An arbitrary grouping of stars which form a pattern. The 12 zodiacal constellations, Orion, the Big Dipper, and the Pleiades are familiar to most people. The ancients named 48 constellations, mostly for objects or heroes of mythology. Now the sky is divided into 88 constellations. These vary in size and shape from Hydra, the sea monster, which is the largest at 1,303 square degrees, to Crux, the cross, which is the smallest at 68 square degrees.