If you look at one of the points of light in the night sky, chances are good it’s not a star—it’s likely two or more stars, so close together they look like one from far away. These binary, trinary, and more complex star systems formed together out of a single cloud of gas and dust.
This formation process is hard to observe because protostars have not yet begun nuclear fusion and are thus very dim. Furthermore, in many of these systems, the protostars are so close together that only an extremely sensitive instrument could resolve their separation. Fortunately, a very sensitive instrument now exists. A team of researchers turned to the Atacama Large Millimeter/submillimeter Array (ALMA) and imaging a multi-star system during its formation. The team’s observations shed light on the mechanisms that create these systems.
Researchers have proposed two main mechanisms by which multiple-star systems might form. Both may account for some of these systems, though it’s not known what percentage each mechanism is responsible for. The first mechanism is a fragmentation of a nebula’s gas over large scales, which happens due to turbulence in the gas. The second mechanism is the gas fragmenting at smaller scales. When this happens, the fragmentation occurs due to gravitational instabilities in the spinning disk that forms the star and any planets.