The grouping of stars into constellations 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 Bo÷tes) for which the quadrantid meteors are named. Various other less official patterns have existed alongside the constellations called asterisms, such as the Big Dipper (known in the UK as The Plough) and the Little Dipper.
See also: list of constellations
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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.