star Bulk stellar propertiesastronomy

Temperatures of stars can be defined in a number of ways. From the character of the spectrum and the various degrees of ionization and excitation found from its analysis, an ionization or excitation temperature can be determined.

A comparison of the V and B magnitudes (see above Stellar colours) yields a B − V colour index, which is related to the colour temperature of the star. The colour temperature is therefore a measure of the relative amounts of radiation in two more or less broad wavelength regions, while the ionization and excitation temperatures pertain to the temperatures of strata wherein spectral lines are formed.

Provided that the angular size of a star can be measured (see below Stellar radii) and that the total energy flux received at Earth (corrected for atmospheric extinction) is known, the so-called brightness temperature can be found.

The effective temperature, T_eff, of a star is defined in terms of its total energy output and radius. Thus, since σT⁴_eff is the rate of radiation per unit area for a perfectly radiating sphere and if L is the total radiation (i.e., luminosity) of a star considered to be a sphere of radius R, such a sphere (called a blackbody) would emit a total amount of energy equal to its surface area, 4πR², multiplied by its energy per unit area. In symbols, L = 4πR²σT⁴_eff. This relation defines the star’s equivalent blackbody, or effective, temperature.

Since the total energy radiated by a star cannot be directly observed (except in the case of the Sun), the effective temperature is a derived quantity rather than an observed one. Yet, theoretically, it is the fundamental temperature. If the bolometric corrections are known, the effective temperature can be found for any star whose absolute visual magnitude and radius are known. Effective temperatures are closely related to spectral type, and range from about 40,000 K for hot O-type stars, through 5,800 K for stars like the Sun, to about 2,800 K for the coolest stars.