Stars are enormous spheres of ignited gas that light the cosmos, and seed it with the materials for rocky worlds and living beings. They come in many different types and sizes, from smouldering white dwarfs to blazing red giants.
Stars are often classified according to spectral type. Although they emit all colors of light, spectral classification considers only the peak of this emission as an indicator of the star’s surface temperature. Using this system, blue stars are the hottest, and are called O-type. The coolest stars are red and are called M-type. In order of increasing temperature, the spectral classes are M (red), K (orange), G (yellow), F (yellow-white), A (white), B (blue-white), O (blue).