dense core

The term dense core (in discussions of star formation) refers to a dense molecular cloud or a dense region of a molecular cloud, a stage in the development of stars from clouds. Current star-formation models presume such a stage, and apparent examples are observed, e.g., in well-known clouds in Orion. The term refers to a mass of molecular hydrogen that is gravitationally bound (essentially, that the Jeans criterion is met) or very close to that state. Current research aims to understand their formation and evolution into stars. A coherent dense core is such a region that is relatively calm, i.e., has thermal kinetic energy but lacks the degree of turbulence otherwise evident in the region around it.

As far as I can tell, in the context of star formation, the word core (without the word "dense") can refer to a dense core, or (by context) can refer to the core of a subsequent protostar.

The phrase dense core is also seen regarding subsequent stellar evolution, merely referring to a stellar core that is particularly dense. It is similarly seen referring to the cores of planets.