A constellation is a pattern of stars which appear to form the outline of an image or picture in the sky. The best-known of these constellations are relatively easily located, have highly recognizable shapes, and have well-known names, like the Big Dipper (and the Little Dipper), Orion the Hunter, and Cassiopeia. It is important to remember that the only thing the stars in a constellation necessarily have in common is that they look like they are related from the perspective of an observer on Earth; in reality, these stars may be vast distances apart, and often are.
- Major Constellations -
At this point, the sky has generally been divided up so that a large portion of the major stars are part of one constellation or another, which create a quiltwork of recognizable patterns across the entire field of the night sky to the trained stargazer. In the past, this was especially useful because it allowed early astronomers to track stars and other celestial objects and identify them to each other in a jargon that could be quickly understood and translated. Today, stargazers searching for constellations are usually amateurs simply enjoying the clear night sky: professional astronomers have advanced, computer-operated telescopes to do much more sophisticated research work.
In the northern hemisphere, most of the constellations still bear the names first assigned to them by the ancient Greek and Roman astrologers, which were grouped into the zodiac and assigned based on rough animal shapes: the Ram, the Bull, the Twins (Gemini), the Crab (Cancer), the Lion (Leo), the Virgin (Virgo), Libra, Scorpio, the Archer (Sagittarius), the Fish (Pisces), and so on. Other Greek constellations existed outside the zodiac. Orion the Hunter, for example, stretches roughly from the Earth's equator on a line straight outward; therefore, it is unusual in that it can be viewed from both hemispheres.
Southern hemisphere constellations differ from northern ones. (In addition, the dust layer of the Milky Way galaxy can be observed more easily from the southern hemisphere, allowing stargazers in some conditions to detect cloud shapes, or dark cloud constellations, within the interstellar medium of the galaxy itself.) The best-known of the southern constellations is the Southern Cross, which plays a role in navigation analogous to that of Polaris, the North Star, in the northern hemisphere.
- Constellations Move -
In general, constellations simply appear as they are from Earth; the stars usually have very little in common, and may even be hundreds of light-years apart in terms of real space. In W-shaped Cassiopeia, for instance, Eta Cassiopeiae (also known as Achird) is just 19 light-years away from us, very close in astronomical terms, but V509 Casseiopeiae is a massive hypergiant star 7800 light-years away. One of the stars in the constellation, Cassopeia A, actually is no longer a star at all, but a remnant left over from the massive supernova of Tycho, detected in the 16th century. (Because Tycho is 11,000 light years away, the supernova itself actually occurred around the time that our distant ancestors were first inventing agriculture.)
Because stars in constellations can occupy such disparate positions in space, over long periods of time constellations can change, move, and even gradually move apart. For this reason many constellation designations previously used in the past have been abandoned, and new ones created in their place.