Asterism

An asterism is a recognizable, informal pattern of stars in the sky that is not one of the 88 official constellations recognized by the International Astronomical Union, though it is often made up of stars that belong to one or more of those constellations. The most famous example is the Big Dipper, a widely recognized asterism formed from seven of the brightest stars within the much larger official constellation Ursa Major, the Great Bear. Other well-known asterisms include the Summer Triangle, formed by three bright stars from three separate constellations (Vega, Deneb, and Altair), and the Winter Hexagon, a large ring of bright winter stars spanning multiple constellations. Asterisms are useful and enduring precisely because they are easy to spot, making them popular tools for beginning stargazers to orient themselves in the night sky and star-hop toward fainter, harder-to-find objects nearby. Unlike constellations, which have official, permanently fixed boundaries covering the entire celestial sphere, asterisms are informal and culturally defined, meaning different traditions and cultures throughout history have recognized different patterns among the same stars, even when those patterns cross the modern, official constellation boundaries.