Cataclysmic Variable Star
This is an illustration of a class of double star called a cataclysmic variable. The system consists of a white dwarf star - a dense, burned-out star that has collapsed to the size of Earth and a companion that is a normal star, similar to but smaller than the Sun. The stars are so close together gas flows from the normal star onto the dwarf where it swirls into a pancake-shaped disk.
When the disk of gas periodically collapses onto the white dwarf, it unleashes a burst of kinetic energy. Once dumped onto the dwarf's surface, hydrogen accumulates until it undergoes thermonuclear fusion reactions that eventually trigger the classical nova. After the detonation the "fueling" of the white dwarf starts again.
Spectroscopic studies by Hubble Space Telescope have allowed astronomers to measure directly the white dwarf's temperature, spin rate and chemical composition in two such systems. Hubble has surprised researchers by finding that the white dwarf stars at the heart of the fireworks are cooler than expected and spin more slowly than thought.