Surface temperature
A star’s surface temperature—also known as its effective temperature—is the temperature of its outer radiating layer, the photosphere. It directly determines the star’s color and spectral type. Cool stars are red, hot stars are blue, and stars of intermediate temperature, such as the Sun, are yellow or white. It ranges from less than 3,000 K for the coldest red dwarfs to more than 50,000 K for the hottest blue stars.
The color-temperature relationship
A star’s color is a direct consequence of its surface temperature, according to the Wien’s law: the hotter a star is, the more its peak radiation is emitted at short wavelengths. A star at 3,000 K emits primarily in the infrared and red. A star at 10,000 K emits primarily in the blue and ultraviolet regions. At 5,778 K, the Sun emits primarily in the yellow-green region of the visible spectrum.
Spectral Classification
Stars are classified according to their spectral type, designated by the letters O, B, A, F, G, K, and M in descending order of temperature. This sequence, memorized by astronomers using the mnemonic phrase Oh Be A Fine Girl Kiss Me, ranges from O-type stars (over 30,000 K) to M-type stars (less than 3,500 K). The Sun is a G2-type star, with a temperature of 5,778 K.
Importance in Astronomy
Surface temperature is one of the two fundamental characteristics that define a star’s position in the Hertzsprung-Russell diagram. Together with luminosity, it allows us to determine the star’s size, evolutionary stage, and age. It is measured precisely using spectroscopy and multiband photometry.
Concrete example
The star Betelgeuse, a red supergiant in Orion, has a surface temperature of about 3,500 K—much cooler than the Sun. Yet it is hundreds of thousands of times more luminous, because its radius is about 700 times that of the Sun. This is a perfect illustration of the relative independence between temperature and luminosity for giant stars.
Frequently Asked Questions
Is the surface temperature the same as the star’s temperature?
No. The interior of a star is much hotter: the core of the Sun reaches 15 million degrees. The surface temperature—a few thousand degrees—is that of the photosphere, the outer layer from which light can escape. The temperature gradually increases toward the center.
How is a star’s temperature measured?
Mainly by analyzing its light spectrum. The position of the emission peak (Wien’s law) and the characteristic absorption lines make it possible to determine the temperature with precision. Photometry using multiple color filters provides a less precise but faster estimate.
Are the hottest stars the brightest?
Generally yes, for main-sequence stars, where temperature and luminosity are correlated with mass. But this isn’t always true: a white dwarf can be very hot (50,000 K) yet not very bright because it is very small. A red giant can be very bright while still being cold because it is enormous.