What is a Constellation?

A constellation is more than just an image drawn between a few stars.  It is actually a specific area of the sky with boundaries much like the borders of states or countries on a map of the world.  (In the illustration of the constellation Cassiopeia below the constellation boundaries are in blue.)  These boundaries are defined using celestial coordinates and have been standardized worldwide since 1930 by the International Astronomical Union.  Every star in the night sky has an address within the boundaries of just one constellation.  Within the boundaries of a constellation there may be a few dozen to several dozen objects visible to the naked eye, but in reality there are are many thousands, millions, even billions of objects within the boundaries (although most can be seen only with powerful instruments).

Cassiopeia Boundaries

The easiest way to locate constellations is by finding recognizable patterns in the stars within their boundaries.  Such a pattern is called an asterism. (The figures in yellow in the illustration are asterisms.)  While the constellation boundaries are standardized, the asterisms and their mythologies vary between cultures and over time.  (To learn about the various mythology of the constellations click here.)

Because of the large number of stars that can be found within the boundaries of a constellation it would be impractical to assign each a proper name. There are approximately 6,000 stars visible to the naked eye but only a few of the very brightest have a proper name. Instead, in 1603 Johann Bayer devised a system to label the stars within a constellation using a Greek letter followed by the name of the constellation in which they are found.  Initially Bayer intended that letters be assigned in order of brightness but there are many exceptions to that convention.  The use of Greek alphabet letters to designate individual stars, however, has endured and these assignments are fairly well standardized.

Using this system and the stars and asterisms within defined constellations it is possible to locate a great many deep sky objects.  For example, when explaining where to find the Crab Nebula we would direct the observer to look approximately 1° NW of Zeta Tauri, the Zeta star in the constellation Taurus.

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