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White dwarfs are Earth-sized, billion-ton remnants of low-to-intermediate mass stars, supported by electron degeneracy pressure. Composed of carbon and oxygen (or helium), they cool slowly over billions of years. The age of the oldest white dwarfs provides a lower limit on the age of the Galaxy, making white dwarfs cosmic chronometers.
From stellar evolution, you know that a star's fate depends on its mass. Stars like our Sun spend billions of years fusing hydrogen into helium on the main sequence, then swell into red giants as they exhaust core hydrogen and begin shell burning. For low-to-intermediate mass stars (roughly 0.5 to 8 solar masses — which includes the vast majority of all stars), the story ends not in a dramatic supernova but in a slow, quiet transformation into a white dwarf. During the red giant and asymptotic giant branch phases, the star's outer layers are expelled as a planetary nebula, leaving behind only the dense, hot core.
That remnant core is astonishing in its extremity. A typical white dwarf packs roughly 0.6 solar masses — more than half the mass of our Sun — into a volume about the size of Earth. A teaspoon of white dwarf material would weigh several tons. At these densities, ordinary gas pressure is irrelevant. Instead, white dwarfs are held up by electron degeneracy pressure, a quantum mechanical effect arising from the Pauli exclusion principle: no two electrons can occupy the same quantum state, so as matter is compressed, electrons are forced into higher and higher energy states, generating an outward pressure that resists further collapse. This pressure depends on density rather than temperature, which is why a white dwarf can support itself even as it cools — unlike a normal star, which would contract if it stopped generating heat.
There is, however, a limit. The Indian-American astrophysicist Subrahmanyan Chandrasekhar showed that electron degeneracy pressure can only support a white dwarf up to about 1.4 solar masses — the Chandrasekhar limit. Beyond this mass, the electrons would need to move faster than light to provide sufficient pressure, which is impossible. White dwarfs above this limit cannot exist as stable objects; they would collapse further into neutron stars or undergo thermonuclear detonation (as in Type Ia supernovae). This mass limit is not just a curiosity — Type Ia supernovae, triggered when a white dwarf accretes matter from a companion star and approaches the Chandrasekhar limit, all reach roughly the same peak luminosity, making them invaluable standard candles for measuring cosmic distances.
Because white dwarfs generate no new energy through fusion, they simply radiate away their stored thermal energy over billions of years, gradually dimming and cooling from incandescent white through yellow, red, and eventually — given enough time — to a hypothetical cold, dark black dwarf (though the universe is not yet old enough for any to have reached this state). This cooling is remarkably predictable: theoretical cooling curves relate a white dwarf's luminosity to its age. By finding the faintest, coolest white dwarfs in the Milky Way's disk or in globular clusters and reading their temperature from the cooling models, astronomers can establish a minimum age for the stellar population that produced them. The oldest known white dwarfs have been cooling for approximately 11–12 billion years, providing an independent lower bound on the age of the Galaxy that agrees well with other dating methods.