The coma is the bright region surrounding the nucleus of a comet. It is formed by molecules that have evaporated
from the nucleus and then been dissociated by sunlight. These molecule fragments or "radicals" (e.g., CN, NH2, C2) are excited
by sunlight and emit radiation as they return to lower energy levels. The emission "lines" mostly appear as broad "bands", which is characteristic
of molecules.
Spectrum of Comet Coma