White dwarfs are formed when a low-mass star runs out of fuel and can no longer support fusion reactions in its core. As the star's core collapses, the outer layers of the star are expelled, forming a planetary nebula. The collapsed core then becomes a white dwarf.
White dwarfs are very hot objects, with surface temperatures of up to 100,000 Kelvin. However, they are not very luminous, as they emit most of their energy in the form of ultraviolet light.
White dwarfs are stable objects, and they can last for billions of years. However, they will eventually cool down and become black dwarfs.
Here is a more detailed explanation of the process of how white dwarfs form:
1. As a low-mass star ages, it begins to run out of fuel in its core. This causes the core to contract and the outer layers of the star to expand. The star becomes a red giant.
2. As the core contracts, it becomes hotter and denser. Eventually, the core reaches a temperature and density at which carbon fusion can begin. This process releases energy, which halts the collapse of the core.
3. The star then enters a phase called the horizontal branch phase. During this phase, the star burns carbon in its core and helium in a shell surrounding the core.
4. Eventually, the star runs out of carbon in its core and the core collapses again. This time, the collapse is not halted by fusion reactions. Instead, the electrons in the core become so tightly packed that they can no longer resist the gravitational pull of the nucleus. This causes the core to collapse into a very dense object called a white dwarf.
5. The outer layers of the star are expelled during the collapse of the core, forming a planetary nebula.
White dwarfs are very stable objects, and they can last for billions of years. However, they will eventually cool down and become black dwarfs.
