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The transfer of energy from a photon to an atom or molecule. If the energy of the photon is just sufficient to raise an electron from one energy level to another, the result is an absorption line at a particular wavelength; this is called a bound–bound transition. Absorption at a wide range of wavelengths is called continuous absorption, and occurs when the absorbed photons have energies greater than the minimum needed to eject an electron completely; this is called a bound–free transition, or photoionization. Absorption may occur whenever radiation traverses a gas, for example the outer layers of a star, the Earth's atmosphere, a nebula, or interstellar space (see Interstellar Absorption).

Subjects: Astronomy and Astrophysics.

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