Black body

A black body is an idealized object that absorbs all incident electromagnetic radiation and emits a spectrum determined solely by its temperature.

Black-body radiation: The electromagnetic radiation emitted by an ideal absorber in thermal equilibrium, with a spectrum set by its temperature. It is the characteristic emission whose spectrum defines a black body.
Thermal equilibrium: A state in which systems in contact exchange no net heat and share a common temperature. Black-body spectra describe radiation in thermal equilibrium with matter.
Color temperature: The temperature of a black body whose color most closely matches a given light source. It expresses the apparent color of a source by comparison with black-body emission.
Gray body: An idealized surface whose emissivity is constant across wavelengths but less than one. Unlike a black body, it emits less radiation than the ideal at the same temperature.
Planck's law: A law giving the spectral radiance of black-body radiation as a function of frequency and temperature. It predicts the black body's emission spectrum across frequencies and temperatures.
Electromagnetic radiation: Energy propagated through space as oscillating electric and magnetic fields. The ideal object absorbs and emits every frequency of electromagnetic radiation.
Effective temperature: The temperature of a black body that would emit the same total power per unit area as a celestial object. It lets astronomers assign a temperature to stars using their total radiation.
Line spectrum: A spectrum containing discrete frequencies emitted or absorbed by atoms or molecules. Discrete spectral lines differ from the smooth thermal continuum of a black body.
Stefan–Boltzmann law: A law stating that a black body's total emitted power per unit area is proportional to the fourth power of its temperature. It gives the total energy emitted across the black body's spectrum.
Emissivity: A measure of a surface's emitted radiation relative to an ideal black body at the same temperature. It quantifies how closely a real surface approaches black-body emission.

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