Definition of a White Dwarf
A white dwarf is the very dense remnant of a star that has run out of nuclear fuel. These stars, typically with an initial mass less than about 8 times that of our Sun, shed their outer layers, leaving behind a core composed mainly of carbon and oxygen, sometimes helium.
Key Characteristics and Formation
White dwarfs are incredibly compact, roughly the size of Earth but with a mass comparable to the Sun. Their density is extreme: a teaspoon of white dwarf material would weigh several tons. They form after a star like our Sun exhausts the hydrogen fuel in its core, expands into a red giant, and then expels its outer envelopes to form a planetary nebula, leaving the hot, inert core behind.
An Illustrative Example
Our Sun is expected to become a white dwarf in about 5 billion years. After swelling into a red giant and shedding its outer layers, its core will collapse into a white dwarf. Sirius B, the companion star to Sirius, is one of the closest and best-studied white dwarfs, providing a real-world example of this stellar corpse.
The Fate and Importance of White Dwarfs
White dwarfs no longer undergo nuclear fusion; they simply cool down over billions of years, radiating their residual heat. Eventually, they are theorized to become 'black dwarfs,' though none are thought to exist yet due to the universe's age. They are crucial for understanding stellar lifecycles and are often found in binary systems, sometimes accreting matter from a companion star, which can lead to supernova events.