forbidden line

A forbidden line (or nebular line) is a spectral line appearing in objects that is not reproduced in labs, which cannot sufficiently rarefy enough gas to produce an observable line. Mechanisms producing such lines are termed forbidden mechanisms. The lines form from electron orbit-transitions (forbidden transitions) occurring at a timescale much longer than that of collisions between atoms which trigger more substantial transitions, making the forbidden transitions rare, and the phrase highly improbable line would be more descriptive than forbidden line. They generally occur when an electron's state of excitation is metastable, i.e., energy is required to "get over a hump" to make the move to a lower-energy state that would produce the line, and outside influences such as collisions are likely to trigger other transitions. In space, a gas cloud can be thin enough to reduce collisions, yet so large that these improbable forbidden transitions are numerous enough to form detectable lines. Such lines are seen in the thin upper Earth atmosphere as well as HII regions and planetary nebulae. Examples:

• neutral atomic hydrogen 21-cm line
• nitrogen [N II] at 654.8 nm and 658.1 nm
• sulfur [S II] at 671.6 nm and 673.1 nm
• oxygen [O II] at 372.7 nm and [O III] at 495.9 nm and 500.7 nm

The square-bracket form [N II] indicates forbidden transitions by the given species (e.g., "N II" being nitrogen with a state of ionization of 1, i.e., singly ionized).