White Dwarfs are already relatively common in the Galaxy, but as the Universe ages they will proliferate. About 200 billion will form before the gas runs out for star formation in the Milky Way. But by then the Milky Way and Andromeda’s M31 will merge as ‘Milkomeda’ – a largish Elliptical Galaxy – roughly doubling the numbers. Stars will age and brighten as Milkomeda ages at ever smaller stellar masses, until all the fusible gases are depleted and stars are too small to fuse.
In the Long Dark that follows, every 100 billion years, star corpses and wannabe stars, the brown dwarfs, will collide with sometimes spectacular results. A brown dwarf and white dwarf collision will probably result in either a renewal of fusion burning for the white dwarf or a nova explosion. Two brown dwarfs colliding could produce an low mass star or a renewed hot brown dwarf glowing from the collision’s kinetic energy. Two white dwarfs colliding could have a number of outcomes – with enough energy the helium or carbon fusion Main sequences can be triggered. Alternatively a mass above the Chandrasekhar Limit, or close to it, can produce a thermonuclear detonation, with the stars totally disrupted in a Type Ia Supernova.
According to the Fertile Cosmos proposal of R.J.Spivey each Type Ia conflagration produces sufficient heavy elements to make roughly 450 thousand Earth mass ocean planets. These, in turn, are warmed via neutrino pair-annihilation in their iron cores, sufficient to keep their sub-glacial oceans warm for a 100 billion trillion years.