Leavitt, Henrietta Swan
Leavitt, Henrietta Swan (1868-1921) was an American astronomer whose work revolutionized human understanding of the relative brightness and variability of stars. Her greatest achievement was in establishing the period-luminosity relationship of variable stars—stars whose brightness fluctuates over regularly occurring periods. This significant discovery became the basis for the ability of astronomers to calculate the distance of stars from the earth.
Leavitt was born in Lancaster, Massachusetts, the child of Henrietta Swan Kendrick and George Roswell Leavitt, a Congregationalist minister. After attending public school in Cambridge, where her father's parish was located, she entered Oberlin College in Ohio in 1885. In 1888, she transferred to the Society for the Collegiate Instruction of Women, now Radcliffe Institute for Advanced Study, part of Harvard University. She received her B.A. degree in 1892, but remained in school an additional year to take further astronomy courses.
In the 1890's, Leavitt worked as a volunteer research assistant at Harvard College Observatory under the astronomer Edward Pickering, who had initiated a research program on the measurement of stellar magnitudes. In 1902, Pickering hired Leavitt to the Observatory's permanent staff, and she worked there until her death in 1921.
Leavitt discovered 2,400 new variable stars, about half of those known at the time, as well as four novae. But her discovery of the relation between the luminosity, or brightness, of a certain group of pulsating stars known as Cepheids and their variability, or periods of change in pulsation, made it possible to accurately determine stellar distances, in particular vastly greater distances than had ever before been possible.
Leavitt's investigations and consequent discoveries grew out of her being asked by Pickering in 1907 to establish a north polar sequence of star brightnesses, which would then be used as a standard for the whole sky. Because the brightness of stars as seen in photographic images is not proportional to their actual brightness, and because each telescope gave varying results for different colored stars, such a standard was greatly desirable.
While studying the two hazy areas of the sky in which the Cepheids reside, known as the Magellanic Clouds, Leavitt noticed that the brightness of these stars varied in a steady, rhythmic pattern over a period of days or weeks. She found that the overall brightness of any Cepheid determined the time it took for its magnitude to change. She then deduced that the longer the period of pulsation, the brighter the star, thus establishing the groundbreaking period-luminosity relation.
She published her initial findings in 1908 and by 1912, having proved that the Cepheids' apparent brightness increased linearly with the logorithm of their periods, published a table that indicated the period lengths of 25 Cepheids.
Her system of calculation was adopted in 1913 by the International Committee on Photographic Magnitudes, and she went on to establish brightness sequences for 108 areas of the sky. After Pickering established 48 Harvard standard regions in the sky, Leavitt then derived their secondary brightness standards, which were applied internationally until they were superseded by more modern methods. Before the establishment of the period-luminosity relation, it had only been possible to calculate cosmic distance to about 100 light-years, but Leavitt's findings allowed for the calculation of distances out to 10 million light-years, a number almost incomprehensible to astronomers of the time.
Her discoveries also revealed that the Magellanic Clouds are two irregular galaxies beyond the Milky Way, something Leavitt had not been aware of. Astronomers much later applied her findings in determining a new value for Hubble's Constant, a mathematical equation from which the age of the universe is derived. They calculated a new age for the universe, now put at between 8 and 12 billion years old. This new number was announced to the world in 1994.