Star Color Temperature Calculator

Stellar temperature by spectral class and Wien's law

Look at a star's color and you're reading its surface temperature, which is exactly what the OBAFGKM spectral classification was built to track. Each class covers an effective-temperature range: O ≥ 30,000 K (blue), B 10,000–30,000 K (blue-white), A 7,500–10,000 K (white), F 6,000–7,500 K (yellow-white), G 5,200–6,000 K (yellow, with the Sun sitting at ≈ 5,778 K), K 3,700–5,200 K (orange) and M 2,400–3,700 K (red). What ties color to temperature is Wien's displacement law: λ_max · T = b, where b = 2.898×10⁻³ m·K. That puts the Sun's peak near 501 nm (green-yellow) and Betelgeuse (≈3,500 K) all the way out at ~828 nm, deep red.

Applications

Knowing T_eff lets you place a star on the Hertzsprung–Russell diagram (luminosity against T_eff), which feeds the models of how stars evolve. Spectral type also gives you a handle on mass and lifetime — O-type stars burn out in a few Myr while M dwarfs keep going for over a trillion years. The same temperature sets where an exoplanet's habitable zone falls, and the strengths of spectral lines, which depend on T, let you read off composition.

FAQ

Why are hot stars blue and cool stars red? As T rises, blackbody emission shifts toward shorter wavelengths. That's Wien's law in action.

What is the B−V color index? It's the magnitude difference between the blue and visual bands, and it tracks T_eff closely (the Sun sits at B−V ≈ 0.65).

What about classes L, T, Y? These run cooler than M and cover brown dwarfs and sub-stellar objects (T < 2,400 K). They were added to the scheme after 1999.