- One of the possible remnants which can be formed from stars after they cease producing energy by fusion. If less than 1.4 solar masses of material remain after the star leaves the main sequence and loses mass, it will collapse into a white dwarf when the outward radiation pressure which holds active stars up is removed. White dwarfs are held up instead by the mutual repulsion between the electrons in their atoms. They emit small amounts of residual heat and when this is gone reach the black dwarf stage – or they will, since the process takes so long that no star in the universe has completed it yet. The coolest white dwarfs are still at about 3,500°. White dwarf material has a density of many tonnes per cc. The interior of a white dwarf can be active because of sloshing vibrations as heat seeps to the surface from the interior, causing their brightness to vary over a timescale of hours.