A giant star is a star with much larger radius and luminosity than a main-sequence star of the same surface temperature.[2][3]
Giant stars are up to a few hundred times the diameter of the Sun and between 10 and a few thousand times brighter than the Sun. They don't last as long as most main sequence stars. Stars still more luminous than giants are referred to as supergiants and hypergiants.
A hot, luminous main-sequence star may also be referred to as a giant.[4]
There are a wide range of giant class stars, and sub-divisions are often used to identify particular types. Astronomers use such terms as: sub-giants, bright giants, red giants, yellow giants and blue giants. The giant luminosity class is given the Roman numeral III, for bright giants it is II.
These have spectral types K to M.
These have spectral types F and G.
The Cepheid variables are yellow giants.
These have spectral types A, B, and O.
Giants nearly as bright as supergiants. They can have many spectral types.