What is the main sequence?

A band on the HR diagram where stars fuse hydrogen in their cores. About 90% of all stars lie on it. Stars spend most of their lives here.

Where do red giants appear?

Above and to the right of the main sequence: cool but very luminous because of their enormous size.

Below and to the left: hot but very faint because they are tiny (roughly Earth-sized).

Estimate a main-sequence star's luminosity and radius from its effective temperature using empirical scaling relations.

Effective Temperature

Luminosity (L_sun)

1.0000 L_sun

Radius (R_sun)1.0000 R_sun

Luminosity (L_sun) vs Effective Temperature

The HR diagram plots stellar luminosity against temperature. For main-sequence stars, hotter stars are larger and more luminous.

Radius: R / R_sun ≈ (T / T_sun)^0.8

Luminosity from Stefan-Boltzmann: L / L_sun = (R/R_sun)² × (T/T_sun)⁴

These are rough main-sequence estimates. Giants, supergiants, and white dwarfs fall off the main sequence.

A star with T = 10 000 K (A-type).

Compute luminosity from temperature and radius.

Estimate temperature from colour index.

Estimate how long a star stays on the main sequence.