HARVARD COLLEGE OBSERVATORY / SCIENCE PHOTO LIBRARY HARVARD COLLEGE OBSERVATORY / SCIENCE PHOTO LIBRARY
Leavitt's work on Cepheid variables, 1900. Photographic plate of stars in the Large Magellanic Cloud, with markings made by US astronomer Henrietta Leavitt (1868-1921) during her research on Cepheid variable stars. Leavitt measured and catalogued the brightness of these stars and the period of their variability, discovering a standard relationship that allowed them to be used to measure the distance of faraway galaxies. Leavitt's research, published in 1908, led in turn to Hubble's discovery in the 1920s that the universe is expanding. This plate was obtained on 18 December 1900 from Arequipa, Peru, with a 60-minute exposure.
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