Here's why:

* Small: White dwarfs are the leftover cores of stars that have exhausted their nuclear fuel. They are incredibly dense, packing the mass of our Sun into a sphere roughly the size of Earth.

* Hot: While they are small, they are incredibly hot, with surface temperatures reaching tens of thousands of degrees Celsius. This is due to their high density and leftover heat from their star's previous life.

Important Note: While small and hot, white dwarfs are not part of the main sequence, which is where most stars reside on the Hertzsprung-Russell diagram. They are classified as a separate category due to their unique properties and evolutionary stage.