The small white star on the left is the sun the way it is now. The large red star on the right is what the sun will look like when it turns into a red giant.
A **red giant** is a giant star that has the mass of about one-half to ten times the mass of our Sun. Red giants get their name because they appear to be colored red and they are very large. Many red giants could fit thousands and thousands of suns like ours inside of them. 

Right now, our Sun is a main-sequence star, not a red giant. However, five billion years from now, scientists believe our sun will become a red giant. It will be about 200 times bigger in diameter than it is now. It will become so big it will swallow up Mercury, Venus and possibly the Earth.

How a star becomes a red giant
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See the main article: Stellar evolution
All new stars change hydrogen to helium through nuclear fusion. This makes a lot of energy (e.g. light and heat). In a normal star, like our Sun and all other main-sequence stars, this change happens at the very center of the star. Sooner or later, almost all of the hydrogen at the center has changed to helium. This causes the nuclear reaction to stop. The center will start to get smaller due to the star's gravity. This makes the layer just outside the center get hotter. This layer still has hydrogen. This hydrogen will fuse to make helium.

Changes of a star like the Sun in brightness and temperature as the Sun grows old. You can see the three red giant phases.
With this new source of power, the outer layers of the star will get much, much bigger. The star will get brighter, sometimes as much as ten thousand times as bright as when it was on the main sequence. Since the outside of the star is bigger, the energy will be spread over a much larger area. Because of this, the temperature of the surface will go down and the color will change to red or orange.

There are often three main parts of a red giant stage: firstly, the red giant's center is plain helium while the layer outside it has hydrogen fusing into helium. This is called the red giant branch or RGB phase of the star. Eventually, the helium in the center will ignite and start to turn into carbon. This ignition is called the helium flash, and after this happens, the red giant branch star becomes a horizontal branch star. For stars with the same mass than the Sun, the horizontal branch phase is called the red clump phase. Red clump stars are smaller and hotter than red giant branch stars, and can appear yellow or orange in color. For stars more massive than the Sun, they become much hotter so that their color will change from red, to yellow, and then to blue-white. Their horizontal branch phase is called a blue loop.

Soon (in only hundreds of millions of years) horizontal branch stars will start to fuse helium to make other elements like carbon, nitrogen and oxygen. They expand back into red giants even larger than the red giant branch phase. This is called the **asymptotic giant branch** or AGB phase.

The red giant phases are temporary. They are shorter than the billions of years a star spends on the main sequence. Some of their outer layers will blow away, leaving interstellar gas and dust circling the star. In time, most red giants will become white dwarfs. Very large red giants (red supergiants) become neutron stars or black holes.

Examples
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### Red Giant Branch (RGB)

* Pollux
* Arcturus
* Aldebaran

### Red Clump (Horizontal Branch)

* Capella
* Beta Ceti
* Delta Andromedae

### Asymptotic Giant Branch (AGB)

* Mira
* Alpha Herculis
* Chi Cygni
* Rho Persei

Related pages
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* Stellar evolution
* Main sequence
* White dwarf
* Red dwarf