where do atoms in nebulae get energy from
Planetary nebula Physicists showed in the 1920s that in gas at extremely low densities, electrons can occupy excited metastable energy levels in atoms and ions that would otherwise be de-excited by collisions that would occur at higher densities.[10] Electron transitions from these levels in nitrogen and oxygen ions (O+, O2+ (a.k.a. O iii), and N+) give rise to the 500.7 nm emission line and others.[6] These spectral lines, which can only be seen in very low density gases, are called forbidden lines. Spectroscopic observations thus showed that nebulae were made of extremely rarefied gas.[11]