As described in the previous chapter, theory predicts that the protostellar envelope should collapse at near free-fall velocities to form the stellar core and disk. It is not easy to detect this collapse directly; on large scales, the infall velocities are small and difficult to isolate from the complex supersonic motions in the surrounding cloud, while freeze-out or other chemical effects remove some of the standard tracers in the inner envelope. In addition, the presence of high-velocity bipolar outflows cause further confusion. Nevertheless, an increasing body of evidence generally supports the rapid collapse model of protostar formation.

Infrared imaging and spectroscopy have provided the most broadly based indications of protostellar collapse. While dust emission does not directly measure infall motion, the presence of dust in the near environs of very young stars, as shown either directly in scattered light or inferred through detection of warm dust emission, demands a dynamical explanation. As the material is too cold to be thermally supported, it must be either falling in or flying out (the envelopes are mostly not in flattened disks, though somewhat flattened “toroids” are observed). The required infall rates are plausible from the collapse theories discussed in the previous chapter, while wind mass loss rates would have to be implausibly large (because the same density implies a larger mass flux for a larger velocity) – and, in any event, outflows have distinctive bipolar geometries, not toroidal expansion.