An asterism is a distinctive pattern of stars lying within a constellation — or, in some cases, one made up of stars from two or more adjoining constellations. Undoubtedly the best-known and most easily recognized of all such grouping is the Big Dipper in the constellation of Ursa Major.
Contrary to popular belief, the Big Dipper is not a constellation itself, but rather an “asterism” or distinctive figure within a constellation — in its case, Ursa Major, the Great Bear of the heavens. And as such, it’s surely the most famous asterism in the entire sky!
Perhaps you have one of those neat rotating star charts, like the Edmund Scientifics’ Star and Planet Locator (which has just been revised and printed on heavier stock than the original one). By turning the chart, you’ll see how the Big Dipper wheels around the north pole of the sky and that it’s at its highest in the heavens in spring — during the evening hours of April (and also May).
Most skywatchers know that the Big Dipper serves as a convenient pointer to the North Star, or Polaris, a line drawn through the two stars at the end of its bowl pointing northward to the celestial pole. But the Dipper can do far more than just that. Of the 88 officially-recognized constellations, 66 can be seen — all or in part — from mid-northern latitudes. And this asterism can be used to find every one of them with the help of that rotating star chart!
To illustrate this, if you follow the curve of its handle southward, you’ll come to the radiant golden-orange star, Arcturus, in the kite-shaped constellation Bootes. Continuing this arc brings you to the bright icy-blue star, Spica, in the Y-shaped constellation Virgo. The color difference between these two luminaries is obvious even at a casual glance. And it’s actually real, a result of differences in their temperature — Arcturus being a relatively cool sun, while Spica is a very hot one. Amazingly, without any optical equipment, the human eye just looking up on a clear night can tell something of the physical nature of stars far out in the depths of interstellar space!
As another example of the Dig Dipper’s use as a celestial pointer, let’s go back to those two stars at the end of its bowl. Instead of following them upward as before, extend a line through them downward and you will arrive at the constellation of Leo, the Lion, with its bluish-white luminary Regulus. The right-hand portion of Leo looks like a backwards question-mark and represent the Lion’s head, while the left-hand part consists of a triangle marking its hindquarters. In a similar way, you can use your rotating chart to find other stars of the Big Dipper to serve as pointers to many additional constellations during this and other seasons of the year.
Former assistant editor at Sky & Telescope and author of five books on stargazing.