grid-based codes which must resort to nested or adaptive grids [@BurBod93; @Trueloveetal97; @Trueloveetal98]. With SPH, if the number of particles used is sufficient to resolve the minimum Jeans mass, a calculation can be followed to arbitrary densities with the required spatial resolution given automatically with increasing density.

Recently, this ability of SPH was used to perform the first three-dimensional calculations ever to follow the collapse of a molecular cloud core to stellar densities [@Bate98]. The calculations followed the collapse of an initially uniform-density molecular cloud core of mass $M=1 {\rm \ M}_\odot$ and radius $R=7 \times 10^{16} {\rm \ cm}$.

The minimum resolvable mass in the SPH code was $\approx 2 N_{\rm neigh} = 100$ particles. Thus, to enable the minimum Jeans mass during the calculation ($\approx 4 \times 10^{-4}\ {\rm M}_{\odot}$) to be resolved, the calculation used $3 \times 10^5$ equal-mass particles.

The code did not include radiative transfer. Instead, to model the behaviour of the gas during the different phases of collapse, a piece-wise polytropic equation of state, $P=K \rho^{\gamma}$, was used, where $P$ is the pressure, $\rho$ is the density, $K$ gives the entropy of the gas, and the ratio of specific heats, $\gamma$, was varied as 