Star Groupings and Asterisms

Constellations are imaginary groupings of stars on the celestial sphere, but some of the more familiar "constellations" are technically not constellations at all. For example, the grouping of stars shown in the adjacent figure that is known as the Big Dipper is probably familiar to most, but it is not actually a constellation. The Big Dipper is part of a larger grouping of stars called the Big Bear (Ursa Major) that is a constellation. A well-known grouping of stars like the Big Dipper that is not officially recognized as a constellation is called an asterism. In the adjacent figure the Big Dipper is an asterism contained in the constellation Ursa Major.

In most cases the stars in constellations and asterisms are each very different distances from us. They only appear to be grouped because they lie in approximately the same direction. This is illustrated in the following figure for the stars of the Big Dipper, where their physical distance from the Earth is drawn to scale (but the size of Earth and of the stars is greatly exaggerated). The actual position of the stars is shown to the left, and the projection that we see on the celestial sphere is shown to the right. The numbers give the distance of each star from Earth in light years, which is the distance that light can travel in a single year and is a common measure of distance to stars.