What are red giants?

earthsky.org
7 min read
fairly easy
What are red giants? Most main-sequence stars, like our sun, will become one. Red giants swell to such a large
What are red giants?

A red giant is a star in its death stages. It has slowly swollen up to many times its original size. Once at the red giant stage, a star might stay that way for up to a billion years. Then the star will slowly contract and cool to become a white dwarf: Earth-sized, ultra-dense star corpses radiating a tiny fraction of their original energy. Eventually, after billions of years, these stars will become totally cold and radiate no energy. They'll end their lives as a so-called black dwarf: a tiny, burned-out, virtually-invisible cinder.

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To become a red giant, a particular star must have between half our sun's mass, and eight times our times our sun's mass. Astronomers call such stars low- or intermediate-mass stars. So you can see that our sun is one of the stars that will inevitably, someday, become a red giant.

So it's our sun's destiny to become a red giant star (and afterwards a white dwarf, and then a black dwarf). But what processes will drive the sun's evolution to the red giant stage? And what will happen exactly, inside the star, as it changes? Let's examine the fate of low- or intermediate-mass stars such as our sun, as they evolve to the red giant phase.

Hydrogen: A star's 1st fuel

Stars radiate energy by converting hydrogen to helium via nuclear fusion. It's this process that causes our sun to radiate light, heat and other forms of energy as a byproduct. But nuclear fusion in stars at first requires hydrogen. And stars don't have an infinite supply of hydrogen. Our sun converts around 600 million tons of hydrogen into helium every second. If that sounds as if the supply should therefore soon exhaust itself, just remember that the sun is a star nearly a million miles across. And if you have trouble visualizing that, imagine boarding a jet airliner for a flight that is going to last 226 days. That is how long it…
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