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A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The radiation emitted by a black body in thermal equilibrium with its environment is called black-body radiation. The name "black body" is given because it absorbs all colors of light.
Stars emit less ultraviolet radiation than a black body with the same B−V index. Although stars are not perfect blackbodies, to first order the spectra of light emitted by stars conforms closely to a black-body radiation curve, also referred to sometimes as a thermal radiation curve.
For a black body (a perfect absorber) there is no reflected radiation, and so the spectral radiance is entirely due to emission. In addition, a black body is a diffuse emitter (its emission is independent of direction). Blackbody radiation becomes a visible glow of light if the temperature of the object is high enough. [19]
Blacksmiths work iron when it is hot enough to emit plainly visible thermal radiation. The color of a star is determined by its temperature, according to Wien's law. In the constellation of Orion, one can compare Betelgeuse (T ≈ 3800 K, upper left), Rigel (T = 12100 K, bottom right), Bellatrix (T = 22000 K, upper right), and Mintaka (T = 31800 K, rightmost of the 3 "belt stars" in the middle).
The luminosity of the star, which can be measured from observations of the star's apparent brightness, [7] can then be written as: L = 4 π R s t a r 2 σ T s t a r 4 {\displaystyle L=4\pi R_{\rm {star}}^{2}\sigma T_{\rm {star}}^{4}} where the flux has been multiplied by the surface area of the star.
In principle, the temperature of a star can be calculated directly from the B−V index, and there are several formulae to make this connection. [6] A good approximation can be obtained by considering stars as black bodies, using Ballesteros' formula [7] (also implemented in the PyAstronomy package for Python): [8]
But more importantly, it relied on a new theoretical postulate of "perfectly black bodies", which is the reason why one speaks of Kirchhoff's law. Such black bodies showed complete absorption in their infinitely thin most superficial surface. They correspond to Balfour Stewart's reference bodies, with internal radiation, coated with lamp-black.
By treating the star as an idealized energy radiator known as a black body, the luminosity L and radius R can be related to the effective temperature T eff by the Stefan–Boltzmann law: = where σ is the Stefan–Boltzmann constant. As the position of a star on the HR diagram shows its approximate luminosity, this relation can be used to ...