A white dwarf is the compact “dead core” of a star, while a main sequence star is a living star still burning hydrogen in its core.

Core idea in one line

  • Main sequence star : Actively fusing hydrogen to helium in its core → stable, shining “normal” star.
  • White dwarf : Fusion has stopped → ultra‑dense, Earth‑sized stellar remnant slowly cooling over time.

How they form in a star’s life

  • Main sequence stars are stars in the long middle phase of life, where gravity pulling inward is balanced by energy from hydrogen fusion pushing outward.
  • When a low‑ or medium‑mass star runs out of core hydrogen, it swells into a red giant, sheds its outer layers, and the leftover core becomes a white dwarf.
  • So a typical white dwarf is literally what many main sequence stars will eventually turn into.

Size, mass, and density

  • Main sequence stars (like the Sun) are roughly the size of the Sun, with radii hundreds of thousands of kilometers.
  • White dwarfs pack a similar fraction of a Sun’s mass into a radius about the size of Earth, making them incredibly dense; a ton of white‑dwarf material could fit in a matchbox.

What’s happening inside

  • In a main sequence star, the core is hot plasma where hydrogen nuclei fuse to helium, generating continuous energy.
  • In a white dwarf, fusion fuel is spent; the star’s matter is in a degenerate state (mostly carbon and oxygen nuclei in a sea of electrons), and it shines only by leftover heat as it slowly cools.

In simple terms

If you imagine a star’s life like a human life:

  • The main sequence star is the adult—active, burning fuel, living its “normal” life.
  • The white dwarf is the compact, cooling remnant—no longer “living” in the sense of fusion, just fading away over billions of years.

Main difference to remember: a main sequence star is actively burning hydrogen and in its long, stable life phase; a white dwarf is the small, dense, burned‑out core left after that active life ends.