A nova is an explosion from the surface of a white-dwarf star in a binary system. A nova occurs when the white dwarf, which is the dense core of a once-normal star, "steals" gas from its nearby companion star. When enough gas builts up on the surface of the white dwarf it triggers a thermonuclear explosion. For a brief time, the system can shine up to a million times brighter than normal. As long as it continues to take gas from its companion star, the white dwarf can produce nova outbursts at regular intervals.