At the same time, gravity will attempt to contract the system even further, and will do so on a free-fall time

where G is the universal gravitational constant, is the gas density within the region, and is the gas number density for mean mass per particle (1=''u'' = 3.9 × 10^^-24 g is appropriate for molecular hydrogen with 20% helium by number). When the sound-crossing time is less than the free-fall time, pressure forces temporarily overcome