The color of a star is determined by that part of the visible spectrum where the peak amount of radiation is emitted.
Blue stars are extremely hot, red stars are relatively cool.
The color of a star is determined by measuring its color index . It is important to remember temperature and luminosity for a star are not strictly related. We will see below that this means the star must be very large to be both bright and cool. Stars are divided into a series of spectral types based on the appearance of their absorption spectra. Between the classes there were 10 subdivisions numbered 0 to 9.