Red Giant Star

A red giant is a luminous giant star of low or intermediate mass. A mass like our sun, but is in a late phase of its life. Because the inner fusion reactions of the star have become much less intense, there is a gravitational collapse of the core as the outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower. The appearance of the red giant is from yellow orange to red, including the spectral types K and M, but also class S stars and most carbon stars.

Red giants are stars with radii tens to hundreds of times larger than that of the Sun. They have exhausted the supply of hydrogen in their cores and switched to fusing hydrogen in a shell outside the core. Main sequence stars of spectral types A through K are believed to become red giants. The most common red giants are the so-called red giant branch stars (RGB stars) whose shells are still fusing hydrogen into helium, while the core is inactive helium.