Constellations and Asterisms
Constellations are groups of stars which have been designated as representing a particular figure in the sky. Different cultures have done this in different ways, and many ancient and modern constellations are no longer in use. Eighty-eight of the traditional and modern Western constellations (based on Babylonian and Greek constellations of two to five thousand years ago) have been designated as "official" constellations by the IAU (the International Astronomical Union).
Constellations can be represented by stick figures, some of which are traditional, and others which are created by the illustrator of one book or another. The latter are often copyrighted (e.g., those used in The Stars
by H.A.Rey). In the past, constellations were usually depicted by artistic drawings based on allegorical figures, and different books typically had different drawings, which didn't always include exactly the same stars. In 1930 the IAU defined constellation boundaries which enclose the regions traditionally occupied by allegorical figures, so that every part of the sky is inside one constellation or another. Stars which are inside the boundary of a constellation are usually said to be "in" that constellation, meaning that they are in a direction enclosed by that boundary.
Asterisms are groups of stars such as the Little Dipper, Big Dipper and the Pleiades, which are not constellations but have well-known names of their own. The Big and Little Dippers are part of the Big and Little Bears (Ursa Major
and Ursa Minor
). The Pleiades are an example of a cluster
of stars which happens to be visible to the eye in the constellation of Taurus
(the Hyades is another asterism, also in Taurus, which is also a cluster).
Two of the stars in the bowl of the Big Dipper have nearly the same right ascension (an east-west position similar to longitude on the Earth); as a result, a line through them, being nearly north-south, points toward Polaris, the Pole Star. As a result, Dubhe and Merak are often referred to as the Pointers. (Jerry Lodriguess, Catching the Light, apod070108)
Allegorical Figures (immediately below) /
Even though a well-known group of stars, the Pleiades is not a constellation, but simply part of the constellation of Taurus
. Many people confuse the Pleiades with one of the Dippers (which are also asterisms). The misty glow around the stars is light reflected from clouds of dust that the cluster happens to be passing through, and has nothing to do with the dust which surrounded them during their formation. (ESA, AURA/Caltech, NASA)
Ways Of Showing Constellations
Until fairly recently maps in celestial atlases showed allegorical figures which more or less encompassed the region and stars traditionally associated with the constellations in that region. Examples of these are shown on most of the constellation pages in the Celestial Atlas
that I am working on, and a few samples are shown immediately below: