What's the Difference Between a Constellation and an Asterism?

As with so many other examples, the word "constellation" is often used differently depending on whether it's being used colloquially or academically. In many situations the term is interchangeable with "asterism," but both have a specific meaning. Here's the difference!

Asterism

An asterism is a collection of stars in the sky that form some kind of recognisable pattern. There are no official asterisms (you can make your own up!), though many names are known across populations and through time. Some have given their names to actual constellations as well. One example of this is Orion, one of the more easily identified asterisms in northern skies. In the diagram below you can see the Orion asterism as a collection of stars joined together with lines to form a simple picture that many feel looks like a hunter. There are other stars (as well as other bits and bobs) scattered around and within the image, but they are not part of the asterism. Also within the image can be seen bits of the asterisms known as Taurus, Gemini, Canis Major, and others.

Orion, the Constellation and Orion, the Asterism.
IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg) / CC BY


Constellation

A constellation is a defined region of the sky which is often named after an asterism that it contains. An example in the image above is that of the constellation Orion, marked out with a white background against the surrounding grey. The constellation of Orion includes the stars of the asterism as well as everything else within the defined boundaries.
There are 88 official constellations which fit together like a jigsaw puzzle to form a complete map of the sky. You can download all 88 constellation charts from the IAU here.

Another example

A very well-known "constellation", the plough (or the big dipper) is actually an asterism of 7 stars within a larger asterism known as Ursa Major (The Great Bear), which is marked in the chart below. Can you find the plough as well?

Ursa Major, the Constellation and the Asterism.
IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg) / CC BY 





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