What is the primary characteristic of a white dwarf?

The primary characteristic of a white dwarf is that it is the remnant core of a star that has exhausted its nuclear fuel. When a star similar in size to the Sun completes its life cycle, it sheds its outer layers and leaves behind a hot, dense core made mostly of carbon and oxygen. This core is what we refer to as a white dwarf.

After the red giant phase, during which the star undergoes fusion of heavier elements, it loses its outer layers, and what remains is primarily this inert core no longer undergoing fusion reactions. As a result, white dwarfs are typically very small but have extremely high temperatures, glowing due to residual heat.

In contrast, the other options describe different stellar phenomena. The final stage of a massive star typically leads to a supernova explosion, which is not how white dwarfs are formed. While white dwarfs can contain helium, they are primarily made of carbon and oxygen from the original star’s fusion processes. Moreover, white dwarfs are not formed via a supernova; rather, they are the leftover cores of less massive stars that didn't undergo such explosive ends.