How exactly does the accretion disk form?
Matter from a nearby companion star gets attracted gravitationally to the white dwarf, and some of this matter can go into orbit around the white dwarf. As the matter (say, a stream of gas) falls toward the white dwarf, it orbits faster and faster, according to the law of conservation of angular momentum. Eventually a disk of swirling matter, called an accretion disk, forms. This disk can heat up from friction (the individual particles in it, with different orbital velocities determined by Kepler’s laws, rub against each other), and can glow quite brightly from thermal radiation. When chunks of matter fall in to the disk, there can be bursts of radiation from gravitational energy getting converted to thermal energy.
