Ask someone in the Northern Hemisphere to name a constellation and there is a good chance that their response will be the Big Dipper, or the Plough, as it is also referred to.The Big Dipper is after all one of the easiest of constellations to identify in the night sky, but there is a problem, in reality the Big Dipper is not a constellation.
In the strictest sense of the word there are only eighty-eight constellations, identified as such by the International Astronomical Union. The Big Dipper is classed as an asterism, a group of easily identifiable stars, rather than a constellation.
No matter by what classification the Big Dipper is known, the grouping of stars is one that is visible to the naked eye every clear night in the Northern Hemisphere. Indeed the Big Dipper can often be seen even in the middle of a big town; the brightness of the stars overpowering any light pollution that masks less bright stars.
The Big Dipper is part of the grouping known as the Circumpolar stars, something which is explained later, but most of the stars within the Big Dipper are part of the Ursa Major, the Great Bear Constellation.
The Big Dipper is made up of seven visible stars, starting at the end of the ladle’s handle with –
Dubhe and Alkaid are not part of Ursa Major.
There are additional stars within the Big Dipper, most notably Alcor, but these are not visible to the naked eye. Alcor is so close to Mizar, that the brightness of the latter outshines the former.
The constellations are changing, and over time the stars move in different directions, they are after all many light years apart. That being said the Big Dipper has remained a recognisable symbol of the night sky for thousands of years, and it has been used as a guide to other constellations, and as directions during the night. To this end Dubhe and Alkaid can be used as pointer stars, pointing the way to the North Star.
As with many elements of the night sky, the stars of the Big Dipper have been given much focus in ancient mythology, in particular Greek mythology, although it is based on the story of Ursa Major. It is the story of Callisto, a lover of Zeus, who was transformed into a bear during one of Hera’s rages. Callisto though had already given birth to a son, Actas, who turned into a hunter. One day it seemed destined that Actas would kill the bear that was in truth his mother, but Zeus intervened to place both the mother and son, in the form of bears, into the night sky.
The Big Dipper is named after the ladle, although the name of the Plough is equally apt.
The Big Dipper is part of the Circumpolar stars because unlike most other stars, the stars within it do not move across the night sky, but instead seem to rotate around the North Star.