Constellation

Modern astronomy, a constellation is an internationally defined area of the celestial sphere. Historically, the term was also used to refer to a perceived pattern formed by prominent stars within apparent proximity to one another, and this practice is still common today.

colloquial usage, a constellation is a group of celestial bodies, usually stars, which appear to form a pattern or picture in the sky. Astronomers today still utilize the term, though the current system focuses primarily on constellations as grid-like segments of the celestial sphere rather than as patterns. A star-pattern that is not officially classed as a constellation is referred to as an asterism. One famous example is the asterism known as the Big Dipper, a term unused by the International Astronomical Union (IAU) as the stars are considered part of the larger constellation of Ursa Major.

stars within a constellation rarely have any substantial astrophysical relationship to each other, and their apparent proximity when viewed from Earth disguises the fact that they are far apart, some being much farther from Earth than others. However, there are some exceptions: many of the stars in the constellation of Ursa Major (including most of the Big Dipper) are approximate to one another, a phenomenon known as the Ursa Major moving group.