Henrietta Leavitt - 150th anniversary

July 4th is the 150th anniversary of the birthday of American astronomer Henrietta Swan Leavitt.

Henrietta Leavitt isn't a well known name, but a century ago she made one of the most important discoveries of 20th century astronomy. Previously, astronomers could only measure distances up to 100 light years, but her work extended that to 10 million light years.

Henrietta Swan Leavitt

In the early 20th century an astronomer made a revolutionary discovery. Yet her life left almost no footprints on history. "Miss Leavitt's Stars" contrasts the solidity of her professional accomplishment with the butterfly touch of her life. Miss Leavitt isn't even the star of her own biography.

Miss Leavitt's Stars - book review

Infographic on the work of Henrietta Leavitt, by Abigail Malate (for the American Institute of Physics)

Leavitt's Law
Most people have never heard of Leavitt's Law. Yet it is one of the most important relationships in astronomy, as it lets astronomers determine distances in space. Edwin Hubble relied on it in the work which showed that Andromeda was a separate galaxy, not part of the Milky Way.

Some stars are variable, meaning their brightness changes. Leavitt discovered that a type of star called a Cepheid variable pulsates in a regular way and that the period of pulsation depends on how luminous it really is (intrinsic luminosity). Light gets dimmer in a predictable way as the distance increases, so if you know how bright a star is, you can work out the distance to it by comparing the actual brightness with the apparent brightness.

Henrietta Swan Leavitt (1868-1921) had been hired by Harvard Observatory as a computer (someone who did measurements and calculations in pre-electronic days). Nonetheless she devised what became the international standard for determining the magnitudes of stars in photographs. She also discovered over a thousand variable stars, half of those known in her lifetime. And she discovered what was called <em>the period-luminosity relation</em>, not Leavitt's Law. However in 2009 the American Astronomical Society did officially agree to encourage people to call the relationship the Leavitt Law and that has become somewhat more common.

And you can find out about some more astronomical names in What's in a Name .

If Henrietta Leavitt had done no more than discover Leavitt's Law, she would still deserve a place in astronomical history. However she did more than that.

Quote

By the death of Miss Leavitt on December 12, 1921, the Observatory lost an investigator of the highest value. She had obtained a comprehensive experience in photographic photometry, and had developed a clear appreciation of the difficulties involved in the theory and practice of this important research. Her work on standard magnitude sequences was nearly concluded at the time of her death, but she had hardly begun work on her extensive program of photographic measures of variable stars. In the foregoing summary no mention has been made of Miss Leavitt's work on standard photometry...

Harlow Shapley, Seventy-seventh Annual Report of the Director of the Astronomical Observatory of Harvard College for the Year ending September 30, 1922 (1923)

It was studying variable stars in the Small Magellanic Cloud that lead to Leavitt's law. She discovered that the time of the variability (period) and the brightness (luminosity) were related. Since all the stars were at about the same distance from Earth, this relationship must be something intrinsic to the stars.

Image credit: ESO/VISTA VMC