Cecilia Payne-Gaposchkin, the first person to earn a Ph.D. from Radcliffe College, and later the chair of Harvard's department of Astronomy. However, her work is largely unknown today – and having just spent an hour reading through some of it, this is something which needs to be fixed.
Her most critical work was actually her Ph.D. thesis in 1925, in which she calculated the chemical composition of the stars. (http://articles.adsabs.harvard.edu//full/1925PhDT.........1P/0000001.000.html – thanks to Rajini Rao for finding the link. Be warned, it's long!) The use of spectroscopy to determine which elements are present in the stars had started in the 1860's (and had led to the discovery of Helium), but figuring out how much of those elements is present was much more complicated. The idea is simple: each element (and each molecule, and each ionized atom, etc) absorbs light at a particular pattern of frequencies, which can be measured in the lab; hot objects emit light in a particular pattern; put them together, compare with the light you see coming from a star, and you should be able to figure out what's there. The hard part is that the precise patterns of absorption (etc) depend on things like the structures of temperature and pressure inside the star, which are rather hard to measure directly. Payne did the theoretical work of figuring out how to estimate all the critical quantities (using only the scientific knowledge of the 1920's! Remember, this is when quantum mechanics itself was in its infancy, and the idea of being able to calculate these things from first principles seemed impossible) and build a model of how the patterns of light should correspond to the abundances of the elements, and used that to compute just what the stars are made of.
Some of her big results:
- All stars are made of roughly the same material, no matter what color they are.
- The composition of the Sun is apparently very similar to the composition of the Earth, lending support to the idea that they were made out of the same material; but
- The Sun also contains an enormous amount of Hydrogen and Helium, and is in fact almost entirely made out of those two elements.
Under pressure from Russell (a noted astrophysicist of the day) she stepped back from that last conclusion, saying that the high abundances of those materials suggests that there must be an error in her method. There wasn't; that's what stars are made of.
(NB: Several years later, Russell got the same answer using other methods, and concluded that she was in fact right. He was quite up-front about giving her credit, but others tended to ignore this, and give Russell most of the credit for the discovery)
It's hard to overstate the importance of this work. Pretty much all of modern astrophysics (and no small part of nuclear physics) derives from our ability to measure these quantities. The famous B2FH paper, for example, which explained how the chemical elements were formed in the first place (inside stellar furnaces and supernovae) is built pretty much entirely on these sorts of calculations, and the techniques which Payne pioneered.
Despite this, Payne's work has remained mostly unknown. I myself worked in a fairly related field – high-energy physics – and had never heard of her until today. (And so my estimate of the work's significance comes from digging it up and actually reading it, and knowing the big picture of where those results are used – results which I had known for years without ever thinking about who originally found them)
In fact, this paper has a few weaknesses – the weaknesses that you see when the author can't get the full support of the community. She backs away from the most significant result; all of the most important results are fairly buried in chapter 13, rather than placed front and center. I've seen many papers do this, and it's quite clear that if her advisors (Shapely and Eddington, two of the preëminent astronomers of the day) had backed her more firmly, or if the community had followed Russell's lead and properly credited the work when it was confirmed by Russell's results years later, this paper would have been more forcefully stated, and its significance would be better-known today. Given the impact of these methods on astrophysics, it is highly likely that she would have been awarded the Nobel Prize.
(As it is, credit for this discovery seems to have diffused out across many researchers in the astronomical community, and no one person is considered "the" discoverer. But having read the paper and knowing its date, I think that it's more than fair to mark Payne as such; she worked out all of the details years before anyone else)
So today, let's take some time to remember this extraordinary discovery. We know what's on the insides of stars because of her.