Astronomy Answers
From the Astronomical Dictionary

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The description of the word you requested from the astronomical dictionary is given below.

the coma

komè = [Greek] hair

The coma of a comet is a cloud of water vapor and dust particules around the still frozen center of the comet. The coma develops when the comet gets close enough to the Sun (within about 2.5 AU) that the water ice at the surface of the comet sublimates (changes into water vapor). The coma can grow to be many hundreds of thousands of kilometers (or miles) in size.