Asterism

An asterism is a pattern of stars recognized in the night sky that does not correspond to any officially recognized constellation. Asterisms are informal groupings, often composed of relatively bright stars, used to help observers navigate the sky or identify other stars and deep-sky objects. Unlike constellations, which have formal boundaries defined by bodies such as the International Astronomical Union, asterisms are not IAU-designated and can vary by culture and tradition. The term derives from the Greek aster, meaning star, with a suffix indicating a form or figure, and entered English via French astérisme.

Well-known examples include the Big Dipper, the seven-star asterism in Ursa Major, which helps locate the North Star via its pointer stars Merak and Dubhe. The Summer Triangle, formed by Vega, Deneb, and Altair, spans three different constellations and serves as a practical reference for finding other bright stars. Other familiar asterisms include the W-shaped pattern Cassiopeia within Cassiopeia, the Teapot in Sagittarius, and the Winter Hexagon, a rough six-star figure that links several bright stars such as Sirius, Betelgeuse, Rigel, Aldebaran, Capella, and Procyon.

Asterisms can be contained wholly within a single constellation or cross constellation boundaries, and they are widely used in star charts, planispheres, and naked-eye astronomy to aid in orientation and observation. Cultural differences can lead to different asterisms or emphasis on different star patterns. The concept highlights practical patterns in the sky distinct from formal celestial boundaries.