stellar core

The stellar core is the middle of the star, an element of its stellar structure. For a main sequence star the term is generally used to indicate the region where fusion is taking place, or where most of the fusion is taking place. The boundary of what is cited as the "core" appears to be at the radius where there is a significant transition from the conditions in the center of the star. These center-conditions depend on the size of the star.

For small stars such as red dwarfs, the core is the location of the fusion, specifically proton-proton chain reactions, which occur at the temperature produced by the small star's gravity.

Larger stars achieve a hotter temperature which triggers the CNO cycle, thus the "transition" from the central conditions is to a surrounding area that lacks the CNO cycle, but which is still sufficiently hot to trigger some proton-proton chain fusion. So the star's core can be described as including the bulk of the star's fusion, but not all.

The center of the core may include some material that is not fusing, that has the products of fusion, but without the density and temperature to further fuse the particular material. This happens in phases of post-main-sequence giant stars. The locations and types of fusion evolve as such a star ages.

The core withstands extremely high pressure from its surroundings due to its gravity acting on the mass of the rest of the star. It counteracts this with its own high pressure, due to its compression, aided by the heat of its fusion. In more massive (early) stars, when all fusion eventually ceases, core collapse results, which can produce a core collapse supernova.

Protostars (by definition) have no fusion yet. Thus the "core" of these would not be distinguished by the same characteristics used for main sequence stars.

The solar core is the stellar core of the Sun.