Red giant star expanding

Red giant stars represent the final stage of stellar evolution, where the core contracts and the outer layers expand.

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A red giant star is an advanced phase in the life of low to intermediate mass stars, like the Sun. For millions of years, the star fuses hydrogen into helium in its core. When core hydrogen is exhausted, fusion ceases and the core contracts under gravity, heating up. This heats the outer layers, which begin hydrogen fusion in a shell around the inert helium core. The increased pressure and temperature cause the outer layers to expand enormously, while the surface cools, giving the star a reddish color. The diameter can increase hundreds of times, while luminosity can rise by thousands of times.

Notable examples of red giants include Betelgeuse in Orion and Arcturus in Boötes. Betelgeuse is a red supergiant that may explode as a supernova within 100,000 years. Arcturus is a closer, bright red giant visible to the naked eye. During the red giant phase, stars like the Sun will undergo intense mass loss, ejecting outer layers that form planetary nebulae. The remaining core will become a white dwarf.

Interestingly, the Sun will become a red giant in about 5 billion years. Its expansion will likely engulf Mercury, Venus, and possibly Earth. Earth's atmosphere will be vaporized much earlier. This stage lasts a few hundred million years, relatively short on cosmic scales. Red giants are key to recycling matter in the universe, as their stellar winds enrich the interstellar medium with heavy elements formed in the stellar interior.

Frequently Asked Questions

How long does a star remain a red giant?

The red giant stage lasts a few hundred million years, a short period compared to the star's total lifespan (billions of years).

What happens after the red giant phase?

After shedding its outer layers as a planetary nebula, the exposed core becomes a white dwarf, which slowly cools.

Will the Sun become a red giant?

Yes, in about 5 billion years. The Sun will expand, possibly engulfing the inner planets, including Earth.