Background image: The Horsehead nebula, a stellar nursery. Jeans was attempting to explain how the instability of such large clouds could give rise to stars.
The instability or “Jeans” problem, named for James Jeans was first noted by Poincaré in the context of Newtonian gravity in 1902. Jeans showed that for any gas a size could be found such that no cloud of greater size would be stable, and so the cloud would collapse under any perturbation, no matter how small. In cosmology this meant that a static Newtonian cosmology was not possible, for an infinite space would always be unstable under perturbation. Newton, in correspondence with Bishop Bentley in the 1690s, had anticipated this result. Einsteinian cosmologies are likewise unstable, a problem overlooked by Einstein when he introduced the cosmological constant in order to obtain a static solution to his field equations.
The instability found by Jeans was originally considered as a method for the production of stars and galaxies from a primordial fluid, however the analysis bears much similarity to that of Chandrasekhar and Oppenheimer in considering the formation of neutron stars and black holes.