Stellar evolution

In astronomy, stellar evolution is the sequence of changes that a star undergoes during its lifetime, the millions or billions of years during which it emits light and heat. Over the course of that time, the star will change radically.

The middle years of a star's life

After millions to billions of years, depending on its initial mass, a star has exhausted all the hydrogen in its core. Larger and hotter stars consume their hydrogen much more rapidly than cooler and less massive ones. Once the core's ready supply of hydrogen is gone, nuclear processes there cease.

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Without the outward pressure generated by these reactions to counteract the force of gravity, the outer layers of the star begin to collapse inward on the core. The temperature and pressure increase as during formation of the protostar, but now to even higher levels, until helium fusion begins at core temperatures of around 100 million kelvin.

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The very hot core causes the outer layers of the star to expand enormously; the star becomes as much as 100 times larger than it was during its main sequence lifetime. It is now a red giant, and the helium burning phase lasts for a few million years. Almost all red giants are variable.

Related Topics:
Red giant - Variable

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What happens next depends, once more, on the star's mass.

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