What does the color of a star indicate?

The color of a star mostly indicates a star's temperature, and it can also suggest the star's age.

Temperature

Astronomers have long used stars' color to identify their temperature, and they created a classification system called spectral classes to do so, which places stars into a category based on their temperature:

Class O stars, which are blue in color, are the hottest, and class M stars, which are red in color, are the coldest. This can be tricky to remember, as we typically think of blue as a cool color and red as a hot one. Thus, it's helpful to understand the reason blue stars are hot and red ones are cool:

Remember that the temperature of an object is a direct measure of the amount of motion within it. The hotter the star, the faster its particles move and the more energy they radiate.

Cool stars radiate most of their energy in the red and infrared region of the electromagnetic spectrum — which means shorter wavelengths and less energy.

Thus, they appear red, while hot stars — in which particles are moving much more quickly — emit more energy and thus emit mostly at blue and ultra-violet wavelengths, making them appear blue or white.

Age

Throughout most of a star's life, it is burning hydrogen at its core, which creates lots of energy and thus makes it appear blue. As stars age, they run out of hydrogen to burn, decreasing the amount of energy they emit. Thus, younger stars can appear bluer while older ones appear more red, and in this way, a star's color can tell us something about that star's age.