A white dwarf becomes a nova when it accretes enough mass from a companion star to trigger a thermonuclear runaway reaction in its outer layers. This process is known as a nova eruption, and it can cause the white dwarf to briefly outshine its companion star by a factor of thousands or even millions.

Nova eruptions are thought to be caused by a combination of two factors:

1. The accumulation of hydrogen-rich material on the surface of the white dwarf from the companion star.

2. The compression and heating of this material due to the strong gravitational pull of the white dwarf.

When the hydrogen-rich material reaches a critical density and temperature, it undergoes a thermonuclear fusion reaction, releasing a tremendous amount of energy in a very short period of time. This energy outburst causes the white dwarf to undergo a rapid expansion, and the outer layers are ejected into space.

The ejected material from a nova eruption is rich in heavy elements, including carbon, nitrogen, and oxygen. These elements are synthesized in the thermonuclear reaction that powers the nova eruption, and they are subsequently dispersed into the surrounding interstellar medium. Nova eruptions are therefore thought to play an important role in the chemical enrichment of the galaxy.

Nova eruptions can also have a profound impact on the companion star. In some cases, the nova eruption can cause the companion star to lose a significant amount of mass, which can lead to its eventual destruction. In other cases, the nova eruption can actually trigger the companion star to evolve into a red giant star.

Nova eruptions are a fascinating and important phenomenon in the universe. They are a reminder of the delicate balance that exists between gravitational collapse and thermonuclear fusion in white dwarf stars.