Our own single star may be atypical, which leaves us with a series of questions. What determines whether a single star or one with companions forms? What determines the distribution into systems of two, three, or more stars? What determines whether a companion to a star is a planet, a brown dwarf, or another star? There may be several processes at work to form binary and multiple systems — fission, capture, and sub-fragmentation.
Two supermassive stars orbiting one another in the open cluster Pismis 24. Click here for original source URL
Astronomers speculate that fission may create binary systems. Fission is a process in which a rapidly spinning proto star can split in two, leaving a very close pair or a contact binary. As evidence in favor of fission, close binaries show a preference for roughly equal masses, while widely separated binaries have the mass ratios expected from random pairings of stars. On the other hand, there are mechanisms that might cause widely spaced pairs to evolve into contact binaries, and once they are in close proximity stars might equalize their mass by the transfer of material. Most astronomers think that fission is not a plausible cause for most binaries.
Another possible explanation for binary systems is the capture theory. According to capture theories, binaries can form when one star enters the sphere of gravitational influence of another star. Unfortunately, the chance of encounters among random stars in the sky is far too slight to explain the observed numbers of binaries. Furthermore, randomly paired stars would be of widely different ages, but this is not observed among binaries. Where could stars of similar ages interact in a closely packed group? A newly formed star cluster is a very suitable environment — as many as half of the protostars in a cluster undergo collisions or close encounters. Many wide pairs probably formed in this way. Once two stars are bound together in an orbit, their combined gravity may attract further stars. Capture theories can probably explain multiple systems of three or more stars.
In a third category of theories, a contracting cloud shrinks because of its own gravity, but instead of forming a flat disk with a central star, its mass distribution or angular momentum distribution may make it split into two or more clouds that orbit each other. These smaller clouds then collapse into separate stars. Since the smaller clouds were part of a large cloud that was gravitationally bound, the stars that form out of the smaller clouds are bound to each other also.
Adding to the puzzle of how binaries and multiples form is the question of how likely it is that they will harbor planets. While we can't fully answer this question, stars have been found in numerous binary and multiple star systems, indicating that planet formation is possible. The planets of our solar system formed by a condensation of dust and ice particles followed by accumulation during their collisions; the surrounding gas medium eventually blew away. We can think of planets as debris left over from star formation. There is no reason that this debris should not exist around binary or multiple stars, allowing star formation to occur in the same way. As multiple star and multiple planet systems continue to be discovered, they will place limits how planets form in these systems.
Each of the three theoretical processes may produce binaries of a certain type. A complication in sorting out binaries of different types is that orbits of binary and multiple stars evolve through gravitational influences. Mass transfer in close pairs can alter orbits. Widely spaced pairs formed inside larger star clusters can evolve into closely spaced pairs as the clusters break apart. In one theoretical study of some 800 imaginary triple-star systems, about 97% were found to be gravitationally unstable, eventually kicking out one star and becoming binary systems. Thus the observed statistics of binaries and multiples may not reflect their original characteristics.