Path on the Hertzsprung-Russell diagram corresponding to the changes that a Self-luminous object held together by its own self-gravity. Often refers to those objects which generate energy from nuclear reactions occurring at their cores, but may also be applied to stellar remnants such as neutron stars. undergoes after He burning ceases in the In the context of planetary formation, the core is the central region of a large differentiated asteroid, planet or moon and made up of denser materials than the surrounding mantle and crust. For example, the cores of the Earth, the terrestrial planets and differentiated asteroids are rich in metallic iron-nickel. Click on Term to Read More. At this stage, the C core shrinks and drives the expansion of the envelope, and the star becomes a red supergiant.