White Dwarfs
The dense, cooling remnants of Sun-like stars supported by electron degeneracy pressure, with a maximum mass of about 1.4 solar masses.
A white dwarf is the final state of stars up to about eight times the mass of the Sun — including our Sun — after they exhaust their nuclear fuel. During the red giant phase, such a star expels its outer layers as a glowing planetary nebula, leaving behind the exposed, compressed core: a white dwarf. The first white dwarf discovered was the companion of Sirius (1915), which packs roughly a solar mass into a volume about the size of Earth.
A white dwarf shines by residual heat, not fusion. Its matter is compressed to densities of about a tonne per cubic centimeter, and the pressure that supports it against gravity is electron degeneracy pressure — a quantum mechanical effect: electrons cannot occupy the same quantum state, so squeezing them harder does not shrink the star further as long as they remain non-relativistic. The interior is mostly carbon and oxygen, the ash left by stellar nucleosynthesis in the precursor star, with a thin hydrogen or helium atmosphere.
There is a limit to how massive a white dwarf can be. Subrahmanyan Chandrasekhar showed in 1930 that above about 1.4 solar masses — the Chandrasekhar limit — electron degeneracy pressure cannot resist gravity, and the star collapses further. This is the origin of type Ia supernovae: a white dwarf in a binary system can accrete matter from its companion until it crosses the limit and detonates. Because they explode at nearly the same mass, type Ia supernovae serve as standard candles for measuring cosmic distances.
A white dwarf is not the end of the line for every star. More massive progenitors skip this stage and collapse directly to neutron stars or black holes. White dwarfs themselves cool and fade over billions of years, eventually becoming cold, dark objects called black dwarfs — a stage so slow that none are expected to exist yet, because the universe is not old enough. They are the most common endpoint of stellar evolution in the Milky Way.
Tags
astronomy degenerate matter stellar evolution white dwarfs
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