Few science writers have done more to explicate the work of astronomy for the public than Dava Sobel, 69, who has written extensively about the emergence of the discipline in the modern era, including two major studies, Galileo's Daughter: A Historical Memoir of Science, Faith, and Love (2000) and A More Perfect Heaven: How Copernicus Revolutionized the Cosmos (2011).

The former New York Times writer turns her attention to the 19th and early 20th centuries in her new work, The Glass Universe: How the Ladies of the Harvard Observatory Took the Measure of the Stars.

Released last Tuesday, the lively narrative tells the story of a group of pioneering women who used the new technique of photography to make major discoveries about stars.

Many were hired as "computers," the name given to a skilled albeit low-level research position that had them crunch numbers. Encouraged by a series of progressive directors to make contributions of their own, the women quickly distinguished themselves.

Sobel will talk about the book at 7:30 p.m. on Tuesday, Dec. 13, at the Free Library of Philadelphia's Central Library. It's part of a double program about science that also will feature a talk by astrophysicist Priyamvada Natarajan about her new book, Mapping the Heavens: The Radical Scientific Ideas That Reveal the Cosmos.

As a reporter, you covered virtually every scientific field, but it's clear from your books you are especially passionate about astronomy. Why this particular field?

I had a broad interest in science, but then in the early 1970s, it was probably 1972, I went to a public lecture by Carl Sagan. It was before he was super-famous. And it really changed my life. I felt what he was talking about was the most interesting thing I'd ever heard.

A short time later I got to interview him, and I've been a space cadet ever since.

So did he do his whole "billions and billions and billions" thing?

No, that was later, and it was mostly on [The Tonight Show Starring] Johnny Carson. I know Sagan was a really good sport about it.

You know, he had this incredible ability to speak to a popular audience, and an audience of children sometimes, to communicate his passion for the work. That really inspired . . . people. I have interviewed so many astronomers, and I ask them the same question you asked me, and I cannot tell you how many of them answer, "Because I watched [Sagan's TV show] Cosmos."

Edward Charles Pickering, who ran the Harvard Observatory, began hiring women in the 1880s as "computers" or research assistants to help with the hundreds of photographs that were being taken of the stars. What was their job?

They looked at glass [photographic] plates with thousands of of tiny dots and tried to assess the brightness of each, or how it changed over time. It was a painstaking affair.

Was Pickering breaking new ground by using photographs in tandem with the telescope?

It was very new. Astronomy until then was making visual observations of the stars to calculate stellar positions for navigation. Astronomy was in the service of navigation, and only amateur astronomers - wealthy individuals who could afford to build their own telescopes - were interested in looking into the actual physics of the stars.

So photography helped astronomy become a theoretical science?

Pickering, who took over in 1870, was a physicist, and he was really interested in what the stars are made of, how they work, and why some are brighter than others. He had a brother, William, who was interested in photography, and he really came up with the idea of using photography as a major research tool.

Many astronomers were opposed to it. "How could a picture be better than first-hand-observation?" they'd say.

Pickering had a new method, a well-funded observatory, and an influx of new assistants who were encouraged to pursue goals way above their pay grade. Sounds like a perfect creative environment.

These ladies might have been called computers in the beginning, but once they got those glass plates in their hands, they made discoveries that are still of value today. Henrietta Leavitt made the discovery that serves as the basis for calculating a star's distance from Earth. Two of the women worked on the stellar classification system that's still in use.

Right: It was developed by Williamina Fleming and later refined by Annie Jump Cannon, who studied at Wellesley College. I noticed that many of the women were graduates of the new women's colleges that were springing up in the Northeast.

Yes, that was a wonderful part of this confluence of events. Here were all these women coming out of colleges like Radcliffe, Smith, Wellesley, who had a solid theoretical background and a real practical knowledge of the new science of spectroscopy, where you divide and disperse the light from a star into a strips of color [according to its] wavelength.

The book features detailed profiles of more than half a dozen women. Who are your favorites?

There's Cecilia Helena Payne, a graduate student from England who realized she couldn't get very far as a scientist at home, so she came to America.

She found that stars are made almost entirely of hydrogen and helium [and] not rock. That was a huge discovery, and at first people couldn't believe it.

She also had an interesting personal life. She was traveling in Europe when she met this Russian émigré [Sergei Gaposchkin] in Germany who was stateless. He couldn't go back home or stay in Germany, so she got him a job at the observatory and eloped with him.

Williamina Fleming was initially hired as a maid, right?

She was a schoolteacher in Scotland . . . [though] her education didn't go much beyond high school. But she was really good at math, which Pickering saw straight off. She became the first woman from the observatory to hold a university title when she was named curator of the photographic collection.

There seems to be a real push to recover the history of women's contributions to science. Margot Lee Shetterly's Hidden Figures, for one, is about African American female mathematicians who worked as computers at NASA. But I suspect it'll take some time before the folks you write about are household names.

I sent the manuscript of the book out to 10 astronomers to check it for scientific accuracy. There was one woman astronomer who is on the faculty at Harvard, and she was really surprised. She'd heard some of the names, but she said "I always thought it was just something cute or quaint and that they really didn't do anything important. I was surprised to read that they were doing real science."