White Dwarfs
A white dwarf is the small, very dense, cooling leftover core of a dead star. When a star roughly the size of the Sun runs out of fuel and dies, it puffs off its outer layers as a beautiful glowing shell called a planetary nebula, and the bare core that remains is a white dwarf. A white dwarf is roughly the size of Earth but packs in as much mass as our entire Sun. They are some of the densest objects in the universe, second only to neutron stars and black holes.
- Typical sizeEarth-sizedRoughly 12,000 km across
- Typical massapprox. 0.6 solar massesBut up to 1.4 (the Chandrasekhar limit)
- Densityapprox. 1 million times waterA teaspoon weighs about 5 tonnes
- Surface temperature8,000 to 100,000 °CHot when young, cooler with age
- Famous exampleSirius BOrbits Sirius, 8.6 light years from Earth
- FateSlowly coolsTrillions of years to fade away
What is a white dwarf?
When a star like our Sun finishes its life, gravity pulls its leftover core in until it is squeezed into a ball about the same size as the Earth. At that point a strange physics rule called electron degeneracy pressure takes over and stops the core shrinking any further. The result is a white dwarf: a city-sized lump of stripped atomic nuclei, roughly Earth-sized but containing the mass of a whole star.
White dwarfs do not do any nuclear fusion. They simply glow with the leftover heat from when they were a star. They start out incredibly hot (up to 100,000 °C) but with very little surface area, so they fade quickly at first and then more and more slowly over trillions of years.
How does a white dwarf form?
About 97% of all stars in the Milky Way will end up as white dwarfs. The process goes like this:
- The star uses up the hydrogen in its core, then expands into a red giant as helium fusion takes over.
- The red giant slowly puffs off its outer layers into space, forming a glowing planetary nebula.
- The bare core is left behind, glowing white-hot. It is now a white dwarf.
- Over billions of years, the white dwarf radiates away its heat and slowly turns cooler and redder.
- After trillions of years, in theory it should cool completely and become a cold, dark black dwarf.
Step 5 is theoretical only: the universe has not been around long enough for any black dwarfs to have formed yet.
How dense is a white dwarf?
Cramming the mass of our Sun into a ball the size of Earth makes white dwarfs extraordinarily dense. A teaspoonful of white dwarf material would weigh around 5 tonnes, about the weight of an elephant. The escape speed from the surface of a white dwarf is several hundred kilometres per second. The gravitational pull on the surface is around 100,000 times stronger than on Earth.
Inside a white dwarf, atoms are crushed so close together that the normal rules of chemistry break down. Electrons no longer belong to particular atoms but slosh around freely, like a sea of negative charge holding the structure up against gravity. This is the "electron degeneracy" that gives white dwarfs their unique behaviour.
The Sun's future
In about 5 billion years, our Sun will run out of hydrogen and start its final stages. It will swell into a red giant large enough to swallow the orbits of Mercury, Venus and probably Earth, then puff off its outer layers as a beautiful planetary nebula. The Sun's core will collapse into a white dwarf about the size of Earth, with around half the Sun's current mass. From whatever is left of our solar system, the dead Sun will appear as a brilliant pinprick of light, slowly fading over the next many billions of years.
The Chandrasekhar limit
A white dwarf cannot be heavier than a certain mass. The Indian-American astronomer Subrahmanyan Chandrasekhar calculated this limit in 1930 (he was 19 at the time): the maximum mass is around 1.4 times the Sun. Above that, electron degeneracy pressure is no longer strong enough to hold up the star against gravity, and the white dwarf collapses further.
This Chandrasekhar limit also leads to a spectacular event called a Type Ia supernova. If a white dwarf is part of a close binary star system and slowly steals gas from its companion, the white dwarf can grow towards the limit. When it reaches it, the whole star explodes in a runaway thermonuclear reaction, leaving nothing behind. Type Ia supernovae always have nearly the same brightness, so astronomers use them as "standard candles" to measure distances across the universe.
Deeper dive: electron degeneracy pressure explained
White dwarfs and neutron stars are held up against gravity by a strange quantum-mechanical effect called degeneracy pressure. To understand why, you need a tiny bit of quantum physics.
One of the rules of the quantum world is called the Pauli exclusion principle: two electrons cannot occupy exactly the same state at the same time. Squeeze a gas of electrons hard enough, and they will resist being forced into the same state, even at zero temperature. The harder you squeeze, the more they resist. The push-back you feel is called electron degeneracy pressure.
In a white dwarf, the gravity of the dead star is enormous, but the electron degeneracy pressure can just about hold it up. The Indian-American astronomer Chandrasekhar showed that this only works up to about 1.4 solar masses. Above that, the gravity wins. Electrons are crushed into protons to form neutrons, and the star turns into a different, even denser object: a neutron star, held up by an even stronger degeneracy pressure of neutrons. Above about 3 solar masses, even that gives way, and you get a black hole. Each dead star is essentially the end-point of one of these three quantum stand-offs against gravity.
For other ways stars die, see neutron stars and supergiants. For how the whole life cycle works, see life cycle of a star.