What do "forbidden" lines reveal about interstellar space?

Question

Quizplus · Accepted Answer

Forbidden lines are spectral emission lines that arise from atoms or ions in conditions where collisions are not sufficient to populate excited energy levels. These lines are useful for studying interstellar gas because the density or pressure of the gas emitting the lines is much lower than can be produced in a laboratory. Forbidden lines are emission lines that are not allowed by the laws of quantum mechanics in laboratory conditions. They occur in space when atoms are in very low-density environments, such as interstellar gas clouds. Therefore, the presence of forbidden lines indicates that the density or pressure of this gas is much lower than can be produced in a laboratory.

What Do "Forbidden" Lines Reveal About Interstellar Space