Spectral classes of star, computer artwork. There are seven main classes of star, categorised according to their temperature and spectrum. These are, from left to right, called O, B, A, F, G, K and M. The surface temperature determines their colour, and decreases from left to right. O stars are extremely hot and emit much of their radiation in the ultraviolet. B stars are slightly cooler but still extremely powerful, and A stars are cooler still but still appear white. F stars appear slightly yellow, while G stars, of which the Sun is one, appear yellower still. K stars appear orange, while M stars, the most common type in the galaxy, appear red.