Forbidden emission line
= Forbidden emission line
A <forbidden emission line> is suppressed by the electric-dipole <selection rules> but can occur through weaker radiative channels such as magnetic-dipole or electric-quadrupole emission. Its relatively long-lived upper atomic level is vulnerable to collisional de-excitation in dense gas. Low-density astrophysical plasmas can therefore display strong forbidden-line emission even when laboratory dense sources do not.