Historically, constellations were groupings of stars that were thought to
outline the shape of something, usually with mythological significance. There
are 88 recognized constellations, with their names tracing as far back as
Mesopotamia, 5000 years ago.
The Historical Constellations
In some cases one can discern easily the purported shape; for
example, the constellation Leo shown on the right might actually look like a
lion with the dots connected as they are. In other cases the supposed shape is
very much in the eye of the beholder, as the example of Canis Minor (The Little
Dog) shown on the left indicates. This certainly could be a little
dog, or a cow, or a submarine, or . . .
Star Groupings and Asterisms
Some of the more familiar "constellations" are technically not
constellations at all. For example, the grouping of stars known as the Big
Dipper is probably familiar to most, but it is not actually a constellation. The
Big Dipper is part of a larger grouping of stars called the Big Bear (Ursa
Major) that is a constellation.
A well-known grouping of stars like the Big Dipper that is not officially
recognized as a constellation is called an asterism.
Constellations Are Not Physical Groupings
The apparent groupings of stars into constellations that we see on the celestial sphere are not physical groupings. In most cases the stars in constellations and asterisms are each very different distances from us, and only appear to be grouped because they lie in approximately the same direction. This is illustrated in the following figure for the stars of the Big Dipper, where their physical distance from the Earth is drawn to scale (numbers beside each star give the distance from Earth in light years).
The relative distances to stars in the Big Dipper |
It is important to make this distinction because later we shall consider groupings that are physical groupings, such as star clusters and binary star systems.
The Constellations of the Zodiac
The zodiac is an imaginary band 18 degrees wide and centered on the ecliptic. The constellations that fall in the zodiac are called the 12 constellations of the zodiac. They were at one time thought to have great mystical and astrological significance. Astrology is bunk, but the constellations of the zodiac are still of importance because the planets, as well as the Sun and Moon, are all near or on the ecliptic at any given time; thus, they are always found within one of the zodiac constellations.Constellations in Modern Astronomy
In modern astronomy, the significance of constellations is no longer mythological, but practical: constellations define imaginary regions of the sky, just as the individual states each define an imaginary region of the United States. Thus, to say that a planet is in the constellation Leo is to partially locate the planet on the celestial sphere, just as saying that Knoxville is in Tennessee is to partially locate the city on the surface of the Earth. As for states, modern constellations have irregular boundaries that have been agreed upon for various reasons, perhaps not always completely logical.
Information supplied by: http://csep10.phys.utk.edu