  Gravitational lensing may render individual high-mass stars detectable out to
cosmological distances, and several extremely magnified stars have in recent
years been detected out to redshifts $z\approx 6$. Here, we present Muspelheim,
a model for the evolving spectral energy distributions of both metal-enriched
and metal-free stars at high redshifts. Using this model, we argue that lensed
stars should form a highly biased sample of the intrinsic distribution of stars
across the Hertzsprung-Russell diagram, and that this bias will typically tend
to favour the detection of lensed stars in evolved stages characterized by low
effective temperatures, even though stars only spend a minor fraction of their
lifetimes in such states. We also explore the prospects of detecting
individual, lensed metal-free (Population III) stars at high redshifts using
the James Webb Space Telescope (JWST). We find that very massive ($\gtrsim 100\
M_\odot$) Population III stars at $z\gtrsim 6$ may potentially be detected by
JWST in surveys covering large numbers of strong lensing clusters, provided
that the Population III stellar initial mass function is sufficiently
top-heavy, that these stars evolve to effective temperatures $\leq 15000$ K,
and that the cosmic star formation rate density of Pop III stars reaches
$\gtrsim 10^{-4}\ M_\odot$ cMpc$^{-3}$ yr$^{-1}$ at $z\approx$ 6-10. Various
ways to distinguish metal-free lensed stars from metal-enriched ones are also
discussed.
