| Stars form in dense clouds of gas and dust, and this material greatly obscures the central cores of still-forming stars. However, some radiation does escape from their circumstellar envelopes and disks, allowing some study of these youngest stars. This even includes Class 0 protostars, stars so young that they have yet to accrete the majority of their mass. We have recently observed the first absorption spectrum of the photosphere of a Class 0 protostar, S68N in Serpens. The K-band spectrum shows a very red continuum, CO absorption bands, weak or non-existent atomic metal absorptions, and H2 emission lines. The near-IR H2 emission is consistent with excitation in shocks or by X-rays but not by UV radiation. We model the absorption spectrum as a stellar photosphere plus circumstellar continuum emission with wavelength-dependent extinction. A Markov Chain Monte Carlo analysis shows that the most likely model parameters are consistent with a low-temperature, low-gravity photosphere with significant extinction and no more than modest continuum veiling. Its effective temperature is similar to that of older, more evolved pre-main-sequence stars, but its surface gravity is approximately ten times lower. This implies that the radii of these still-forming protostars are a factor of ~ 3 larger than that of young but fully-formed T-Tauri stars. The modest continuum veiling and the high extinction is consistent with most of the circumstellar material being in a cold envelope, as expected for such young protostars. Future Keck and JWST observations of more Class 0 protostars will reveal more about their stellar properties and how they accrete mass. |