What is a red giant?
A red giant is a large, cool, glowing star in a late stage of life. It forms when a star like the Sun runs out of hydrogen in its core and its outer layers swell to enormous size.
For most of its life, a star fuses hydrogen into helium in its core. When the core's hydrogen runs low, the core contracts and heats up, while hydrogen continues to burn in a shell around it. This extra energy pushes the outer layers outward, making the star expand hundreds of times wider. Its surface cools, giving it a reddish-orange color.
Well-known red giants include Arcturus and Aldebaran. Even larger stars become red supergiants, such as Betelgeuse in Orion, which is so large that if it were placed where the Sun is, it would extend past the orbit of Mars or even Jupiter.
The Sun's future
In about five billion years, the Sun will become a red giant. It will likely swallow Mercury and Venus and may reach Earth's orbit. Eventually, it will shed its outer layers, forming a planetary nebula, and leave behind a small, hot core called a white dwarf.
Did You Know?
Despite their size, red giants have a low average density. Their outer layers are much thinner than the air we breathe.