How does a nova occur?

A nova occurs when a white dwarf star accretes material from a companion star, causing a thermonuclear explosion.

Novae are transient astronomical events that occur in binary star systems. They are caused by a white dwarf star accreting material from a companion star, usually a red giant. As the white dwarf accretes material, it becomes more massive and its temperature and pressure increase. When the temperature and pressure reach a critical point, a thermonuclear explosion occurs on the surface of the white dwarf.

The explosion releases a tremendous amount of energy, causing the white dwarf to brighten significantly. The explosion also ejects material into space at high speeds, creating a shell of gas around the white dwarf. This shell can be observed as a nebula surrounding the nova.

Novae are different from supernovae in that they do not completely destroy the white dwarf star. Instead, the white dwarf remains intact and can continue to accrete material from its companion star, leading to future nova events.

The study of novae is important for understanding the evolution of binary star systems and the role of white dwarf stars in the universe. Novae can also be used as distance indicators, allowing astronomers to measure the distances to other galaxies.