Flash in the

A red giant may continue on its unstable career for a few hundred million years, outwardly expanding and inwardly shrinking. At some point, however, the shrinking of the core raises its temperature high enough to finally ignite the helium that has been waiting there patiently. It's like the piston in your car engine on the compression stroke. Eventually, the combustion reaction is triggered. Only, in a star, it's a fusion reaction, not chemical combustion. Now, along a timeline that has been measured in tens of millions of years, something very sudden occurs. In a process that consumes only a few hours, not millions or billions of years, helium starts to burn in an explosion of activity called the helium flash—the explosive onset of helium burning in the core of a red giant star. The helium is fused into carbon, and the star settles into a (short-lived) equilibrium.

After the helium flash, the helium in the core is rapidly fused into carbon and (occasionally) oxygen. The star's core begins to heat up again, and with helium being fused in the core comes another equilibrium. The star's outer layers shrink, but become hotter, so the star becomes bluer and less luminous. Once helium is exhausted in the star's core, it's nearly the end of the road for a low-mass star. It won't be able to again raise the core temperature sufficiently to fuse carbon into heavier elements.

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