When a star like the Sun dies, it does not go quietly. It swells into a red giant, sheds its outer layers into space, and leaves behind a tiny, blazing core. That core is a white dwarf. It is roughly the size of Earth, with the mass of the Sun. A teaspoon of its material would weigh several tons on Earth.

White dwarfs no longer fuse anything. They shine because they are hot, and they are slowly cooling. A white dwarf with a surface temperature of 100,000 Kelvin may take billions of years to fade into a cold, dark cinder. None have had time to cool completely, so every white dwarf ever formed is still glowing.

The physics inside a white dwarf is strange. Electrons are packed so tightly that quantum pressure, not heat, holds the star up against gravity. That pressure has a limit. If a white dwarf accumulates too much mass from a companion star, it can exceed the Chandrasekhar limit of about 1.4 solar masses and explode as a Type Ia supernova.

White dwarfs are common. The nearest is Sirius B, about 8.6 light-years away. Astronomers study them to understand the history of star formation and the future of our own Sun, which will become one in about 5 billion years.