Cecilia Payne Gaposchkin
It was while studying at Newnham College, Cambridge, in 1919 that her interest in astronomy was ignited by a lecture from Sir Arthur Eddington on Einstein’s new science of general relativity. Determined to follow her newfound passion after leaving Cambridge, she applied to Harvard College Observatory. Alongside images of the sky, the Observatory’s catalogue also included the spectra of countless stars, made by spreading out starlight according to its wavelength and capturing it on photographic plates. These rainbow-like bands were usually crossed by a variety of dark ‘absorption lines’ – chemical fingerprints created by the lightabsorbing properties of various elements somehow associated with the stars themselves. The goal of her PhD thesis would be to explain exactly how.
One of the most puzzling aspects of stellar spectra was the huge variety in the strength and number of their absorption lines, implying great variation in the elements they contained – by proper application of the equation, Cecilia was able to show that most of this variety was due to differences in the surface temperature of the stars, not to their composition. Higher temperatures strip away more of the electrons from around atoms in a star’s atmosphere, leaving ‘ions’ with increasingly positive electric charges that interact differently with light escaping from the star’s surface.
Applying these principles to the atmospheres of various stars,
Payne showed that they contained similar amounts of relatively heavy elements such as silicon, carbon and oxygen to those found on Earth. But there was a troubling discrepancy when it came to the lightest elements: helium and especially hydrogen seemed to be vastly more abundant in the stars than they are on Earth.
Throughout a long and distinguished career spent entirely at Harvard, Cecilia was able to see the fruits of her breakthrough in understanding the composition of stars. Understanding that they are predominantly made of hydrogen is the key to understanding the fuel source that powers them, the conditions inside them and the various tracks along which they can evolve during their lifetimes.