  We study the blackbody properties and the thermodynamic equilibrium
quantities of a photon gas in the framework of nonlinear electrodynamics. In
this vein, we take into account the photon propagation in a uniform external
magnetic field in the weak field approximation, where an angular anisotropic
energy density distribution appears in the frequency spectrum. The particular
case when the photon propagates perpendicular to the background magnetic field
is also discussed, which allows us to probe the strong field regime. We then
derive a modified blackbody spectral distribution and the Stefan-Boltzmann law
in this situation. Considerations about Wien's displacement law and the
Rayleigh-Jeans formula are contemplated as well. Deviations from the
thermodynamic quantities at thermal equilibrium such as energy, pressure,
entropy, and heat capacity densities are obtained from the Helmholtz free
energy. As an application, we study three nonlinear electrodynamics, namely,
the Euler-Heisenberg, the generalized Born-Infeld, and the logarithmic
electrodynamics. Possible implications on stellar systems with strong magnetic
fields such as magnetars are discussed.
