Let’s start with an easy target. Look in the northwestern sky for the seven stars of the Big Dipper. Although it’s one of the most recognizable star patterns, the Big Dipper is not a constellation. That is, astronomers don’t recognize it as one of the official 88 constellations that cover the sky.
Rather, the Big Dipper is an asterism — a widely known but unofficial group of stars. Asterisms can sit in a single constellation, like the Dipper, or they can contain stars from several constellations. One example of the latter is the Summer Triangle, which I’ll discuss later.