The mantle of gas surrounding a protostar is called the protostellar envelope. This material is freely collapsing onto the central star and its surrounding disk. The envelope is the densest portion of a molecular cloud core that went into gravitational collapse. Infalling, envelope material impacts the protostar and disk, creating a radiating shock front. The envelope gas contains solid dust grains, which absorb all optical and ultraviolet radiation from the star and accretion shock, reradiating it at infrared and longer wavelengths. Within about 1 AU from the star, the dust grains are thermally destroyed by sublimation. The dust photosphere (10 AU) is the radius at which the grains are so sparse that radiation streams passed them unimpeded. The emergent spectrum resembles a cold blackbody.