noun
Refers to a recognizable pattern of stars that is not an official constellation, such as the Big Dipper or the Summer Triangle.
北斗七星は、おおぐま座の一部をなすアステリズムです。
The Big Dipper is an asterism that forms part of the constellation Ursa Major.
星座 means 'constellation', an officially recognized star pattern. アステリズム is a more informal star pattern, often part of a constellation or spanning multiple constellations.
Borrowed from English 'asterism', ultimately from Greek 'asterismos' (a grouping of stars).