A constellation is a pattern of stars that people have linked together and named. For astronomers today, it is also an official region of the sky, like a country on a map, so that every star and galaxy belongs to exactly one constellation.
Stars in a constellation are not neighbors
The stars of a constellation only appear close together from Earth. They usually lie at very different distances. In Orion, for example, Betelgeuse is about 550 to 700 light-years away and Rigel about 860. From another star system, the patterns would look completely different. Over tens of thousands of years, the stars’ own motions will slowly distort the shapes too.
Where the names come from
- Many Northern Hemisphere constellations were first described in Mesopotamia and later adopted by the Greeks, who attached myths to them: Orion the hunter, Perseus, Andromeda, Cassiopeia.
- Ptolemy listed 48 constellations in the 2nd century CE, see the history of astronomy.
- European navigators and astronomers of the 16th to 18th centuries added southern constellations, often named after instruments or animals: Telescopium, Microscopium, Pavo the peacock.
- Many other cultures have their own sky patterns, such as the Chinese lunar mansions or the Aboriginal Australian “Emu in the Sky”, formed by dark dust lanes in the Milky Way.
The official 88
In 1922, the International Astronomical Union adopted a list of 88 constellations, and in 1930 it published precise boundaries. Some are huge, like Hydra; others, like Crux (the Southern Cross), are tiny.
Asterisms
Familiar patterns are not always official constellations. The Big Dipper (the Plough in the UK) is an asterism within Ursa Major. The Summer Triangle joins bright stars from three constellations. The Big Dipper is also the classic guide to the North Star, see the significance of the North Star.
The zodiac
The zodiac constellations lie along the ecliptic, the Sun’s apparent path through the year. Astronomically, the Sun also crosses Ophiuchus, and because of Earth’s slow wobble (precession), the dates no longer match those used in astrology. Astrology is not a science; constellations in astronomy are simply sky maps.
Learning the sky
Start with a few bright patterns in the current season (Orion in winter, Scorpius in summer for mid-northern observers), use a free planetarium app or a planisphere, and expand from there. Meteor showers are named after the constellation they appear to come from, see meteor showers.
