The release of a photon from an atom or ion when an electron jumps to a lower energy level. Emission from an astronomical object indicates that there must be a source of energy within it. A hot gas radiates because the motion of the atoms causes collisions which excite electrons in the atoms to higher energy levels, or even causes them to break free of the atoms, producing ions. When the electrons return to lower energy levels, they emit the energy difference in the form of photons of specific wavelengths, giving rise to emission lines. If the electron is free (unbound) and recombines with an atom or ion, then the emission can occur at a continuous range of wavelengths, producing a continuous spectrum. See also Bound–bound Transition; Free–bound Transition
Subjects: Astronomy and Astrophysics.