What Is A White Dwarf?
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A white dwarf is the dense, Earth-sized remnant of a star like the Sun. White dwarfs contain about a solar mass packed into a sphere the size of Earth. Sirius B is the most famous white dwarf. Most stars in the universe will eventually become white dwarfs.
A white dwarf is the typical end state for most stars in the universe. After billions of years on the main sequence and an expansion into a red giant phase, stars between about 0.5 and 8 solar masses collapse into incredibly dense, Earth-sized remnants. White dwarfs are made of strange degenerate matter and slowly cool over trillions of years. They're a window into the long-term fate of most stars.
How does a white dwarf form?
From the dying core of a low to medium mass star. According to NASA, white dwarfs are the leftover remnants of stars similar to the Sun. After a star exhausts its nuclear fuel and passes through the red giant phase, the outer layers are gently expelled as a planetary nebula. The remaining dense core collapses under gravity until it reaches a stable size, supported by electron degeneracy pressure. About 97% of stars in our galaxy will eventually end up as white dwarfs.
How dense are they?
Incredibly dense, but not as dense as neutron stars. A typical white dwarf contains about a solar mass packed into a volume roughly the size of Earth, about 7,500 miles across. A teaspoon of white dwarf material would weigh about 5 tons. The high density gives white dwarfs enormous surface gravity, about 100,000 times Earth's. The matter is in a strange state called electron-degenerate matter, where electrons are packed so tightly that quantum mechanics determines their behavior.
What is Sirius B?
The most famous white dwarf. Sirius B is the small companion of Sirius A, the brightest star in our night sky. Sirius B has about 98% the mass of the Sun packed into a sphere only 7,500 miles in diameter. The two stars orbit each other every 50 years at distances between 8.2 and 31.5 astronomical units. Sirius B was first detected gravitationally in 1844 and directly observed in 1862.
What happens to white dwarfs?
They cool down over trillions of years. White dwarfs aren't burning fuel; they're glowing from leftover heat. Over many billions of years, they gradually radiate this heat into space, cooling from white to yellow to red. Eventually they would cool to black, but the universe isn't old enough (13.8 billion years) for any white dwarf to have cooled completely. The Chandrasekhar limit (1.4 solar masses) sets the maximum mass; above that, the remnant collapses into a neutron star instead.
A white dwarf is the dense, Earth-sized remnant of a star like the Sun after it has exhausted its nuclear fuel. About 97% of stars in our galaxy will eventually become white dwarfs. They contain about a solar mass packed into a sphere the size of Earth, made of electron-degenerate matter. Sirius B is the most famous example. White dwarfs slowly cool over trillions of years, eventually becoming black dwarfs (none exist yet because the universe is too young).
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