Astronomy Magazine
Red giant
What a star becomes when the hydrogen in its core runs out.
With the core fuel spent, fusion moves to a shell around it and the outer layers swell enormously and cool to a deep orange-red. The star can grow to hundreds of times its former size — the Sun will reach past the orbit of Mercury and possibly Venus — while the core contracts and heats until it can start fusing helium. It is a late and relatively brief stage, but a spectacular one, and a good number of the brightest stars in the night sky are in it.
Where it has come up lately
Astronomy Magazine
Related terms
Magnetar
A neutron star with a magnetic field a thousand trillion times Earth's.
Supernova (Type Ia and Type II)
Two unrelated ways for a star to explode, useful for two entirely different reasons.
The main sequence
The long, stable middle age a star spends fusing hydrogen in its core.
Hertzsprung–Russell diagram
The single plot that organises everything about stars.
Questions
What is Red giant?
What a star becomes when the hydrogen in its core runs out.
Why does it matter?
The star can grow to hundreds of times its former size — the Sun will reach past the orbit of Mercury and possibly Venus — while the core contracts and heats until it can start fusing helium. It is a late and relatively brief stage, but a spectacular one, and a good number of the brightest stars in the night sky are in it.