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Welcome to SciShow Space! In this episode Caitlin Hofmeister will talk about Cecilia Payne-Gaposchkin, one of the most influential women in astronomy!

Hosted by Caitlin Hofmeister

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There was a time, not long ago when everyone thought the planets and the stars were all made of the same stuff as Earth; the same elements and compounds, all in the same proportions. This was also a time when we thought that we had invented the best possible tools to observe stars and planets in close detail, but it turned out we were wrong. It wasn't until 1925 that a young astronomy student found the key to observing what was really out there and she changed the way we look at the universe in more ways than one.

(Intro)

Cecilia Payne was born in England in 1900, not a time when women were welcome in the world of science, but she was smart and passionate and pursued her true love: astronomy.

In 1923, Payne immigrated to the US to study at Radcliffe College, now Harvard College Observatory, one of the only academic institutions at the time that accepted female students in the discipline of physics and in just two years she became the first person to earn a PH.D. at the college with what's been called the "most brilliant thesis ever written in astronomy".

In it, 23 year-old Payne laid the groundwork for basically everything we know about the stars and for the first time she corrected our assumption that the whole Universe shares Earth's elemental makeup.

Before Payne, everyone assumed that the stars were made of basically the same 100 or so elements found on Earth. It stood to reason at the time that the stuff we know here must be the same as the stuff out there, but once we started to use spectrographs, tools that allow us to read the elements in the stars, the Universe started to look confusing.

Spectrography was first developed in the late 19th century, when scientists realized that if they passed light through a medium like a gas and broke it into a spectrum, certain wavelengths of color would be missing from that spectrum.
After doing this enough, they were able to tell which elements absorbed which wavelength and the science of spectrography was born.

This was clearly useful for observing the elements in the atmospheres of stars, a medium that lay between us and the constant source of starlight, and it appeared that the stars tended to have all the same elements as Earth in more or less the same proportions. But soon a weird pattern emerged. The spectra from all known stars consistently formed seven different absorption sequences.

This seemed to suggest that there are seven kinds of stars, each composed of the same 100-odd elements as earth but with some slight variations.

But why would there only be seven different types of stars, when there are more than a million possible combinations of those 100-or-so elements? It just didn't add up and no one had a good explanation.

But Payne suspected that we were looking at the problem in the wrong way. She hypothesized, the seven spectrum patterns we were seeing weren't the result of different combinations of elements but they were created by seven different temperature ranges.

How could that be?

Well, in matter, atoms are normally swarming around each other and colliding. The higher the temperature is, the more atoms move around, and the more collisions there are. And in really exceedingly high temperatures, sometimes the atoms can collide so fast, that one of their electrons will essentially break off, creating an ion of that atom.

So, for example, if an atom of helium, which typically has two electrons, is in one of these high-temperature mosh pits, and it loses one of its electrons, it'll turn into a helium ion. So now, because they have new slightly different chemical signatures, these ions absorb different wavelengths of light than their parent elements do.

Payne hypothesized that since hotter temperatures mean more ionization, that could explain the different absorption patterns that astronomers were finding.

But then the questions was: Ions of what?

She went about determining which ions could create those mysterious absorption patterns, and at what temperature range they could each exist. That could have taken forever, because Payne was dealing with not just the original 100-odd elements, but also countless variations of each.

But as soon as she began studying ions of hydrogen everything fell into place. The seven different spectrograph patterns created by starlight perfectly corresponded to those made by seven groups of hydrogen ions, each of which could only exist in its own temperature range.

After identifying those groups, Payne was able to study other ions that could also exist at those temperatures, and determined which ones could make the spectral patterns that everyone was seeing.

The thing was, most of what could be found in those lines were ions of Helium but not much else.

For the first time it was clear that the stars were not made of the same elements as Earth, but overwhelmingly of just the two latest elements, Hydrogen and Helium.

Hydrogen, Payne realized, was about one million times more abundant than any other element in the stars. And from this new understanding scientists were able to form all sorts of new theories that address some of the biggest issues in the cosmos.

Like that the universe was originally made of Hydrogen, and then the stars created heavier atoms by fusing Hydrogen atoms together, and the heavier atoms that followed.

Payne's research made it possible to read the histories of stars, by knowing not only their chemical makeup but their temperatures and densities too.

Still, it was 1925, and Cecilia Payne was a woman. Her advisor urged her not to publish her findings in her thesis because they were too controversial. One professor said they were clearly impossible. Regrettably, she took their advice.

But then a few years later, her advisor published the same results that Payne first discovered and presented to him, and is still sometimes credited with her work to this day.

That's why we're doing this episode about her. Payne eventually became Payne-Gaposchkin and spent most of her life working as a "technical assistant" at Harvard.

She taught a full course load and many of her students went on to have prominent careers in astronomy. For decades she continued to pursue research into the evolution of stars, as well as other exotic bodies like Pulsars.

And, in time, she did publish her revolutionary findings in a book called "Stellar Atmospheres", which convinced many of her colleagues not only what stars are made of, but that she was the one who had figured it out.

Finally, in 1956, more than 30 years after her discovery, and despite the grumblings of her former advisor, she became the chair of the astronomy department at Harvard.

She broke the field of astronomy's glass ceiling, for which I will forever be grateful.

Thank you so much for joining me for SciShow Space. If you want to learn how you can help us keep exploring the universe together, go to subbable.com/scishow and don't forget to go to youtube.com/scishowspace and subscribe.