There are two types of plates. The majority are photographic: clear glass sheets scattered with dark specks of stars. Today, these plates draw the most interest from scientists. But about one in five is a spectrographic plate, with each star depicted as a grey smear (representing the rainbow of visible light) perhaps a quarter inch long. In the early days of the collection, these spectra were the cutting edge of astronomy.
The first major developments to come out of the Harvard plate collection were systems that classified stars based on the tiny white lines slicing the spectrum’s grey rainbow. “It’s almost incomprehensible that this was all done visually by examining the plates under magnifying glasses,” said Josh Grindlay, an astronomer at Harvard. “Thank heavens they did because it was not something that just immediately popped out, what this spectral classification system was.”
The first system was designed by Williamina Fleming, whom Pickering originally hired as a housemaid only to discover her astronomical potential. Fleming’s system, published in 1890, sorted more than 10,000 spectra into an alphabetical sequence of 15 letters. A second, independent system—complicated and criticized for its complexity—was created by Antonia Maury and used 22 letters, with further subclasses denoting how wide or narrow certain spectral lines were. (Years later, astronomers realized some of these characteristics identify binary stars and supergiants.).
These two systems were expanded and reconciled by Annie Jump Cannon, whom Grindlay calls a “wonder woman.” Over her career, Cannon classified more than 350,000 spectra on the Harvard plates, sometimes managing a hundred in one day. “She was an extraordinary woman who did one thing extremely well all her life,” said Hearnshaw. In her free time, she also spotted 300 variable stars and five novae Cannon built on Fleming’s and Maury’s work to create the O, B, A, F, G, K, M sequence that still underlies stellar classification.
Nevertheless, other astronomers likely could have tackled the classification problem. “This would have been done, but not in the concerted way that it was done here,” Grindlay said. Hearnshaw agreed, pointing particularly to observatories in California and Germany. “Harvard was a pioneer, but they had other people chasing on their tails, and of course that’s good for science.”
The spectrographic plates had another secret hidden among their streaks: the recipe for stars, deciphered by Cecilia Payne-Gaposchkin. Scientists elsewhere had tied specific elements and their charged varieties to the wavelengths of light they absorb, which match the white lines chopping through a spectrum; Payne-Gaposchkin applied this work to translate the Harvard spectra into elemental ingredients.
“What she found was that the stars all seemed enormously uniform in composition,” Gingerich said. “The spectra looked very different, but that was because of the temperature difference of the stars.” Hot hydrogen and very hot hydrogen have very different spectra, but both are hydrogen. “That was a very important finding,” he added. Payne-Gaposchkin’s work also proved that the vast majority of the universe is hydrogen and helium.
Even while the spectra were still unveiling their secrets, the photographic plates were beginning to shine, thanks to what is widely recognized as the single most important discovery to come out of the Harvard plates.
Astronomers wanted to sort out how bright different stars were — but there’s usually no way to determine whether a star looks bright because it’s nearby or because it’s genuinely a bright star. Harvard astronomer Henrietta Leavitt looked at the Large and Small Magellanic Clouds and saw clusters of stars.
“She could see there was this big clump of what looked like a gazillion stars,” said Grindlay. “It was clearly one thing.” Stars clumped together must all be about the same distance from Earth—which in turn meant the ones that appeared brighter really were brighter. By studying a specific type of variable star, called Cepheids, in the Magellanic Clouds, Leavitt realized that brighter Cepheids took longer to dim and brighten, establishing a relationship between intrinsic luminosity and period.
That relationship meant astronomers could reverse the process: Measuring how quickly a Cepheid brightens and dims would reveal “how many watts were in the lightbulb,” said Grindlay, and from there, “out pops the distance.” That conversion required other techniques to pin down how far away specific Cepheids were, Gingerich added. But Edwin Hubble was still able to use Leavitt’s work to prove the Milky Way and Andromeda were two separate galaxies.
The collection’s full-sky scope was crucial for Leavitt, since the clouds are only visible from the often-ignored southern hemisphere. Other galaxies could have yielded the same realization, but they were too far away for the small telescopes of the time to truly see into them. “It would have been very difficult and I would say essentially impossible without having the Magellanic Clouds,” Grindlay said.