What’s the difference between a constellation and a galaxy?

Dear Cecil: What is the effing difference between a constellation and a galaxy? My understanding of the cosmos is at stake. G-Man, Montreal, Quebec, Canada

Cecil replies:

Get a grip, ace. A constellation is an apparent cluster of stars (that is, they look close together to us easily-deceived sods on earth), while a galaxy is a real cluster of stars, i.e., a bunch of stars that actually are (in astronomical terms) close together. The stars in many constellations are actually quite distant from one another, but we don’t realize it because when we look at the stars we can’t perceive depth. Never caused us to think less of John Wayne, so why should we hold it against the Big Dipper? I’ll tell you why: because the Big Dipper isn’t a constellation. It’s an asterism, which sounds like something you might want to dab with Preparation H but actually signifies a star cluster within a constellation, Ursa Major (the Great Bear) in the case of the Big Dipper. Ain’t this column educational?

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.