The constellations are totally imaginary things that poets, farmers and astronomers have made up over the past 6,000 years (and probably even more!). The real purpose for the constellations is to help us tell which stars are which, nothing more. On a really dark night, you can see about 1000 to 1500 stars. Trying to tell which is which is hard. The constellations help by breaking up the sky into more managable bits. They are used as mnemonics, or memory aids. For example, if you spot three bright stars in a row in the winter evening, you might realize, "Oh! That's part of Orion!" Suddenly, the rest of the constellation falls into place and you can declare: "There's Betelgeuse in Orion's left shoulder and Rigel is his foot." And once you recognize Orion, you can remember that Orion's Hunting Dogs are always nearby. Then you might recognize the two bright stars in the upper and lower left of the photograph as Procyon in Canis Minor and Sirius in Canis Major, respectively. Astronomers off ...more

In modern astronomy, constellation refers to an area of the celestial sphere, defined by exact boundaries. The term "constellation" can also be used loosely to refer to just the more prominent visible stars that seem to form a pattern in that area.

Constellations are normally the product of human perception rather than astronomical realities. 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, there are some exceptions. The famous star pattern known as the Big Dipper is almost entirely created by stars that are genuinely close together in astronomical terms; they are known as the Ursa Major moving group.

The grouping of stars into constellations is essentially arbitrary, as different cultures have seen different patterns in the sky, although a few of the more obvious ones tend to recur frequently, e.g. ...more