Constellation

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 Boštes) 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 Đ they are just along roughly the same lines of sight, and are
typically very far apart.

Star names

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.

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