This blogpost is part of Ada Lovelace Day, which is an international day of blogging to draw attention to the achievements of women in technology and science. Women’s contributions often go unacknowledged, their innovations seldom mentioned, their faces rarely recognised.

Cecilia Payne-Gaposchkin (May 10, 1900 – December 7, 1979) was an English-American astronomer who in 1925 was first to show that the Sun is mainly composed of hydrogen, contradicting accepted wisdom at the time.

Cecilia Payne wrote a doctoral dissertation, and so in 1925 she became the first person to earn a Ph.D. in astronomy from Radcliffe (now part of Harvard) for her thesis: "Stellar Atmospheres, A Contribution to the Observational Study of High Temperature in the Reversing Layers of Stars". Astronomer Otto Struve characterized it as "undoubtedly the most brilliant Ph.D. thesis ever written in astronomy". By applying the ionization theory developed by Indian physicist Megh Nad Saha she was able to accurately relate the spectral classes of stars to their actual temperatures. She showed that the great variation in stellar absorption lines was due to differing amounts of ionization that occurred at different temperatures, and not due to the different abundances of elements. She correctly suggested that silicon, carbon, and other common metals seen in the Sun were found in about the same relative amounts as on Earth, but that helium and particularly hydrogen were vastly more abundant (by about a factor of one million in the case of hydrogen). Her thesis thus established that hydrogen was the overwhelming constituent of the stars. When her dissertation was reviewed, she was dissuaded by Henry Norris Russell from concluding that the composition of the Sun is different from the Earth, which was the accepted wisdom at the time. However, Russell changed his mind four years later when other evidence emerged. After Payne-Gaposchkin was proven correct Russell was often given the credit.

She published several books including:
"Stars of High Luminosity" (1930),
"Variable Stars" (1938),
"Variable Stars and Galactic Structure" (1954),
"Introduction to Astronomy" (1956),
"The Galactic Novae" (1957)
"Cecilia Payne-Gaposchkin : an autobiography and other recollections" (1984) ed. Katherine Haramundan)

She was also a Unitarian and a member of First Parish and Church in Lexington, Massachusetts. According to Owen Gingerich:

Payne-Gaposchkin was a many-sided personality known for her wit, her literary knowledge, and for her personal friendships with individual stars. She became the first woman in the history of Harvard University to receive a corporation appointment with tenure, and the first woman department chair in 1956. 

My other Finding Ada blogposts:
Lisa Barone
Hedy Lamarr