White dwarf

When a star like the Sun runs out of fuel, it puffs off its outer layers into space and leaves behind its exposed core. That core is a white dwarf: typically about 0.6 times the mass of the Sun, packed into a ball roughly the size of Earth. A teaspoon of its material would weigh several tonnes.

What holds it up

A white dwarf no longer produces energy by fusion. Nothing is burning inside it. What stops gravity crushing it further is electron degeneracy pressure: a quantum effect that prevents electrons from being squeezed into the same state. It works only up to a point. In 1931 the young Indian physicist Subrahmanyan Chandrasekhar calculated that above about 1.44 solar masses, no white dwarf can support itself. This Chandrasekhar limit earned him a share of the 1983 Nobel Prize in Physics.

A white dwarf that gains enough mass from a companion to approach the limit can be destroyed in a Type Ia supernova, the kind used to measure the expansion of the universe. Exactly how this happens is still debated.

The future of most stars

Over 97% of the stars in the Milky Way, including the Sun, will end as white dwarfs. Only the most massive stars become neutron stars or black holes. The first white dwarf recognised as unusual was 40 Eridani B, found in 1910 to be white-hot yet very dim. Sirius B followed. The name “white dwarf” was coined by Willem Luyten in 1922.

Slowly freezing

A newborn white dwarf can be hotter than 150 000 K (149 730 °C). With no fuel, it simply cools, very slowly. In 2019 Pier-Emmanuel Tremblay and colleagues used data from ESA’s Gaia satellite on more than 15 000 white dwarfs and found a pile-up at a certain brightness: the signature of their cores crystallising into solid as they cool, releasing extra heat and slowing the process by up to 2 billion years.

Eventually a white dwarf should cool until it no longer glows at all, becoming a black dwarf. That takes many trillions of years. The universe is only 13.8 billion years old, so no black dwarf exists yet.