Red Giant

1. What is a Red Giant?

A Red Giant is a late phase in the life cycle of a star.
It occurs when a star has exhausted the hydrogen fuel in its core and begins to burn hydrogen in a shell around the core, causing the star’s outer layers to expand and cool.
The star becomes much larger and cooler on the surface, which gives it a reddish color, hence the name “Red Giant.”

Size: Red Giants are enormous — their radius can be tens to hundreds of times larger than the Sun’s.
Temperature: Surface temperature drops to about 3,000 to 5,000 K, cooler than the Sun’s ~5,800 K, giving a reddish hue.
Brightness: Despite the cooler surface, their large size means they are very luminous, often hundreds or thousands of times brighter than the Sun.
Color: The cooler surface temperature causes the star to glow red or orange.

When a star like the Sun uses up hydrogen in its core, nuclear fusion stops in the core, causing it to contract under gravity.
This contraction heats the shell around the core, igniting hydrogen fusion in this shell.
The outer layers expand enormously and cool down, forming a Red Giant.

The Red Giant phase occurs after the main sequence phase for stars with masses roughly 0.5 to 8 times that of the Sun.
It is a relatively short phase in stellar evolution (millions to a billion years).
Eventually, the core temperature rises enough to start helium fusion (helium burning), leading to the next evolutionary phases (Horizontal Branch or Asymptotic Giant Branch, depending on star mass).

Betelgeuse (in Orion) is one of the most famous red giants, visible to the naked eye as a bright reddish star.
Aldebaran (in Taurus) is another well-known red giant.

For stars like the Sun, after the Red Giant phase, the outer layers are expelled to form a planetary nebula, and the core becomes a white dwarf.
More massive stars might continue fusion to heavier elements, possibly ending in a supernova.