 The star does not have enough mass to either burn much heavier elements, or to implode into a neutron star or type II supernova as a larger star can, from the force of its own gravity, so it gradually shrinks and becomes very dense as it cools, glowing white and then red, for a period many times longer than the present age of the Universe.Occasionally, a white dwarf gains mass from another source – for example, a binary star companion that is close enough for the dwarf star to siphon sufficient amounts of matter onto itself; or a collision with other stars, the siphoned matter having been expelled during the process of the companion's own late stage stellar evolution.If the white dwarf gains enough matter, its internal pressure and temperature will rise enough for carbon to begin fusing in its core.