# Blue color of ion (plasma) comet tails

What exactly is the cause of blue light of ion (plasma) tails of comets? Somewhere I have read that the source of blue light are CO+ ions which has just acquired the missing electron and became neutral molecule CO. Is this correct?

Close, but not quite right - the blue light is indeed emission from CO$$^+$$, but it's from the CO$$^+$$ ions themselves, with no need for recombination to CO; that (ionized) molecule has a strong set of energy transitions around 425 nm (4250 Angstroms), which is in the blue part of the visible spectrum:
• Very nice answer! +1 The spectrum shown is only about 1.5 nm wide, so it's hard to tell if this represents most of the "blue light" from the tail or just a small fraction of it, but this sounds authoritative enough to be believable. This also finally explains to me why the tails of green comets (C₂ vibronic lines, 1, 2, 3) look blue or violet. Thanks! – uhoh Jul 13 '20 at 2:22