Red Giant

A red giant is a star in an advanced stage of its evolution, characterized by a considerable expansion of its outer layers. When a star like the Sun exhausts the hydrogen in its corecore, fusion continues in an outer layer, causing the star to swell to hundreds of times its original radius. Its surface cools and takes on a reddish-orange hue.

Why does a star expand?

When the corecore of a star exhausts its hydrogen, fusion temporarily stops. Without radiation pressure to counteract gravity, the corecontracts and heats up. This additional heat is transferred to the outer layers, which expand enormously. The star becomes a red giant: its surface is much larger but also cooler, hence its characteristic color.

Helium fusion

In the core of the red giant, the temperature eventually reaches about 100 million degrees— enough to trigger the fusion of helium into carbon and oxygen. This phase, called the helium flash for low-mass stars, temporarily stabilizes the star. For massive stars, several stages of fusion may follow one another.

The Final Fate

For low-mass stars like the Sun, the red giant phase ends with the ejection of the outer layers in the form of a planetary nebula, leaving thebare core—a white dwarf. For very massive stars, the expansion is even more extreme (these are known as red supergiants, such as Betelgeuse) and ends in a supernova.

Did you know?

When the Sun becomes a red giant in about 5 billion years, it could swell to the point of engulfing Earth’s orbit. Its brightness will then be about 2,000 times greater than it is today, making the Earth’s surface completely uninhabitable long before that stage.

Frequently Asked Questions

Is Betelgeuse a red giant?

Betelgeuse is, more precisely, a red supergiant—a star much more massive than the Sun, in a similar but extremely amplified phase. It is about 700 times larger than the Sun and could explode as a supernova in the next few thousand or million years.

Has the Sun ever been a red giant?

No. The Sun is currently a main-sequence star, in the stable phase of hydrogen fusion. It will not become a red giant until about 5 billion years from now, when its central hydrogen is exhausted.

Do all stars become red giants?

Almost all stars with sufficient mass go through a giant phase. Very massive stars become supergiants. Very low-mass red dwarfs, on the other hand, burn so slowly that they could theoretically burn for trillions of years without ever becoming giants.