What Would Happen if the Sun Stopped? Part 2: Kelvin and Helmholtz at the Ready

(This is Part 2 of a series on what would happen if the Sun stopped. Read Part 1 first.)

When you crank up your stove to heat a pan to cook your eggs, you have to wait a while. It takes time for the heat to work its way into the metal. Once the pan is hot enough you cook your eggs, and when you're done you can sit and eat them while the pan slowly cools back down on its own.

The Sun is slightly larger than a stovetop pan, but the same basic physics applies. Once upon a time the Sun, or rather the cold cloud of gas and dust that would become the Sun, was cold. Now it is hot. It took time to become hot. And the surface of the Sun is exposed to the frigid vacuum of space, constantly radiating its warmth away into the dark, constantly cooling off.

All of which means fusion isn't the hot pan itself. Fusion is the flame underneath it. Fusion keeps the Sun warm. Take the flame away, and the Sun stays warm anyway, at least for a while, because like every hot thing it takes time to cool down.

In fact, the rate of fusion is tuned with remarkable precision to keep the Sun just barely warm enough to avoid catastrophe, through a not-so-magical process called hydrostatic equilibrium.

Here's how it works. The Sun is a giant ball of gas, so giant that its own gravity is forever trying to pull it tighter. And when a giant ball of gas gets squeezed into a smaller volume, it heats up. Back in the 19th century, astronomers were genuinely stumped about how the Sun had managed to stay warm for so long. So in 1854 a German aristocrat named Hermann von Helmholtz suggested that maybe it kept warm simply by slowly shrinking. Start with a big ball of gas, let it compress, watch it heat up, let that heat escape as lovely sunshine, let it compress a little more, and keep the whole cycle running.

A decade later another aristocrat, because this was very much the age of "if you want to be a scientist it helps enormously to be independently wealthy," took up the problem. William Thomson, the first Baron Kelvin, Lord Kelvin to us plebes, ran more careful calculations to pin down the lifetime of the Sun. And he got the wrong answer.

He flirted with a few possibilities but generally landed somewhere around a few tens of millions of years. A long time, certainly. But badly out of step with what the geologists, and later the biologists, were finding. They were arriving at ages for the Earth in the hundreds of millions, even billions, of years. For decades afterward astronomers were the butt of jokes in scientific circles (I have no interest in discussing any present-day parallels), in