The Color and Temperature of Stars — Starlight Is a Thermometer
Some stars are red, others blue. A star's color is not mere appearance but an accurate thermometer that tells its surface temperature. This is the story of color and temperature, and of the spectral types used to classify the stars.
Like iron heated in a fire
Put iron into a fire at a blacksmith's forge and it first glows a dull red, then, as it grows hotter, turns orange and yellow and finally a dazzling white and blue. Stars are just the same. The color of the light a hot, glowing body emits is determined solely by its "temperature," so a star's color shows the temperature of its surface directly.
That is why a red star is relatively "cool," while a blue star is the hottest of all. Though to our eyes they may look like twinkling white points, look closely and each star has its own subtly different color.
Stellar spectral types — O B A F G K M
Astronomers sort stars into seven classes by surface temperature (color), naming them, from hottest to coolest, types O, B, A, F, G, K, and M. Type O stars are blue-white and hotter than 30,000 degrees, while type M stars are red, around 3,000 degrees. In English this order is often remembered with the sentence "Oh, Be A Fine Girl/Guy, Kiss Me."
Our Sun is a yellow type-G star with a surface temperature of about 5,500 degrees, sitting right in the middle of this ladder. In winter's Orion you can check this difference with your naked eye — compare red Betelgeuse (type M) at the upper left with blue Rigel (type B) at the lower right, side by side.
Color and brightness together — the life of a star
In the early 20th century astronomers plotted stars as points, with color (temperature) on the horizontal axis and true brightness on the vertical. Rather than scattering at random, the stars turned out to fall into distinct bands and groups. This diagram is named the "Hertzsprung–Russell diagram (H–R diagram)" after the two scholars.
Most stars lie on a band called the "main sequence," running from the upper left (hot, bright stars) to the lower right (cool, faint stars). A star spends most of its life on this band, then, growing old, swells into a red giant and moves to the upper right, and finally becomes a small, hot white dwarf at the lower left. A single star's color, then, holds its temperature, its age, and even its destiny.