Looking up at the night sky, we know that a star’s brightness can tell us something about how far away it is, and even what it’s made of. But how do we know that?

As it turns out, our system for classifying stars comes from work done by a group of female astronomers at Harvard more than a century ago. Decades before American women gained the right to vote, the astronomers of the Harvard College Observatory shattered the “glass universe,” analyzing delicate photographic plates to discern patterns in the cosmos.

“They knew they were doing something unusual, that they'd been given a remarkable opportunity, and they ran with it,” says Dava Sobel, who charted the astronomers’ story in her new book, "The Glass Universe: How the Ladies of the Harvard Observatory Took the Measure of the Stars."

Studies of the glass photographic plates were led by Edward Pickering, who directed the Harvard College Observatory from 1877 to 1919. Sobel credits him with hiring many of the earliest female staff at the observatory — although not the first ones.

“He found some women there when he got there because the resident astronomers’ wives, daughters, sisters, had already been put to work. So, there was a precedent,” she says, adding, “They also cost less to hire, an evergreen theme.”

One of the first women Pickering hired to analyze the plates was his own maid, Williamina Fleming. She went on to become a top figure in the stellar classification project, cataloging thousands of stars over a long career that also included the discoveries of tens of nebulae and novae and hundreds of variable stars. Other women followed, leading to a research setting that was unusually open for the times. “By 1896, when Annie Jump Cannon came to the observatory, she was allowed to make her own observations,” Sobel says.

Sobel explains that the female astronomers worked in pairs, with one woman examining the plate and speaking aloud what she was seeing. The plates, some as large as 11 inches by 17 inches, contained images of thousands of stars, each appearing as a little black dot on the glass. “Or, for some pictures, the light got passed through a prism,” Sobel says. “And then instead of a dot, there'd be a little smear of a strip for each star, with shades of black, white and gray.”

“When they were looking at the spectra, they were really looking at the pattern of the lines and trying to figure out which patterns made a logical category,” Sobel says. “And people needed those different categories to be able to get a taxonomy of the stars.”

If it sounds laborious, it was. But Sobel adds that at the time, the glass plates were cutting-edge technology.

“This was a new thing in astronomy, to be able to take long-exposure photographs and discover things that couldn't be seen even through the most powerful telescopes,” Sobel says. “So, they were actually making discoveries. And they were trying to figure out how much they could learn from starlight about the content of the stars, their evolutionary history, their distances.”

In "Glass Universe," Sobel writes that the women led the way here for decades, and even today, their legacy remains intact. “The monumental work of stellar classification known as the Henry Draper Catalogue and Extension, begun under Williamina Fleming in the 1880s and continued through 1940 by Annie Jump Cannon, is still in regular use.”

Sobel says that the classification system led to the understanding that different categories of stars signified different temperatures. “At the beginning, it wasn't known what they signified,” she says. “They just looked different. Now we know there are different stellar temperatures because stars have different lifestyles. And they also helped show the life stage of the star.”

Sobel adds that the Harvard astronomers also studied stellar brightness and constitution, contributing research that showed the abundance of hydrogen in the stars. Another knockout astronomer at the observatory, Henrietta Leavitt, studied relative distances in space. According to Sobel, Leavitt’s work changed our conception of the universe’s size — and laid the groundwork for major 20th-century discoveries about the cosmos.

“That first led to an understanding of how far away from us the satellite galaxies and the Milky Way [are],” Sobel says. “Her discovery helped establish how far they were and then how big the Milky Way was. And then Edwin Hubble used her discovery to show that the Milky Way was only one galaxy among many. And later he used it again to show that the universe was expanding. So, it was pretty important.”

For Sobel, the work of Harvard’s early female astronomers is nothing short of a launch through the glass ceiling, as the title, "The Glass Universe," suggests.

“At the beginning of this story almost nothing was known about what stars were made of, how they created their heat and light,” she says. “And by the end, just about everything was known.”

This article is based on an interview that aired on PRI's Science Friday. Read an excerpt from "The Glass Universe" on Science Friday’s website.