White Dwarf
The final evolutionary state of roughly 97% of all stars — including the Sun. Not supported by fusion but by electron degeneracy pressure, slowly cooling over billions of years. The first one identified was Sirius B. When a white dwarf exceeds a critical mass, it explodes as a Type Ia supernova — the standard candle that revealed dark energy.
What It Is
When a star of less than ~8 solar masses exhausts its nuclear fuel, it expels its outer layers as a planetary nebula and the core collapses into a white dwarf. The collapse stops not because of fusion pressure but because of electron degeneracy pressure — the quantum mechanical resistance of electrons to being packed together (Pauli exclusion principle). This is a fundamentally different support mechanism from any stage of normal stellar life.
A typical white dwarf contains roughly 1 solar mass compressed into an Earth-sized volume: densities around 10⁶ g/cm³. A teaspoon would weigh several tonnes.
The Chandrasekhar Limit
In 1930, the 19-year-old Indian physicist Subrahmanyan Chandrasekhar calculated that electron degeneracy pressure has a maximum — it can only support a white dwarf up to ~1.4 solar masses. Above this limit, electrons become relativistic and degeneracy pressure is overcome; the star collapses further into a neutron star or, in some cases, triggers a thermonuclear explosion.
This limit — 1.4 M☉ — became one of the most consequential numbers in astrophysics. Chandrasekhar's supervisor Arthur Eddington publicly rejected the finding. Chandrasekhar eventually won the Nobel Prize in 1983.
Type Ia Supernovae — The Standard Candle
A white dwarf in a binary system can accrete mass from a companion. If it reaches the Chandrasekhar limit, carbon fusion ignites explosively throughout the entire star simultaneously — a Type Ia supernova. Because every Type Ia involves the same mass threshold, they all explode with approximately the same intrinsic luminosity.
This makes them standard candles: measure how bright they appear, compare to how bright they should be, and calculate distance with precision. It was Type Ia supernovae observed with the Hubble Space Telescope in 1998 that revealed the universe's expansion is accelerating — the discovery of dark energy (Nobel Prize 2011). See also supernova.
Sirius B — The First Identified
The companion star to Sirius was predicted by Friedrich Bessel in 1844 from perturbations in Sirius's proper motion, and first observed by Alvan Graham Clark in 1862. When its spectrum was measured in 1915, the surface temperature was found to be comparable to the Sun — but its luminosity implied it was tiny. The conclusion: a star about the mass of the Sun compressed into a volume comparable to Earth.
Sirius B was the first white dwarf ever identified. It completed its main-sequence life about 120 million years ago. See sirius.
Cooling History
A newly formed white dwarf may have a surface temperature of 100,000–200,000 K, radiating blue-white light. With no energy source beyond residual heat, it cools continuously over billions of years: white → yellow → orange → red → eventually a cold, dark black dwarf.
No black dwarfs are thought to exist yet — the universe at 13.8 billion years old is too young for any white dwarf to have cooled that far. The coolest known white dwarfs are ~3,500–4,000 K, having formed early in the universe's history.
Composition
Most white dwarfs have carbon-oxygen cores, the ash of helium fusion. Their surfaces show either hydrogen (DA type, ~80%) or helium (DB type, ~20%) — heavier elements sink rapidly in the intense gravity, leaving a pure light-element atmosphere. Some show traces of heavier elements, suggesting they recently accreted rocky material from disrupted asteroids or planets.