The first stars

Primordial clouds are likely to be remarkably uniform over stellar mass-scales in the absence of a pre-existing generation of stars. Thermal instability is found to occur during the collapse of a primordial cloud when the H2 abundance is rising and the H2 optical depth is of order unity. The e-folding rate for fluctuation growth exceeds the free-fall collapse rate by an order of magnitude. Large density fluctuations of mass-scale |~0.1 M_| arise in any collapsing cloud with metallicity ≼ 10−3 of the solar value. Gravitational instability ensures that many of the clumps coagulate to form protostars of masses extending up to the initial Jeans mass when the fluctuations develop, namely |~100 M_|⁠. The primordial IMF should therefore have spanned the mass range from ∼ 0.1 to |~100 M_| but may have been dominated by the more massive stars.