The first sense is the one I know best. The group can be part of a standard constellation (such as the Big Dipper, which is one part of Canis Major) or parts of several constellation (such as the Summer Triangle, which is the brightest stars of Lyra, Cygnus, and Aquila). The second is the star of a star sapphire:
The third is now rarely used. First appears in the 1590s, from Greek asterismós, constellation, from astēr, star.
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