Biographies

Henrietta Leavitt and the variable stars (Cepheids)

Henrietta Leavitt and the variable stars (Cepheids)

Henrietta Swan Leavitt was an American astronomer, known for her study of a type of stars whose brightness varies with regular periods, the so-called "Cepheid variables."

Henrietta Swan Leavitt, daughter of a minister of the American Congress, was born in Lancaster, Massachusetts, on July 4, 1868 and died in Cambridge on December 12, 1921. He studied at Oberlin College and Radcliffe where he graduated from 1892. It was then that he discovered Astronomy.

After graduating he took an Astronomy course, although he could not put his astronomical knowledge into practice until three years later, as a result of a disease that forced him to remain at home during all that time.

In 1895 he entered the Harvard Observatory as a volunteer and seven years later he became part of its staff, under the direction of Charles Pickering. During that time she had the opportunity to carry out theoretical works, but she became the head of the Photographic Department of the Observatory, where, together with her group, she studied the images of the stars to determine their magnitudes.

During his career, Leavitt discovered more than 2,400 variable stars. He then devoted himself to the study of these variable stars, which would mean his greatest contribution to Astronomy: the relationship between the period and the luminosity of the Cepheids.

Cepheids are variable stars that show a regular rhythm of brightness, darkening and brightness when observed in periods of time ranging from a few weeks to a few months. Leavitt noted that the brighter the star was, the longer the pulsation lasted. This means that by observing one of those stars, you can determine the pulsation period and discover how bright the star is.

These stars are pulsating because the areas of hydrogen and ionized helium are near the surface. Cepheids are brighter when they are close to their minimum size and, since all Cepheids are approximately at the same temperature, their size determines their luminosity. In addition, these stars are so bright that they can be seen in very distant galaxies.

Once verified, Leavitt's law was used to measure the distance of very distant objects, such as galaxies. To calibrate it, it was necessary to obtain by other means the distance at which a nearby Cepheid is located, from which its real luminosity is deduced. Using this method, astronomers Shapley and Curtis measured the distance to the M31 galaxy, reaching different conclusions, since they did not agree on the size of the Milky Way nor was the distinction between novae and supernovae known at that time.

Henrietta Leavitt also developed a pattern of photographic measurements that was accepted by the International Committee of Photographic Magnitudes in 1913. To develop this measurement pattern, Leavitt used 299 plates of 13 telescopes and used logarithmic equations to sort the stars over 17 magnitudes of luminosity. Leavitt continued to redefine this work throughout his life.

Due to the prejudices of the time, Henrietta Swan Leavitt could not develop her own working methods in astronomy, so she did not have the opportunity to get the most out of her intellect. In the course of his work he also discovered four new stars and studied some types of binary stars and asteroids.

He was a member of Phi Beta Kappa, of the American Association of the University of Women, of the American Society of Astronomy and Astrophysics, of the Association for the Advancement of Science and honorary member of the Association of Variable Star Observers.

Henrietta Leavitt worked at the Harvard Observatory until her death in 1921 because of cancer. Unfortunately, Henrietta passed away before she could conclude another work on the scales of measurement of the magnitude of the stars. His important contributions to the scientific world were recognized in 1925 posthumously, when he was nominated by the Swedish Academy of Sciences for the Nobel Prize.

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