asterism

1a :  a group of stars that form a pattern in the night sky On October evenings, when the trees have shed their leaves and stars become visible through the twigs, our best-known asterism, the Big Dipper, passes under the north pole. — George Lovi ◆The term asterism does not now usually refer to a constellation but to a star pattern that makes up part of a constellation or that includes stars from more than one constellation. b :  a small group of stars

2 :  a star-shaped figure exhibited by some crystals by reflected light (as in a star sapphire) or by transmitted light (as in some mica)