, which emerged in her senior year, led to her volunteering in the Harvard Observatory.

1902
Leavitt receives a permanent staff appointment. From the start, she worked on the observatory’s great project of determining the brightness of all measurable stars.
There she also was associated with Williamina Fleming (1857 – 1911) and Annie Jump Cannon (1863 – 1941). She soon advanced from routine work to the head of the photographic stellar photometry apartment.

1907
New phase of her work: plan of Edward Charles Pickering (1846 – 1919) to establish photographically standardized values for stellar magnitudes (measurement of brightness used for stars and other objects in space).
The problem was given to her, she began with a sequence of 46 stars which were near the north celestial pole (as it already implies, there’s also a south celestial pole, part of the celestial sphere – shortened with NCP and SCP).

1912
In this year, she made her outstanding achievement. Leavitt discovered that „in a certain class of variable stars, the Cepheid variables, the period of the cycle of fluctuation in brightness is highly regular and is determined by the actual luminosity of the star“. With the calibration of the period-luminosity curve* it permitted astronomers such as Edwin Hubble (1889 – 1953) to determine the distances of other Cepheid stars and thus also star clusters and galaxies in which they were observed in.

*„Once astronomers know how luminous a Cepheid is, they can compare that value to how bright it appears on the sky: the farther the object, the dimmer it will appear“ – NASA

1912 and 1917
Publications of the standards which she developed by constructing new methods of analysis.

1913
An international project called „Astrographic Map of the Sky“ adopts her North Polar Sequence. The project already started in 1887 and used the newly available technology of photography, before that it was done by eye by astronomers looking through the telescope.
This new method made it easier and quicker to catalogue stars; special kind of telescopes were used, known as astrographs (see image below showing The Melbourne Astrographic telescope which is displayed at the Sydney Observatory).

1921
Died in Cambridge, Massachusetts.
When she died she had completely determined magnitudes of stars in 108 areas of the sky, discovered 4 novas and some 2,400 variable stars. The latter is especially remarkable, because the figure compromised more than half of all that were known even by 1930.

The End of Everything

I’m ending this rather short, but hopefully still insightful entry, with a quote of the book where I first learned about her (Katie Mack is a cosmologist, that means her field deals with the ‚interior architecture‘ of our universe): „This discovery was revolutionary, and perhaps one of the most important in the history of astronomy, in that it let us finally measure the scale of the universe around us. It meant that anywhere a Cepheid could be seen, we could get a reliable distance and start to make a usable map. By measuring how quickly a Cepheid pulsed, and how bright it looked from here, Leavitt could tell you with great precision how bright it really was, and thus how distant.“ (p. 118)

Honourable Mention

I’d like to use this opportunity to talk about the women who made the measurements by hand. Known as ‚computers‚, they recorded everything in what is now known as the ‚Ladies Logs‘ (at least in Australia). Leavitt was also part of this group of women who contributed a lot to astronomy.

Their job was to look over the photographic plates of the nightsky and compare them with each other. They were mainly picked by Edward Charles Pickering. Looking at plates for hours on end was considered „boring and unspecialized“ work, which is why Pickering turned to women – who were rarely employed at that time outside of the home -.

These women, hired by Pickering, worked full days for six days a week, but they were only paid between 25 cents and 50 cents an hour – far less than men would have been paid.

Leavitt’s contribution was one part of the legacy. Her work allowed Edwin Hubble to determine the distance to the Andromeda Galaxy (aka M31, 2.5 million lightyears away) and establish that it was an galaxy of its own. Furthermore, it was the first distance measurement for a galaxy outside of the Milky Way.

More about them here (source: space.com).

Sources

Book: „The End of Everything“ by Katie Mack, 2020
ISBN: 978-0-141-98958-7

Timeline
https://www.britannica.com/biography/Henrietta-Swan-Leavitt

Photo of Leavitt at her Desk
https://en.wikipedia.org/wiki/Henrietta_Swan_Leavitt

Photo of Radcliffe College
https://de.wikipedia.org/wiki/Radcliffe_College

What is stellar magnitude?
https://earthsky.org/astronomy-essentials/what-is-stellar-magnitude/

The Celestial Sphere
http://stars.astro.illinois.edu/celsph.html

Cepheids as Cosmology Tools
https://www.nasa.gov/mission_pages/spitzer/multimedia/pia15819.html

The Astrographic Catalogue
https://www.maas.museum/inside-the-collection/2017/08/02/the-astrographic-catalogue/

Harvard’s ‚Computers‘: The Women Who Measured the Stars
https://www.space.com/34675-harvard-computers.html