cloud fragmentation

A single giant molecular cloud holds enough gas for thousands of stars, yet it does not collapse into one monstrous super-star. Instead it shatters into many smaller pieces, each of which makes a star or two. That breaking-up of one big collapsing cloud into many star-sized pieces is cloud fragmentation, and it is why stars are born in clusters rather than one at a time.

Fragmentation follows from the way the Jeans mass falls as a cloud collapses. When a huge cloud begins to fall in, its initial Jeans mass is large, so the whole thing starts to collapse together. But as it gets denser while staying cold (the dust keeps radiating heat away), its Jeans mass shrinks, and now sub-regions within the cloud individually exceed their own, smaller Jeans mass and start to collapse on their own. Each of those pieces, growing denser still, can fragment again. The cloud thus breaks up in a hierarchy, like a dividing cell, ending in a swarm of cores of roughly a stellar mass each. Fragmentation stops when the gas can no longer cool efficiently, so its temperature rises, its Jeans mass stops falling, and the smallest pieces, around a tenth of a solar mass, set the low end of stellar masses.

Fragmentation explains one of the most basic facts about stars: they almost always come in groups, from loose associations to dense clusters, and a large share of them in binary or multiple systems. It is also the bridge from the smooth idea of the Jeans mass to the real, measured spread of stellar masses, the initial mass function. Exactly how cleanly clouds fragment, and how much turbulence and magnetic fields steer the process, remains an active area of computer simulation and debate.