nova

Nova is a transient astronomical event characterized by a sudden, dramatic increase in the brightness of a star. In a nova, hydrogen-rich material is transferred from a companion star to the surface of a white dwarf in a close binary system. When enough material has accumulated, thermonuclear fusion ignites in a shell on the white dwarf’s surface, ejecting material into space and causing the system to brighten for days to weeks.

Two principal varieties are classical novae and recurrent novae. Classical novae undergo a single eruptive cycle on timescales of thousands to tens of thousands of years, while recurrent novae experience repeated outbursts on timescales of decades. A related but distinct class, dwarf novae, arises from instabilities in an accretion disk and does not involve a surface thermonuclear explosion, typically reaching lower peak brightness.

Observationally, novae rise rapidly in brightness, display strong emission lines in their spectra, and fade over weeks to months. The ejected shells expand at hundreds to thousands of kilometers per second. The phenomena require a close binary system with an accreting white dwarf, and the study of novae provides insights into accretion physics, nova nucleosynthesis, and binary evolution. Historically, relations between peak brightness and decline rate have been explored for distance estimates, though calibrations remain uncertain.

The term nova is also used as a general proper noun in various contexts, including organizations, media, and product names.