>>10978384As the core converts hydrogen into helium (thence into successively heavier elements if sufficiently massive) it gets denser and hotter, and no longer depends on the weight of the layers above it to sustain compression. Independently of the rest of the star, it burns ever more fiercely as it shrinks under its own gravity, while the increasing radiation pressure it produces swells the rest of the star outward into a huge envelope with an extremely low density. In stars of the sun's mass, that don't go supernova, the core nevertheless gets so hot that it pushes off the outer layers altogether, producing a planetary nebula, with the naked core visible in the center, shining mostly in the ultraviolet due to how high the temperature remains even at its now exposed surface. It's the exposed core that becomes a white dwarf as it cools.
