A century ago, in 1925, Cecilia Payne—later Payne-Gaposchkin—discovered that stars are almost entirely made up of the first two elements of the Periodic Table: hydrogen and helium.
She analyzed the spectra of stars using existing data to figure it out. Her key insight was to recognize that the absorption lines in stellar spectra represented the presence of hydrogen and helium. Previously it was thought to be caused by iron and other heavy elements. An idea likely influenced by our anthropomorphic tendencies. We wanted to believe the entire universe was made up of the same stuff as Earth. It’s not. Less than 2% of Type I stars, third generation stars, are made up of heavier elements.
At the time, the scientific community was split. The older prevailing thought was that atoms were neutral. They didn’t emit anything. Some thought they emitted things they called ions. Later we discovered ions were electrons and other sub-atomic particles. So, when someone talks about the ionization of atoms, now you know where that came from!
She used the Saha equation, a new formula developed by Indian astrophysicist Meghnad Saha. She demonstrated that the absorption lines were consistent with the presence of hydrogen and helium in stars. Her work provided strong evidence for the abundance of these elements in stars.
Science is about repeatability and four years later in 1929, Henry Norris Russell, an American astronomer, independently confirmed her findings and provided further evidence for the abundance of hydrogen and helium in stars.
So, back in 1925, Cecilia Payne made a groundbreaking contribution to our understanding of ionization and the composition of stars. By 1930, atom ionization had become the prevailing thought in physics, paving the way for our modern understanding of the cosmos.
