What Are Red Giant Stars?

Red Giants

• A red giant star is a phase in the evolution of certain sized solar bodies. Stars that become red giants are of intermediate mass in solar terms, usually between 0.5 and 10 times the size of the sun. When a star reaches red giant status, it also has a much lower temperature than other stars, somewhere in the region of 5,000 Kelvin.

Red Giant Branch Stars

• Most red giant stars are called red giant branch stars (RGB). These are stars have an inactive helium core while the hydrogen atmospheric shell is still forming helium.

Asymptotic Giant Branch Stars

• Asymptotic giant branch stars (AGB) produce carbon from helium through nuclear fusion. This happens to older red giant stars.

What Happens Before?

• A red giant star is formed when the nuclear reaction in the core stops because the hydrogen has run out. This forces the star to start using hydrogen from outside the core, forcing the star to expand. As it produces more energy this way, the color it emits appears red rather than bright yellow.

What Happens Afterwards?

• Eventually the core of the star will collapse as all the hydrogen has been used up and the helium can no longer produce carbon. At this point the outer layers become a planetary nebula and the core turns from a red giant into what is called a white dwarf. This red giant phase in solar evolution usually lasts a few million years, which is relatively short compared to other phases.

Visible Red Giant Stars

• There are a few red giant stars visible from Earth as of 2011. Aldebaran (Alpha Tauri) is part of the Taurus zodiac constellation and is located 65 light years from Earth. Arcturus (Alpha Bootis) is the brightest star in the Northern hemisphere and is located in the Bootes constellation. Gamma Crucis (Gacrux) is 88 light years from Earth and is the third brightest star in the Southern Cross constellation.