A constellation is a group of stars visibly related to each other in a particular configuration. In three-dimensional space, most of the stars we see have little relation to one another, but can appear to be grouped on the imaginary plane of the night sky. Humans excel at finding patterns and throughout history have grouped stars that appear close to one another into constellations. An "unofficial" constellation is also called an asterism.
The grouping of stars into contellations is essentially arbitrary, and different cultures have had different constellations, although a few of the more obvious ones tend to recur frequently, e.g Orion and Scorpius.
The International Astronomical Union (IAU) divides the sky into 88 official constellations with precise boundaries, so that every direction belongs to exactly one constellation. These are mostly based upon the constellations of the ancient Greek tradition, passed down through the Middle Ages.
The first 12 are the zodiac. In addition to these 12, Ptolemy listed another 36 constellations (which now count as 38, due to the break-up of Argo Navis). In more recent times this list has been added to, first to fill gaps between Ptolemy's patterns (the Greeks considered the sky as including both constellations and dim spaces between) and second to fill up the southern sky as European explorers journeyed where they could see it.
Other proposed constellations didn't make the cut, most notably Quadrans Muralis (now part of Boötes) for which the quadrantid meteors are named. Various other less official patterns have existed alongside the constellations called asterisms, such as the Big Dipper and Little Dipper. Note that in any of these figures the stars rarely have anything to do with one another – they are just along roughly the same lines of sight, and are typically very far apart.
See also: list of constellations
Many stars are named using the genitive of the constellation in which they are found. These names include Bayer designations such as Alpha Centauri, Flamsteed designations such as 61 Cygni, and variable star designations such as RR Lyrae. For more information about star names, see Star designations.
See also: List of constellations by area and Chinese constellations.
A WikiProject aiming to provide a standardized template to all the articles of the constellations is being developed. Please visit WikiProject Constellations for more details.