A comet's atmosphere (composed of dust and/or various gases) surrounding its nucleus. The coma is rather tenuous (except very close to the nucleus), and stars can be occasionally easily seen through it, shining from behind. And yet, the coma is usually thick enough that it masks our view of the true nucleus of the comet, as seen from the earth. As a comet's nucleus is usually quite small, it is not able to retain its coma for long periods of time, and the coma material gradually drifts away into space (helped out by the solar wind). Much coma material is thrown back into what we see as the comet's tail. But all coma material originates in the comet's nucleus, and solar sublimation due to heating causes gases to move outward, often in jets, taking dust material with them to form the coma and tail.