A constellation is a group of stars viewed from the Earth. Within each group, imaginary lines to produce patterns can connect the stars. Ancient astronomers saw in these pattern pictures of the animals and heroes of their myths that had been placed in the sky by the gods. More than half the constellations have names that came from the Greeks and Romans, who probably got them from the Babylonians.
There are 88 constellations recognized by astronomers today. The Greek astronomer Ptolemy, who lived in the 2nd century AD, named 48 constellations in a book called the Almagest, and 40 more have been added since. The majority of Ptolemy's names have been retained. European astronomers gave most of the others to the constellations of the southern sky in the 17th and 18th centuries.
Although all the stars in the same constellation look closer together in the sky, they are not necessarily close in space because some may be much further away than others. The constellations exist purely as an accidental result of the way stars appear to us on the Earth.
From one place on the Earth different constellations are seen at different times of the year. This happens because, as well as turning on its axis, the Earth is always moving round the Sun, making one circle each year. A constellation, which is visible during one part of the year, may appear close to the Sun six months later; it would not then appear in the night sky. Some constellations can only be seen from the Northern Hemisphere and some only from the Southern Hemisphere. Thus the constellation Ursa Major cannot be seen from the Southern Hemisphere, while the Southern Cross cannot be seen from the Northern Hemisphere.
The path of the Sun among the stars is called the ecliptic. The twelve constellations, which lie along the ecliptic, form the Zodiac. The other constellations are divided into those north of the Zodiac and those south of it.
Every one of the constellations has some special stars in it. Here we shall mention only some of the most famous constellations and stars. Many of the names are taken from the Greek myths. One of the best known of all constellations is Ursa Major, the Great Bear. Its seven brightest start form a figure which has come to have several names, such as the Plow, the Big Dipper, Charles' Wain, David's Chariot, and the Bier of Lazarus. The two stars furthest from the handle of the Plow are called the "Pointers", because they point at Polaris.
The constellation Orion is usually drawn as a hunter, whose belt is marked out by three bright stars. The bright star above the belt, when viewed from the Northern Hemisphere is Betelgeuse (astronomical name Orionis), a yellowish-red star. The bright white star Rigel (Orionis) is below the belt. The positions of Betelgeuse and Rigel are reversed when Orion is viewed from the Southern Hemisphere.
The hunter has two dogs: the greater, Canis Major, and the lesser, Canis Minor. The most important star in Canis Major is Sirius, the Dog Star (Canis Majoris), the brightest of all the stars we can see, and one of the nearest stars to the Sun.
Also near to Orion is Gemini, the Twins, one of the constellations of the Zodiac. The two brightest stars in Gemini are the mythical twins Castor (Geminorum) and Pollux (Geminnorum).
Another constellation of the Zodiac is Leo, the Lion, supposed to be the lion killed by Hercules. It contains the bright star Regulus (Leonis). Another of Hercules' victims is Draco, the Dragon, near Polaris, supposed to be the dragon guarding the golden apples in the garden of the Hesperides. Near by is the plowman Bootes, with the bright orange-red star Arcturus (Bootes).
The scorpion, which killed Orion, appears as the constellation Scorpio. At its heart is the reddish star Antares (Scorpii), one of the largest stars known. It is many hundred times bigger than the Sun.
The brightest star lying closest to the North Pole marked by Polaris is Vega (Lyrae), in the constellation Lyra, the Harp. It has a very faint star next to it in the sky, but the two stars are not physically close to each other and only appear so from Earth.
Perseus, his wife Andromeda, whom he saved from the sea-monster, and her parents Cepheus and Cassiopeia, are all constellations. Cassiopeia, on the other side of Polaris from the Great Bear, is easy to recognize. It has five bright stars forming a large W. In Perseus is the bright Algol (Persei). Algol means "demon" and the star was probably given this name by ancient peoples who discovered that it is not always of the same brightness. At regular intervals, slightly less than three days apart, it becomes fainter for a while, and then recovers its brightness. This happens because it is really two stars going round each other, one of which is much fainter than the other; when faint star moves between the bright one and the Earth it causes a reduction in brightness.
The brightest stars of the famous constellation Crux, in the southern sky, appear on the flags of Australia and New Zealand.
© 1988 Encyclopaedia Britannica, Inc.