Stellar Classification

Surface temperature and color of object

When the iron is heated, it first turns dark red. As the temperature of the iron increases, it gradually becomes closer to yellow. This is because as the temperature of the iron increases, the color of the light emitted by the iron changes.
Just as the color of the heated iron varies with temperature, the color of the star also depends on the surface temperature. The color of stars varies with the surface temperature as red, yellow, white and blue. That is, the higher the surface temperature, the more blue the star, and the lower the surface temperature, the redder the star. Therefore, you can measure the surface temperature by observing the color of stars.

Stars have a unique spectrum depending on the surface temperature. So, for an accurate surface temperature measurement, we observe the star’s spectrum. Stars are classified into seven types, O, B, A, F, G, K and M, depending on the characteristics of the spectrum. This is called the spectral or spectral type of the star. O type has a high surface temperature, and M type has a low surface temperature.

Spectral classes O B A F G K M
Color Blue Blue White White Yellow White Yellow Light Orange Orange Red
Temperature ≥ 30,000 K 10,000K ~ 30,000K 8,000K ~ 10,000K 6,000K ~ 8,000K 5,000K ~ 6,000K 3,500K ~ 5,000K ≤ 3,500K
A typical star Alnitak (Orion) Rigel, Regulus Sirius, Vega Polaris, Procyon Sun, Capella Arcturus, Aldebaran Betelgeuse, Antares

Blackbody radiation of a star

A black body is an object that completely absorbs and re-emits a given light.
The surface of the star can freely emit radiant energy into space. Therefore, the star is considered an ideal black body.
The German scientist Wilhelm Bein showed the spectral distribution of blackbody radiation as a function of temperature, which is in fact very close to the distribution of stars.
The same mathematical formulas were used for the graphs shown in the above simulation.