Surface temperature and color of object
When the iron is heated, it is initially dark red, but as the iron’s temperature rises, it gradually takes on a yellowish color. This is because the light emitted by the iron changes as the iron’s temperature increases.
Just as the heated iron color varies with temperature, the star’s color also depends on the surface temperature. The color of stars varies with the surface temperature as red, yellow, white, and blue. The higher the surface temperature, the more blue the star, and the lower the surface temperature, the more red the star. Therefore, you can measure the surface temperature by observing the color of the 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 spectrum’s characteristics. 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.
We used the same mathematical formulas for the graphs shown in the above simulation.