Nova. Artwork of a white dwarf star becoming a nova, as seen from a nearby Saturn-like planet. A nova is a star which suddenly increases in bright- ness. White dwarfs are the dim remnants of small to medium- sized stars which have run out of hydro- gen fuel to power their nuclear reactions. If the white dwarf is in a binary system, it may capture hydrogen-rich material from its companion star. This may begin a rapid nuclear reaction on the star's surface. The star typically becomes several thousand times brighter in a matter of hours or days, fading again over months or years. A nova also generally ejects a little of its mass to form a large, gaseous halo.