Star colour
Also called: star color, colour index, B−V.
Stars are coloured by their temperature: blue-white stars are the hottest, orange and red the coolest.
A star glows like a hot coal. The hottest stars, above 10,000 °C, shine blue-white (Rigel, Vega); the Sun, at about 5,500 °C, is yellow-white; cooler stars are orange (Arcturus) or red (Antares, Betelgeuse). Astronomers label them O, B, A, F, G, K and M from hottest to coolest.
Colours are subtle to the eye but easiest to see in contrasting doubles — Albireo's gold and blue — and in slightly defocused bright stars.