An Etymological Dictionary of Astronomy and Astrophysics
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A low-mass → protostar deeply embedded in a → circumstellar dusty envelope and resulting from the → gravitational collapse of a dense → pre-stellar core. This stage in the process of star formation occurs typically a few 10⁴ years after the onset of the collapse. Class 0 protostars represent the earliest stage of → young stellar objects. The → spectral energy distribution (SED) of a Class 0 object resembles a → blackbody spectrum at a temperature below ~ 15-30 K, peaking at → submillimeter wavelengths beyond 100 μm. The central protostar has not yet acquired its final mass, since → accretion is still going on, and the envelope (detected in submillimeter wavelengths) is more massive than the central protostellar mass. Moreover, these objects show powerful → bipolar ejections of material in the form of collimated → carbon monoxide (CO)→ outflows which distinguish them from the pre-stellar phase of star formation. The subsequent evolution of a Class 0 is a → Class I.

→ class; → zero.