Multiple Star Systems

As everyone knows, moons orbit around planets, and planets orbit around stars. But what if, in a developing solar system two clumps of matter form instead of one (information on formation of planetary systems shown here)? When two or more stars begin Hydrogen fusion close enough to each other to have a gravitational link, a multiple star system is born. Being near the Sun, most humans would think that binary or larger star systems are a rare occurrence. But in reality, most stars have a partner. Star systems are separated into two main categories, optical star systems, and physical star systems. An optical star system is present when two stars are right next to each other in the sky, but in distance, they may as well be thousands of light-years apart.

Physical star systems are when stars are actually orbiting one another. However, a star system is only truly binary if the center of gravity lies between the two stars. An example of a binary system (but not star system), is the relation between Pluto and Charon. Since Charon is about half the mass of Pluto, they both orbit around a point just outside Pluto's surface. Otherwise, in these systems are planet-moon systems. But sometimes three or more stars orbit each other. Most large star systems consist of a close binary relationship and a lighter single or binary pair orbiting farther out. The record for physical star systems is six stars. Three pairs of binaries make up this system. Sometimes, one star turns into a black hole (here, here, here and here) and the other loses gas to it, resulting in a nova.