Formation of the first massive stars and the reionization history of the universe

In cosmological models favored by current observations, the first astrophysical objects formed in dark matter halos at redshifts starting at z greater than or similar to 20, and their properties were determined by primordial H, molecular chemistry. These protogalaxies were very abundant, but substantially less massive than typical galaxies in the local Universe. Massive metal-poor stars at their nuclei, together perhaps with massive black holes, reionized the bulk of hydrogen in the intergalactic medium. Reionization may have taken place over an extended redshift interval, ending around z similar to 7. Here we review theoretical expectations on how and when the first massive stars formed.