What are constellations?

StarChild Question of the Month for May 1999

Question:

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Answer:

There is a word for a pattern of stars in the sky which appears to be so distinctive that it is easily identifiable and remembered. That word is "asterism". In ancient times, people saw asterisms and made up all kinds of stories about mythological creatures and characters which they associated with the star patterns. As astronomers subsequently began to make maps of the stars, the named asterisms were included in the maps and called constellations.

As time passed, the sky became filled with constellations, many of which included the same stars. The International Astronomical Union stepped in to gain control and make sense of things in 1925. They adopted 88 official constellations and assigned areas of the sky to specific constellation names. It should be noted, however, that they made no requirement that the constellation be easily seen nor that the legends associated with the constellation make any sense. In fact, most constellations don't really resemble the creatures or characters they are named after. So don't worry if you can't make out the shapes!

Constellations can be a useful way to help identify positions of stars in the sky. Constellations have imaginary boundaries formed by "connecting the dots" and all the stars within those boundaries are labeled with the name of that constellation. However, keep in mind that constellations are not real objects; they are just patterns as seen from our observation point on Earth. The patterns we see are for the most part just by chance. The individual stars in a constellation may appear to be very close to each other, but in fact they can be separated by huge distances in space and have no real connection to each other at all. For example, look at the image below of the stars which make up the constellation Orion. The stars in this easily-observed constellation are at VERY different distances from Earth!

Distance cartoon illustrating how many light-years it is to Orion.

At different times of year, different constellations can be seen in the sky. Different constellations can also be seen depending on where you are on Earth. Here are some places to look to find what is visible each month:

Sky & Telescope's Interactive Sky Chart

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StarChild Authors: The StarChild Team
StarChild Graphics & Music: Acknowledgments
StarChild Project Leader: Dr. Laura A. Whitlock
Curator: J.D. Myers
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