Sun and Fingernails

StarDate: June 9, 2009

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The Sun and your fingernails might not seem like they have much in common. But it turns out that they're growing at the same rate.

Your fingernails grow an inch or two a year, and so does the Sun's diameter. Here's why. The Sun generates energy by "fusing" some of the hydrogen in its core to make helium. It takes four hydrogen atoms to make one helium atom. So as the Sun transforms hydrogen into helium, the number of individual atoms in its core goes down.

The Sun's gravity pulls inward and tries to make the Sun collapse. But the Sun's hot gas exerts an outward pressure. That balances the inward pull of gravity, so the Sun survives.

But the strength of the outward pressure depends on both the temperature of the gas and the total number of atoms. As the Sun converts hydrogen into helium, the total number of atoms decreases. That causes the Sun's core to shrink a little, which makes it hotter and denser. The higher temperature and density in turn speed up the rate at which the Sun converts hydrogen into helium.

This process makes the Sun get brighter. And over time, it adds up: the Sun is about 40 percent brighter today than it was when it was born four and a half billion years ago.

But the Sun doesn't just grow brighter -- it grows bigger, too. Astronomers estimate that the Sun's diameter gains an inch or two a year -- which just happens to be how fast your fingernails are growing.

Script by Ken Croswell, Copyright 2009

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