Hydrogen is the most abundant element in the universe. But how do we know this? Who do we credit for this insight?
I'll answer that for you: Cecilia Payne-Gaposchkin.
In 1925, she was awarded the first Ph.D in astronomy ever. Payne-Gaposchkin was the first to suggest that stars consist primarily of hydrogen—the prevailing theory at the time was that the sun (and therefore all stars) had a chemical composition more or less similar to Earth's. Instead, Pyane-Gaposchkin argued that silicon, carbon, and other metals content was more or less similar, but that helium and hydrogen were far more abundant in the sun than on Earth (for hydrogen, she calculated that it was more prevalent in the sun by a factor of one million). By all rights a discovery as important to science as gravity, special relativity, or evolution, yet it's one that often gets omitted from science curricula.
After a long career of crappy pay and little formal recognition, by 1956 Payne-Gaposchkin was one of the first female professors at Harvard's Faculty of Arts and Sciences. She retired ten years later, at the age of 66, having taught and mentored astronomical luminaries such as Helen Sawyer Hogg, Joseph Ashbrook, Frank Drake and Paul W. Hodge.
If you'd like, you can read her paper "On the Physical Condition of the Supernovae" in its entirety at the website for the Proceedings of the National Academy of Sciences of the United States of America.