Despite the great diversity of cosmological objects and structures, most of them share the same common point:they initially come from a gravitational collapse. This gravitational contraction is initially due to an instability within a gas cloud:the Jeans instability.
The Jeans instability , also called gravitational instability , was first described in 1902 by British physicist Sir James Jeans in an article on the formation and instability of spherical nebulae. The physicist shows that a gas cloud or an area of it can become unstable and collapse in on itself when its internal pressure no longer counterbalances the gravitational effects who tend to contract it. This mechanism is extremely important because it explains the formation of a very large number of astrophysical objects.
For a sufficiently low mass (at fixed temperature, radius, and volume), the gas cloud remains stable. But as soon as this mass reaches and exceeds the critical mass determined by Jeans, a series of gravitational contraction processes begin and the cloud then begins to collapse; possible other physical processes can intervene to stop the contraction and therefore the collapse. This critical mass, called "Mass of Jeans » is a function of density and temperature. A massive, cold and small gas cloud is thus gravitationally very unstable.
Two different forces are present within the cloud and clash. On the one hand, the internal gas pressure resulting from the thermal movement of the atoms and molecules composing the cloud, tends to make the gas expand. On the other hand, the gravitation which is exerted on the cloud and which tends to make it contract on itself. Critical mass is reached when these two forces are in balance. This balance can also be formulated as a critical length (Jeans length). Thelength of Jeans designates the radius that a gas cloud must have to be gravitationally stable; below this radius it is stable, beyond that it begins to collapse. In a sparse environment, the instability of Jeans only occurs on large scales.
The Jeans instability helps to explain the mechanism of star formation from giant molecular clouds as well as their evolution. The phenomenon of gravitational collapse occurs for proto-stars, white dwarfs, neutron stars, black holes and for the formation of larger structures such as galaxies. The birth of a star is explained by the contraction and the initial collapse of a giant molecular cloud which, on reaching equilibrium, breaks up into several pieces. When these pieces exceed the mass of Jeans, they collapse to give a proto-star.
