Star colour and absolute magnitude

A star’s colour depends on its surface temperature: by Wien’s law a hotter star radiates most strongly at shorter wavelengths. Very hot stars (> 10,000 K, for example Rigel ≈ 12,000 K, Sirius ≈ 9900 K) look blue-white, the Sun (≈ 5800 K) yellowish, and cool stars (≈ 3000–4000 K, Betelgeuse ≈ 3600 K, Antares ≈ 3500 K) red-orange. Colour is given quantitatively by the colour index B – V: the difference between a star’s magnitudes through a blue (B) and a yellow-green (V) filter; it is negative (≈ –0.3) for hot stars, ≈ 0.65 for the Sun and ≈ +1.5 or more for cool red stars. The surface temperature found from the spectrum is called the effective temperature – the temperature of a black body radiating the same energy per unit area. Colour does not depend on a star’s distance, which makes it a useful measure.