Final answer:
During their evolution, stars shed mass and can become white dwarfs if their final mass is below the Chandrasekhar limit. The pressure exerted by degenerate electrons keeps white dwarfs from collapsing further.
Step-by-step explanation:
White dwarfs are the remnants of stars that have lost a significant amount of mass during their evolution. When a star exhausts its nuclear fuel, it expands into a red giant and sheds its outer layers. This mass loss can cause the star to become a white dwarf if its final mass is below the Chandrasekhar limit (around 1.4 times the mass of the Sun). The pressure exerted by degenerate electrons within the white dwarf prevents it from collapsing further.
As the outer layers of a dying star are shed, fusion reactions continue in the shells surrounding the white dwarf core. The ashes of these fusion reactions fall onto the core, increasing its mass. A higher mass leads to a smaller core size, as even a degenerate gas is mostly empty space and can contract further. Eventually, the white dwarf will cool off and become a black dwarf, composed mainly of carbon, oxygen, and neon.