Most stars have companions. Two, three or more stars, born from the same gas cloud may orbit each other. Some giant stars are in contact as they orbit each other. The members of other multiple systems are so far apart, they may not belong to one system.
The more massive star of a pair will go through its life cycle more rapidly, and its death may be affected by its companion. In some binary systems, one star has aged until it has become a white dwarf. When the second star at last ages and swells, gas from its out layers can be sucked onto the surface of the white dwarf, which flares up as a bright , seemingly “new” star, a nova, over 100,000 times brighter than the Sun. the white dwarf blows away the excess matter and subsides.