What Are Asterisms?

Star Shapes, Science, and Mythology

Informal patterns of stars, or asterisms, can be easy to spot in the night sky. Some of them are smaller, well-known formations like Orion’s Belt, while others can be a large grouping of stars that are part of one or more constellations.

Sometimes, these star patterns stand out clearly in the night sky. The Big Dipper (also known as the Plough in the UK and Ireland) is one of the brightest asterisms, made up of seven stars in the larger constellation of Ursa Major.

See which asterisms are visible in your night sky

The Orion’s Belt formation is drawn from Greek mythology, with the three brightly-shining stars of Alnitak, Alnilam, and Mintaka easy to identify on a clear, dark night. Nearby, Orion’s Sword is also familiar to stargazers, but many people don’t realize that the middle point of light in this asterism is not a star at all, just the visible glow of the Orion Nebula.

Five Famous Asterisms

Here are some well-known star formations you may be able to spot:

1. The Little Dipper: Located in Ursa Minor, recognizable as a ladle shape, featuring the North Star (Polaris) at the end of the handle.
2. The Summer Triangle: Three stars from three different constellations: Vega (Lyra), Deneb (Cygnus), and Altair (Aquila), create a triangle visible in the Northern Hemisphere’s summer sky.
3. Orion’s Belt: The three bright stars of Alnitak, Alnilam, and Mintaka align to create the “belt” in Orion.
4. The Teapot: Located in Sagittarius, this kettle shape is created by eight stars.
5. The Great Square of Pegasus: A large square shape visible in the autumn sky found in the constellation of Pegasus, with the stars of Markab, Scheat, Algenib, and Alpheratz.

Useful for Creating Coordinates

Asterisms provide important celestial reference points for researchers.

But beyond being very useful for professional astronomers, these formations can help amateur stargazers find their way around the sky and sometimes point toward fascinating information. For example, the Big Dipper asterism can help you tell time without a watch.

A Question of Perspective

Asterisms only exist because our imagination does the work of connecting star locations, and are dependent on our view the sky. But appearances can be deceiving. Ancient cultures often imgagined that stars that seemed to be close to one another were connected and near neighbors in the cosmos.

The reality of distance in space can be quite different. The constellation of Triangulum, for example, contains stars that appear quite close to one another to the naked eye when seen from Earth, but some of these bodies are actually separated by almost 100 light years, or around 946 trillion kilometers.