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Period-luminosity relation of Cepheid stars

Henrietta Swan Leavitt invented one of the most essential standards in the study of space: a rule that allows astronomers to measure distances from Earth to various stars.

Henrietta Born on the 4th of July, 1868, Leavitt was always interested in science. After graduating from Radcliffe College (then called the "Society for the Collegiate Instruction of Women") in 1892, she eventually had to settle for a volunteer Research Assistant position at the Harvard College Observatory (1895).

Being a woman, Leavitt was not taken very seriously by Edward Charles Pickering (1846-1919), then the world's expert in photographic photometry (determining the magnitude of a star from its photographic image). He assigned her the tedious task of cataloguing "variable" stars, whose brightness appears to ebb and flow in predictable patterns.

While examining the Magellanic Clouds (neighbor-galaxies of the Milky Way), Leavitt discovered 1,777 new variable stars. More importantly, in 1912, by comparing different photographs of the same variable star, Leavitt discovered that stars of the "Cepheid" type had bright-dim cycle periods inversely proportionate to their magnitude. That is, the stronger the star, the slower its cycle.

Leavitt realized that the variable stars' cycles must depend not on how bright they appear ("apparent" luminosity), but how bright they really are ("intrinsic" or "absolute" luminosity). Then she constructed a period-luminosity ratio that applies to all Cepheid stars. This ratio allowed astronomers to measure the distance from Earth to any visible Cepheid star in the universe.

Pickering did not allow Leavitt to follow up on her revolutionary discovery, and continued to treat her as a mere lab assistant. But if she had not died of cancer in her mid-50s, Henrietta Leavitt might well have won a Nobel Prize. Her rule allowed for the mapping of the universe, as well as the discovery that it is expanding, and has made her a legend in the history of astronomy.

[March 1998]

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