novas

A nova, plural novae, is a transient astronomical event that causes a dramatic, but temporary, brightening of a star. Novae occur in binary star systems in which a white dwarf accretes hydrogen-rich material from a close companion. The accreted layer builds up on the white dwarf’s surface until thermonuclear fusion ignites explosively, ejecting part of the envelope and markedly increasing the system’s luminosity for days to months before returning to quiescence.

The mechanism involves mass transfer from a donor star to an accretion disk around the white dwarf. Material from the disk settles onto the white dwarf, gradually increasing pressure and temperature in the underlying layer. When conditions reach a critical point, a thermonuclear runaway occurs, causing the accumulated layer to expand and blow off. The white dwarf itself remains intact and can undergo subsequent nova eruptions if accretion continues.

Novae exhibit a range of observational properties. They rise rapidly over days to weeks and then decline over weeks to months. Spectra initially show strong emission lines, often classified as Fe II-type or He/N-type, reflecting the ejected gas’s composition and ionization state. Ejected masses are typically around 10^-5 to 10^-4 solar masses, with expansion velocities from a few hundred to a few thousand kilometers per second. Peak apparent magnitudes vary, but novae can brighten by about 8 to 15 magnitudes relative to quiescence.

Types include classical novae, recurrent novae (with shorter recurrence times between outbursts), and dwarf novae (different phenomena driven by accretion-disk instabilities rather than thermonuclear runaways). Novae occur in the Milky Way and in other galaxies and contribute to the chemical enrichment of the interstellar medium.