On a cloudy night, if we see a star between clouds it is often difficult to identify, if that is the only star we can see. The speck of light from one star is not too different from any other star. But once the clouds clear the situation becomes very different. The stars appear to be arranged in patterns and have been know since antiquity and are called constellations.
Names were given to the constellations, mainly by the Greek astronomers, and many of these names are still in use today. The constellations were named after the people, animals and objects of Greek mythology which they thought they resembled. So we have the constellations of Aries the ram, Taurus the bull, Orion the hunter, Scorpius the scorpion and so on.
In 1603 and 1627 the German astronomer Johann Bayer published catalogues of the heavens in which he retained the old names of constellations given by the early Greeks but he listed stars in each constellation according to brightness with a letter of the Greek alphabet.
So, the brightest star in a constellation is alpha, next beta, gamma, delta, and so on. In Orion, Betelquese becomes Alpha Orionis, Rigel becomes Beta Orionis and Bellatrix becomes Gamma Orionis. This system is still used today. Indeed some bright southern stars which have been observed carefully only since Bayer's time are commonly only known by this system. Rigel Kent, the closest star to the Sun, is better known as Alpha Centauri and Hadar becomes Beta Centauri, the two brightest stars in the constellation of the Centaur.
It is important to remember that constellations are only patterns made up by observers on Earth, the patterns have no real significance and the individual stars within the same constellation are not necessarily close together, and frequently are not, as each constellation is like a wedge of space of infinite depth.
Alpha Centauri as mentioned is 4.3 light years away, whilst its companion in the sky, Beta Centauri is nearly 500 light years away. So the Sun is much more a companion of Alpha Centauri than Alpha is of Beta Centauri.
The well known stars of the Southern Cross that we have on our Australian flag are also at varying distances from us and each other. (All of these measurements have an error of at least 5%.)
- Alpha Crusis is 321 light years away from our sun
- Beta Crusis is 352 light years
- Gamma Crusis is 88 light years
- Delta Crusis is 364 light years, and
- Epsilon Crusis is 228 light years
So from almost any other vantage point in space outside our solar system our Southern Cross would not look like a cross at all. Even the constellations themselves will eventually change shape, as each star in a constellation has its own motion, often at quite high speeds. (It is only the enormous distance that they are from us that makes this motion imperceptible to us.)
Each star therefore has a speed and direction which is different from other stars in the constellation, eventually, over a very long period of time changing the shape that we observe here on Earth. This movement of the stars is called Proper Motion.
There are 88 constellations in the sky, this number and the boundaries of each constellation were fixed by the International Astronomical Union in 1928. The familiar constellations are the "official" constellations but many cultures around the world had their own way of dividing up the sky:
- The Chinese divided the celestial sphere into 28 lunar mansions which encircled 12 zodiacal animals, 4 seasonal constellations are seen - the White Tiger in the west, the Black Tortoise over the north, the Azure Dragon over the east and the Vermillion Bird over the south which encircled the Earth and they incorporated many faint stars into their constellations.
- The Australian Aborigines observed star patterns but also saw patterns in the dark areas of the sky - the Emu is a good example (the head is the Coal Sack region in Crux) follows a dark lane in the Milky Way.
- The Inca culture also saw patterns in the dark areas of the sky.