1590s, "a constellation, a group of stars," from Greek asterismos "a marking with stars," from aster "star" (from PIE root *ster- (2) "star"). Originally any grouping of stars, whether a constellation or not; in modern use usually the latter. The "Big Dipper" is an asterism; Ursa Major is the constellation which contains it. Other examples are the "Summer Triangle," "the sickle" of Leo, "the teapot" of Sagittarius.
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