A constellation is any one of the 88 areas into which the sky – or the celestial sphere – is divided. The term is also often used less formally to denote a group of stars visibly related to each other in a particular configuration or pattern.
Some well-known constellations contain striking and familiar patterns of bright stars. Examples are Ursa Major (containing the Big Dipper), Orion (containing a figure of a hunter), Leo (containing bright stars outlining the form of a lion) and Scorpius (a scorpion). Other constellations do not encompass any discernible star patterns, and contain only faint stars.
The International Astronomical Union (IAU) divides the sky into 88 official constellations with precise boundaries, so that every direction or place in the sky belongs within one constellation. In the northern celestial hemisphere, these are mostly based upon the constellations of the ancient Greek tradition, passed down through the Middle Ages, and contains the signs of the zodiac.
The constellation boundaries were drawn up by Eugène Delporte in 1930, and he drew them along vertical and horizontal lines of right ascension and declination. However, he did so for the epoch B1875.0, which means that due to precession of the equinoxes, the borders on a modern star map (eg, for epoch J2000) are already somewhat skewed and no longer perfectly vertical or horizontal. This skew will increase over the years and centuries to come.
In three-dimensional space, most of the stars we see have little or no relation to one another, but can appear to be grouped on the celestial sphere of the night sky. Humans excel at finding patterns and throughout history have grouped stars that appear close to one another into patterns.
A star pattern may be widely known but may not be recognized by the International Astronomical Union; such a pattern of stars is called an asterism. An example is the grouping called the Big Dipper (North America) or the Plough (UK).
The stars in a constellation or asterism rarely have any astrophysical relationship to each other; they just happen to appear close together in the sky as viewed from Earth and typically lie many light years apart in space. However, one exception to this is the Ursa Major moving group.
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, for example Orion and Scorpius.