  We observe a sharp transition from a singular, high-mass mode of star
formation, to a low-mass dominated mode, in numerical simulations, at a
metallicity of 10^-3 Zsolar. We incorporate a new method for including the
radiative cooling from metals into adaptive mesh-refinement hydrodynamic
simulations. Our results illustrate how metals, produced by the first stars,
led to a transition from the high-mass star formation mode of Pop III stars, to
the low-mass mode that dominates today. We ran hydrodynamic simulations with
cosmological initial conditions in the standard LambdaCDM model, with
metallicities, from zero to 10^-2 Zsolar, beginnning at redshift, z = 99. The
simulations were run until a dense core forms at the center of a 5 x 10^5
Msolar dark matter halo, at z ~ 18. Analysis of the central 1 Msolar core
reveals that the two simulations with the lowest metallicities, Z = 0 and 10^-4
Zsolar, contain one clump with 99% of the mass, while the two with
metallicities, Z = 10^-3 and 10^-2 Zsolar, each contain two clumps that share
most of the mass. The Z = 10^-3 Zsolar simulation also produced two low-mass
proto-stellar objects with masses between 10^-2 and 10^-1 Msolar. Gas with Z >=
10^-3 Zsolar is able to cool to the temperature of the CMB, which sets a lower
limit to the minimum fragmentation mass. This suggests that the second
generation stars produced a spectrum of lower mass stars, but were still more
massive on average than stars formed in the local universe.
