Is the Big Dipper a Constellation or an Asterism? What's the Difference?
As a planetarium presenter, I'll occasionally overhear somebody say, "The only constellation I can find is the Big Dipper," as he or she walks into the planetarium theater. I've also heard people say that the Big Dipper is their favorite constellation.
The Big Dipper (also known as the Plough or the Drinking Gourd) is definitely a well-known group of stars. The stars that make it up are relatively bright, and their brightness combined with a recognizable pattern makes the Big Dipper easy to pick out in the sky, even from the city.
However, is the Big Dipper actually a constellation?
No, the Big Dipper is not a constellation. It is an asterism.
What's the difference between a constellation and an asterism?
Colloquially, a constellation is a recognizable pattern of stars in the sky (in fact, the word "constellation" means "set of stars"). An asterism is also a recognizable pattern of stars in the sky. So, how in the world are they different from each other?
In modern astronomical terms, the word "constellation" refers to a bit more than just the star pattern. To help locate objects in the sky, astronomers (and navigators) imagine what's called the celestial sphere. If you imagine a giant spherical screen around the earth with the stars, planets, the sun,the moon, and anything else you might find in the sky projected on it, you've visualized the celestial sphere. This celestial sphere is broken up into 88 regions (think of states or provinces on a map), and each of these regions is a constellation. The boundaries to these constellations were laid out by the International Astronomical Union in 1922 and ratified in 1928. Of course, people have recognized patterns in the stars for thousands of years, but the classical constellations did not cover the entire sky, and different constellations seen by different cultures could make things confusing.
The term "asterism" refers to any pattern of stars in the sky. Sometimes an asterism is made up of stars from different constellations, and sometimes it's part of a constellation. The Big Dipper is part of the constellation Ursa Major, or the Great Bear (sometimes called the Big Bear). It's made up of the seven brightest stars in the constellation, which represent the bear's hindquarters and tail.
Since most constellations don't look like what they are supposed to be, some asterisms are simply alternate names. The spring constellation Bootes is supposed to be a herdsman, but is sometimes referred to as "The Kite," or "The Ice Cream Cone" due to the pattern in the stars.
An example of an asterism made up of stars from different constellations is the Summer Triangle, which can be found high in the late night summer sky from the Northern Hemisphere. It contains Vega, from the constellation Lyra the Harp, Deneb from Cygnus the Swan, and Altair from Aquila the Eagle - all the brightest stars in their respective constellations. Due to the brightness of these stars and the simple shape they make, the Summer Triangle is easy to spot from either the city or the country.
Constellations are not forever
When thinking about constellations and asterisms, please keep in mind that the stars within them are moving, which causes the patterns to change over many thousands of years. Also, the stars that make up a specific constellation are usually not actually linked together in space. For example, in Cygnus the Swan, the tail star, Deneb, is 3,200 light years away, while the head star, Alberio, is only 380 light years away. An exception to this is Ursa Major, which contains all but one star in the Ursa Major Moving Group - a group of 14 stars of the same age and composition that are heading in the same direction at the same speed. The other star in this group is found in Canis Venatici, which is a faint constellation representing the hunting dogs of Bootes.