Novae

Novae are dramatic explosions on the surface of white dwarfs in close binary systems. In a cataclysmic variable, a white dwarf accretes hydrogen-rich matter from a companion star. When enough material accumulates and the pressure-temperature at the base of the accreted layer reach thermonuclear ignition, a runaway fusion reaction occurs, expelling the outer envelope at high velocity. The white dwarf itself survives, often remaining relatively unaffected, and the system can undergo further outbursts if accretion continues and the surface layer re-accumulates.

Observationally, novae suddenly brighten by several magnitudes, peaking at luminous levels that can outshine the host binary for days to weeks before fading over weeks to months. Their spectra evolve from absorption-dominated precursors to emission-line dominated spectra, featuring hydrogen Balmer lines and often iron (Fe II) or helium–nitrogen (He/N) lines. Ejected material typically travels at hundreds to thousands of kilometers per second. Some novae emit supersoft X-rays after the initial outburst as the white dwarf surface continues to burn hydrogen.

There are two commonly used classes: classical novae, which are single observed outbursts; and recurrent novae, which repeat on timescales of decades to a century. A related but distinct phenomenon is the dwarf nova, caused by accretion-disk instabilities, not a thermonuclear eruption.

Historically, novae were described as new stars appearing in the sky. Notable examples include Nova Aquilae 1918 and Nova Persei 1901. In astronomy, novae are studied for insights into binary evolution, nucleosynthesis, and their potential contribution to white dwarf growth, though they are not the standard path to a Type Ia supernova.