What B-V values correspond to what star types?

B-V 10 000 K). B-V ~ 0.6-0.8 is for Sun-like G stars. B-V > 1.4 is for cool red M dwarfs (<3500 K).

By observing a star through two standard filters (B and V) and computing the magnitude difference: B-V = m_B - m_V.

Is this formula exact?

It is an empirical fit accurate to about 100 K for most main-sequence stars. Heavily reddened or peculiar stars may deviate.

Estimate a star's surface temperature from its B-V colour index using the Ballesteros formula: T = 4600 * (1/(0.92*(B-V)+1.7) + 1/(0.92*(B-V)+0.62)).

B-V Colour Index

Effective Temperature

5,778 K

Effective Temperature vs B-V Colour Index

The B-V colour index measures the difference in brightness between blue (B) and visual (V) filters. Bluer (hotter) stars have lower B-V.

T = 4600 × (1/(0.92(B-V)+1.7) + 1/(0.92(B-V)+0.62)) K

The Sun has B-V ≈ 0.65, giving T ≈ 5780 K.

The Sun: B-V = 0.65.