May 22, 1995: Peering into the heart of two recently exploded double-star systems, the Hubble telescope has surprised researchers by finding that the white dwarf stars at the center of the fireworks are cooler than expected and spin more slowly than previously thought.
Each dwarf - dense, burned-out stars that have collapsed to the size of Earth - is in a compact binary system, called a cataclysmic variable, where its companion is a normal star similar to, but smaller than the Sun. The stars are so close together that the entire binary system would fit inside the Sun. Their closeness allows gas to flow from the normal star onto the dwarf, where it swirls into a pancake-shaped disk [see illustration]. When the disk of gas periodically collapses onto the white dwarf, it unleashes a burst of kinetic energy, called a dwarf nova outburst. Once dumped onto the dwarf's surface, hydrogen accumulates until it undergoes thermonuclear fusion, which eventually triggers an explosion.See the rest: