Saturday, July 4, 2009

July 4: Henrietta Swan Leavitt

Henrietta Swan Leavitt
July 4, 1868 – December 12, 1921

Henrietta Swan Leavitt was an American astronomer and the deaf daughter of a Congregational minister. A graduate of Radcliffe College, Leavitt went to work in 1893 at the Harvard College Observatory in a menial capacity as a "computer", assigned to count images on photographic plates. Study of the plates led Leavitt to propound a groundbreaking theory, worked out while she labored as a $10.50-a-week assistant, that was the basis for the pivotal work of astronomer Edwin Hubble and radically changed the theory of modern astronomy, an accomplishment for which Leavitt received almost no credit during her lifetime.

Leavitt began work in 1893 at Harvard College Observatory as one of the women human "computers" brought in by Edward Charles Pickering to measure and catalog the brightness of stars in the observatory's photographic plate collection. (In the early 1900s, women were not allowed to operate telescopes). She noted thousands of variable stars in images of the Magellanic Clouds. In 1908 she published her results in the Annals of the Astronomical Observatory of Harvard College, noting that a few of the variables showed a pattern: brighter ones appeared to have longer periods. After further study, she confirmed in 1912 that the variable stars of greater intrinsic luminosity – actually Cepheid variables – did indeed have longer periods, and the relationship was quite close and predictable.

Leavitt's discovery is known as 'Period-luminosity relationship.' "A straight line can be readily drawn among each of the two series of points corresponding to maxima and minima," Leavitt wrote of her study of 1,777 variable stars recorded on Harvard's photographic plates, "thus showing that there is a simple relation between the brightness of the variable and their periods."

This relationship provided an important yardstick for measuring distances in the Universe, if it could be calibrated. One year after Leavitt reported her results, Ejnar Hertzsprung determined the distance of several Cepheids in the Milky Way, and with this calibration the distance to any Cepheid could be determined.

At the time, it was not clear that millions of (what we now call) galaxies were actually outside of the Milky Way. Their distance could not be measured until a tool existed. Cepheids were soon detected in other galaxies such as the Andromeda Galaxy (notably by Edwin Hubble in 1923–24). Cepheids were an important part of the evidence that galaxies are far outside of the Milky Way. Our picture of the universe was changed forever, largely because of Leavitt's discovery.

The accomplishments of Edwin Hubble, renowned American astronomer, were made possible by Leavitt's groundbreaking research and Leavitt's Law.
"If Henrietta Leavitt had provided the key to determine the size of the cosmos, then it was Edwin Hubble who inserted it in the lock and provided the observations that allowed it to be turned",
write David H. and Matthew D.H. Clark in their book Measuring the Cosmos. Hubble himself often said that Leavitt deserved the Nobel for her work.

Leavitt worked sporadically during her time at Harvard, often sidelined by health problems and family obligations. But by 1921, when Harlow Shapley took over as director of the observatory, Leavitt was made head of stellar photometry. By the end of that year she had succumbed to cancer, and was buried in the family plot at Cambridge Cemetery in Cambridge, Massachusetts.

The asteroid 5383 Leavitt and the crater Leavitt on the Moon are named in her honor.

No comments:

Post a Comment