black body

The term black body (or blackbody or BB) is used in physics to mean an ideal object that reflects absolutely no light (or any electromagnetic radiation, EMR), absorbing it all, i.e., is perfectly black, the only radiation from it is what it emits due to its temperature, and has had a sufficiently long lifetime to fall into thermodynamic equilibrium, with the energy emitted via EMR matching that which it receives. The spectrum of the radiation of a black body (black-body radiation) is characteristic, known as a black-body spectrum, and depends entirely upon the body's temperature (which determines the relative amounts at each wavelength) and its surface area (which determines the total amount of energy emitted under the constraints of its temperature). The black body concept is useful as a first approximation of objects that are similar in that they emit significant radiation due to their temperature. This includes stars, the Sun, planets, etc. Of interest is matching such a body's spectrum with the temperature of a black body that would produce a close-to-matching spectrum: this is one means of detecting the body's temperature with a spectrograph.

Regarding stars, unlike a black body, they clearly emit more energy than they receive. However, their energy generation is rather constant and their surface is near thermodynamic equilibrium with a stable temperature, and to the outside observer, they do have considerable resemblance to a black body.