Stars and planets tend to be close to perfect spheres. Lagrange points and resonances often lead to simple geometrical patterns of orbiting bodies. (The orbits of most planets in our solar system are also close to perfect circles; see page 973.) Regular spirograph-like patterns can occur for example in planetary nebulas formed by solar mass exploding stars. Unexplained phenomena that could conceivably be at least in part artifacts include gamma ray bursts and ultra high-energy cosmic rays. The local positions of stars are generally assumed to be random. 88 constellations are usually named—quite a few presumably already identified by the Babylonians and Sumerians around 2000 BC.