Introduction

A star is a ball of gas held in static or quasi-static equilibrium by the balance between gravity and a pressure gradient. The pressure can in general be supplied by one or more of a hot perfect ionized gas, radiation and a degenerate electron (or neutron) gas, depending on circumstances. For main-sequence stars like the Sun, nuclear reactions maintain stability over long periods and the pressure is predominantly that of a classical gas at water-like densities. For much larger masses (above about 100 M⊙), radiation pressure leads to instabilities, while for much smaller masses (below about 0.08 M⊙) the central temperature never becomes high enough to ignite hydrogen and the star slowly contracts releasing gravitational energy until halted by degeneracy pressure at densities ∼103 gm cm−3; such stars are called ‘brown dwarfs’. Below about 10−3M⊙ (the mass of Jupiter), ordinary solid-state forces take over from electron degeneracy pressure in supporting the body against gravity at water-like densities, giving a planet rather than a star. The formation of stars is a complicated process, many aspects of which are still poorly understood although it is observed to happen in dense, dusty molecular clouds. A basic concept is the Jeans instability in a uniform medium: gravitational collapse occurs on length scales λ ≥ λJ (the ‘Jeans length’) such that the propagation time for pressure waves λ/cs exceeds the free-fall time (Gρ)−½, where cs is the sound speed and ρ the density.