The coolest stars are red in color. A star's color gives visible information about its surface temperature. The hottest stars are blue, while white and yellow stars indicate mid-range temperatures.
Red is the lowest temperature at which a star can shine in a visible light. As its temperature changes, a star's color changes as well, and the hotter it becomes, the more blue light it produces. Therefore, as it is heated, a star's light moves from red to yellow to white and finally, to blue.
Red stars are also known as M stars, while the blue stars are known as O stars. Between these are five other groups of stars: B, A, F, G and K, which represent different shades of red, yellow, white and blue.