Henrietta Swan Leavitt

Timeline

• 1868 Henrietta Swan Leavitt was born in Lancaster, Massachusetts, USA.
• 1885 She enrolls at Ohio's Oberlin College.
• 1892 After moving back to Massachusetts, Leavitt graduates from the women's 'Harvard Annex'. She begins volunteering at the Harvard observatory as a human 'computer'.
• 1896 She travels to Europe, where an illness damaged her hearing.
• 1898 After returning from Europe, she gets a job as an arts assistant at Beliot College in Wisconsin.
• 1902 Leavitt is hired by Harvard Observatory director Edward Pickering as a fulltime human 'computer' at 30 cents an hour.
• 1908 She publishes a paper, 1777 Variables in the Magellanic Clouds, sumarising her findings on variable stars.
• 1912 She published a graph correlating the brightnesses and periodicity of the Magellanic variables.
• 1913 With Leavitt's discovery in mind, Ejnar Hertzsprung uses paralax to determine the distance to several cepheid variables within the Milky Way. Now the distance to any variable could be calculated.
• 1921 Leavitt is made Head of Stellar Photography at Harvard Observatory, but tragically dies of stomach cancer aged 53.
• 1924 Edwin Hubble uses Leavitt and Hertzsprung's work to calculate the distance to variables in the Andromeda Nebula and discovers it is not part of the Milky Way but in fact a seperate galaxy.
• 1926 Impressed by her pivotal research Magnus Gosta Mittag-Leffler tries to nominate Leavitt for the Noble Prize in Physics, only to be told she died several years prior.

Life

Henrietta Swan Leavitt was an American astronomer, who was employed by Harvard Observatory as a human 'computer'. She was one of many women who were hired to perform tedious work, examining photographic plates to measure and record the brightness of many thousands of stars. Whilst there she made a discovery that would change astronomy.

Pickering, director of the observatory, assigned Leavitt the task of cataloguing variable stars. She discovered thousands of these stars in the Magellanic Clouds, what are now known to be two satellite galaxies of the Milky Way. Whilst examining these stars, Leavitt noted that some of them (now called Cepheid Variables) seemed to show a pattern. Those with greater intrinsic luminosity (brightness) also had a longer period (time taken to cycle from brightest to dimmest). In 1908 she published these findings in the Annals of the Astronomical Observatory of Harvard College.

She then made the leap that these variables in the Magellanic Clouds were approximately the same distance from Earth, and so use their observed luminosity relative to each other as a stand in for their intrinsic luminosity. Leavitt published her work again in 1912, with a graph showing that the log of the star's period is linearly and directly related to the log of the star's intrinsic luminosity.

When combined with the work of Ejnar Hertzsprung, who measured the distance to nearby Cepheid Variables using parallax, Leavitt had now provided a cosmic 'standard candle', where by comparing the observed luminosity with a true luminosity derived from the period, the distance to that star could be derived. This technique would be used by Hubble and others to show that the universe was more than the Milky Way, and in fact was a universe of galaxies.