The expectation is therefore that all Population III.1 stars were massive, with masses typically of the order of $100 \: {\rm M_{\odot}}$ or more, thereby also explaining why none of these stars appear to have survived until the present day. If a Population III.1 star of this mass does form, then it will rapidly dissociate H$_{2}$ throughout the halo, thereby suppressing further star formation [@on99; @gb01]. This fact, together with the lack of fragmentation seen in the simulations, is often taken to imply that only a single Population III.1 star will form in each dark matter halo.

This widely accepted picture for Population III star formation also provides for a second mode of Pop. III star formation. This occurs in metal-free gas which has been ionized by radiation from a previous generation of Population III stars. Following the death of these stars, the gas recombines, and the elevated fractional ionization in the recombining gas allows more H$_{2}$ to form. The enhanced H$_{2}$ fraction enables the gas to cool to a lower temperature, which in turn increases the effectiveness of cooling by the singly-deuterated hydrogen molecule, HD. In the standard Pop. III.1 scenario, HD cooling is of limited importance, but in 