  The formation of the first stars (Population III; PopIII) marks the end of
the dark ages of the universe, a subject of lively scientific debate. Not (yet)
accessible to direct observations, this early stage of the universe is mostly
studied via theoretical calculations and numerical simulations. An indirect
window is provided by integrated present day observables such as the metal
abundance or the diffuse extragalactic photon fields. We aim to derive
constraints on the properties of the PopIII and low metallicity Population II
(LM PopII) stars utilizing limits on the density of the extragalactic
background light (EBL), recently derived from very-high-energy (E > 100 GeV;
VHE) observations. A model calculation for the evolving EBL density produced by
PopIII/LM PopII stars is presented. The model utilizes stellar population
spectra (SPS) for zero and low metallicity stars and accounts for the changing
emission of an aging stellar population. Emission from the dense HII regions
surrounding the stars (nebula) is included. The resulting EBL density for
different scenarios (metallicity, star formation rate, initial mass function)
is compared to the limit on the EBL density. The potential for detecting a
cut-off in HE/VHE spectra is discussed. Assuming a maximum contribution from
PopIII/LM PopII stars to the EBL density of 5 nW m^-2 s-1 at 2 micrometer a
limit on the star formation rate (SFR) of the first stars of 0.3 to 3 M_solar
Mpc^-3 yr^-1 in the redshift range 7 - 14 is derived. The limit depends on the
assumed shape of the SFR and metallicity.
