Here's why:

* White dwarfs are the remnants of stars. They are the dense, Earth-sized cores left behind after a star like our Sun has exhausted its nuclear fuel and shed its outer layers.

* No fusion occurs in a white dwarf. They are essentially "dead" stars, with no internal pressure to support further fusion reactions. Their energy comes from slowly radiating away their remaining heat.

* White dwarfs are incredibly dense. They pack the mass of a Sun-like star into a space the size of Earth, making them extremely dense.

Here's the process:

1. Main Sequence Star: Stars spend most of their lives fusing hydrogen into helium in their cores, like our Sun.

2. Red Giant: When a star exhausts its core hydrogen, it expands into a red giant.

3. Planetary Nebula: The outer layers of a red giant are shed, forming a planetary nebula.

4. White Dwarf: The remaining core of the star becomes a white dwarf.

Important Note: White dwarfs can accrue matter from a nearby companion star, sometimes causing thermonuclear explosions known as Type Ia supernovae. However, this is not the result of ongoing fusion within the white dwarf itself, but rather an explosive reaction triggered by the additional material.