Asterisms

An asterism is a recognizable pattern of stars in the night sky that is not itself a formal constellation. It is a visual grouping formed by stars that can lie across the boundaries of one or more constellations, and it is not an officially defined area of the sky.

Asterisms differ from constellations in that they are not defined by the boundaries of the IAU’s constellations. They arise from observers’ perspectives and cultural traditions, and the same bright stars can participate in different asterisms or be part of different constellations depending on context. Because they are based on pattern recognition, asterisms can vary in different sky charts and across cultures.

Common examples include:

- The Big Dipper, a seven-star asterism in the constellation Ursa Major.

- The Summer Triangle, formed by Deneb, Vega, and Altair across the constellations Cygnus, Lyra, and Aquila.

- Orion’s Belt, three bright stars in Orion aligned in a straight line.

- The Great Square of Pegasus, a large square formed by four bright stars.

- Cassiopeia’s W, a familiar W-shaped pattern within the constellation Cassiopeia.

- The Pointers, the two bright stars Dubhe and Merak in Ursa Major that point toward Polaris.

- The Teapot asterism in Sagittarius, used as a guide to the Milky Way region.

Asterisms are useful in teaching and navigation, aiding observers in locating objects and learning the sky. The term derives from Greek and has been used in astronomy to describe these famous, non-official star patterns.