Women are often overlooked on lists of history's great scientists, an oversight corrected in the latest episode of "Cosmos: A Spacetime Odyssey" with host Neil deGrasse Tyson concentrating on the contributions of women astronomers.

In "Sisters of the Sun," due attention was granted to Henrietta Leavitt, whose work on variable stars allowed astronomers to determine the distance from Earth of the universe's far-flung galaxies; Annie Cannon, whose work was a first serious effort to classify stars by their temperature; and Cecilia Payne, a British astronomer who considered what stars are made of by studying their percentages of helium and hydrogen.

The three made their considerable contributions to astronomy despite having to buck the status quo of social conventions that said what women should and, more to the point, should not be doing.

All of the women profiled in the episode worked at the Harvard Observatory under its director Edward Pickering, whose motive was less than enlightened; he preferred women to act as "calculators" and pore over star images because they had no choice but to accept less pay than men would have.

Still, once at work all three women were instrumental in advancing the astronomical understanding of their time.

Annie Cannon devised the spectral classifying of stars still in use today, and discovered an extensive catalog including 300 variable stars also still in use to measure cosmic distances.

Henrietta Leavitt was a protégé of Cannon and studied a particular class of variable stars, the Cepheids, discovering a relationship between luminosity and the periods of their changing brightness that allowed later astronomers to calculate the universe's age.

Cecelia Payne, joining the observatory in 1923, used spectrographic studies of the stars in her doctoral thesis to reveal their chemical makeup

She would write a book on her research, "Stellar Atmospheres," now a standard astronomical text.

"Payne's 'Stellar Atmospheres' is widely regarded as the most brilliant Ph.D. thesis ever written in astronomy," Tyson said. "It became the standard text in its field ... Cecilia Payne's interpretation of Annie Jump Cannon's sequence of stellar spectra made it possible for us to read the life stories of the stars."

Taken together, the work of the three women pioneers highlighted in the episode did nothing less than absolutely change our understanding of the stars shining in our night skies.

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