For centuries, even after we knew the Sun was a star like any other, we still didn’t know what it was made of. Cecilia Payne changed that.
Inside the Sun, the most powerful source of energy for several light-years in all directions, an incredible process occurs. Deep within its core, hundreds of thousands of kilometers beneath the edge of its photosphere, temperatures exceed a critical threshold of four million degrees, rising up to a maximum of 15 million K in its center. Under these conditions, hydrogen atoms — and specifically, the nuclei of hydrogen atoms — smash into one another, causing their quantum wavefunctions to overlap. Although these collisions are extremely frequent, they most often simply bounce off of one another, failing to create a meaningful, energy-liberating reaction.
But every once in a while, this results in a nuclear fusion reaction, where heavy isotopes (like deuterium or tritium) or even heavier elements (like helium-3 or helium-4) result. These heavier isotopes and elements are more energetically stable than bare protons are on their own, and so as a result of these reactions, energy is liberated. We take for granted, today, that this is the process that not only powers the Sun, but nearly all of the stars (i.e., every star on the main sequence) in the Universe.
It seems hard to imagine it now, but just 100 years ago, we not only didn’t know about this process at all, but we didn’t even know what the Sun (and all stars) are made of: hydrogen and helium. How did we find out? That’s the work of astronomer Cecilia Payne, whose PhD dissertation celebrates its 100th anniversary this year: in 2025. Here’s how this brilliant scientist showed us what the most common luminous object in the Universe is made of, and with it, how stars themselves actually work.
From the time of Newton, we’ve known that the Sun had to be very, very massive: around 300,000 times as massive as planet Earth. Because the Sun is known to be 93 million miles (150 million km) away from us…
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