The phrase "to become a white dwarf" refers to the final stage of a star's life cycle. When a star exhausts its nuclear fuel, it can no longer produce enough energy to support its outer layers. As a result, the star collapses under its own gravity and becomes a white dwarf, which is a dense, hot ball of gas that emits light due to its high temperature. White dwarfs are the remnants of stars like our Sun and are not capable of sustaining nuclear fusion like a star.