White dwarf stars are not normal stars, but the cinders of old stars. They contain most of the star's original mass, but are solid bodies denser than the Earth and about the same size. They do not generate their own energy like a star does via thermonuclear reactions. They are just very hot, solid bodies that are slowly cooling off in time over the course of billions of years. They are 'exotic' but there is nothing that is really peculiar about them. Their masses are only limited by how much weight 'degeneracy pressure' can support against further gravitational collapse, and that is about 1.4 times the mass of the Sun.