  The evolution of radiation emitted during the dynamical collapse of
metal-free protostellar clouds is investigated within a spherically symmetric
hydrodynamical scheme that includes the transfer of radiation and the chemistry
of the primordial gas. The cloud centre collapses on a time scale of about
10^5-10^6 years, thanks to line cooling from molecular hydrogen (H2). For most
of the collapse time, when the evolution proceeds self-similarly, the
luminosity slowly rises up to about 10^36 erg/s and is essentially due to H2 IR
line emission. Later, continuum IR radiation provides an additional
contribution, which is mostly due to the accretion of an infalling envelope
upon a small hydrostatic protostellar core which develops in the centre. We
follow the beginning of the accretion phase, when the enormous accretion rate
(~ 0.1 Msun/yr) produces a very high continuum luminosity of about 10^36 erg/s.
Despite the high luminosities, the radiation field is unable to affect the gas
dynamics during the collapse and the first phases of accretion, because the
opacity of the infalling gas is too small; this is quite different from
present-day star formation. We also find that the protostellar evolution is
similar among clouds with different initial configurations, including those
resulting from 3D cosmological simulations of primordial objects; in
particular, the shape of the molecular spectra is quite universal. Finally, we
briefly discuss the detectability of this pristine cosmic star formation
activity.
