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.[16] 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.[8] 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.[17]
Great Andromeda "Nebula" (M110 to upper left), as photographed by Isaac Roberts, 1899.
https://en.wikipedia.org/wiki/Planetary_nebula
drawfs
Apparently, by late 1968, no X-rays had been detected from the Andromeda Galaxy.[94] A balloon flight on 20 October 1970, set an upper limit for detectable hard X-rays from the Andromeda Galaxy.[95] The Swift BAT all-sky survey successfully detected hard X-rays coming from a region centered 6 arcseconds away from the galaxy center. The emission above 25 keV was later found to be originating from a single source named 3XMM J004232.1+411314, and identified as a binary system where a compact object (a neutron star or a black hole) accretes matter from a star.[96]
https://en.wikipedia.org/wiki/Andromeda_Galaxy
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