The Formation of the First Stars

The first bound star-forming systems in the universe are predicted to form at redshifts of about 30 and to have masses of the order of 10^6 M_sun. Although their sizes and masses are similar to those of present star-forming regions, their temperatures are expected to be much higher because cooling is provided only by trace amounts of molecular hydrogen. Several recent simulations of the collapse and fragmentation of primordial clouds have converged on a thermal regime where the density is about 10^3-10^4 cm^{-3} and the temperature is about 300 K; under these conditions the Jeans mass is of the order of 10^3 M_sun, and all of the simulations show the formation of clumps with masses of this order. The temperatures in these clumps subsequently rise slowly as they collapse, so little if any further fragmentation is expected. As a result, the formation of predominantly massive or very massive stars is expected, and star formation with a normal present-day IMF seems very unlikely. The most massive early stars are expected to collapse to black holes, and these in turn are predicted to end up concentrated near the centers of present-day large galaxies. Such black holes may play a role in the origin of AGNs, and the heavy elements produced by somewhat less massive stars also formed at early times may play an important role in chemically enriching the inner parts of large galaxies and quasars.