How do white dwarf stars form?

White dwarf stars form when medium-sized stars exhaust their nuclear fuel and shed their outer layers.

White dwarf stars are the remnants of medium-sized stars, like our Sun. These stars spend the majority of their lives fusing hydrogen into helium in a process known as nuclear fusion. This process releases a tremendous amount of energy, which causes the star to shine. However, once the star has exhausted its hydrogen supply, it begins to fuse helium into heavier elements such as carbon and oxygen. This stage of a star's life is called the red giant phase.

During the red giant phase, the star expands significantly, often engulfing nearby planets. The core of the star, where the fusion is taking place, becomes increasingly dense. Eventually, the star can no longer sustain fusion because it has run out of fuel. At this point, the outer layers of the star are expelled into space, creating a beautiful cloud of gas and dust known as a planetary nebula.

What remains of the star after this process is a hot, dense core, which is what we call a white dwarf. White dwarfs are incredibly dense; a teaspoon of white dwarf material would weigh as much as an elephant on Earth! They are also very hot, with surface temperatures reaching up to 100,000 degrees Celsius. However, without the energy from nuclear fusion to keep them hot, white dwarfs gradually cool and fade over billions of years.

In summary, white dwarf stars are the end product of the life cycle of medium-sized stars. They form when these stars exhaust their nuclear fuel and shed their outer layers, leaving behind a hot, dense core.