A small constellation or a group of stars that form a recognizable pattern but are not an official constellation.
From Greek aster (star) plus the suffix -ism. This astronomical term was created to describe loose groupings of stars (like the Big Dipper, part of Ursa Major), distinguishing them from formal constellations.
The Big Dipper is a famous asterism—it's not an official constellation, just a pattern of stars that humans noticed. This word shows how even our perception of the night sky is partly human invention and pattern-recognition.
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