A comet's gaseous envelope (which creates the tail) is called what?

The story behind the answer

The coma is the fuzzy, gaseous-and-dusty envelope that surrounds a comet’s solid nucleus when it approaches the Sun. Solar heating causes ices in the nucleus to sublimate, releasing gas and dust that expand to form this glowing cloud. The comet’s tails—an ion tail pushed by the solar wind and a dust tail driven by sunlight—stream out from the coma, not from the nucleus directly. That’s why “coma” is the correct term; “wake,” “backwash,” and “ablative” aren’t used for this feature.

Fun mnemonic: coma sounds like “comb,” and comets were once called “long-haired stars.” In fact, coma comes from the Greek kome, meaning hair—think of the coma as the comet’s “hairy halo” around its head, from which its tails flow.