Accumulation of material from a companion star

In a binary star system, a nova event on the surface of a white dwarf is triggered by the accumulation of material from a companion star. This process occurs in a close binary system where the white dwarf and the companion star are gravitationally bound to each other.

Binary Star System:
- A binary star system consists of two stars orbiting around a common center of mass. In a close binary system, the stars are located in close proximity to each other.
- The more massive star in the system evolves faster and eventually becomes a white dwarf, while the less massive star may still be in its main sequence phase.

Accretion of Material:
- The white dwarf in a close binary system has a strong gravitational pull. As the companion star evolves, it expands and starts transferring material onto the white dwarf.
- The transferred material forms an accretion disk around the white dwarf, composed of gas and dust.
- This material gradually spirals inward due to the gravitational attraction of the white dwarf.

Thermonuclear Runaway:
- As the accreted material accumulates on the surface of the white dwarf, it increases the pressure and temperature in the layer.
- When the temperature reaches a critical point, a thermonuclear runaway reaction is triggered. This reaction involves the fusion of hydrogen into helium.
- The sudden release of a large amount of energy from the thermonuclear reaction causes an explosive eruption on the surface of the white dwarf.

Nova Event:
- The explosion, known as a nova, results in a temporary increase in brightness of the white dwarf by several magnitudes. The nova event can last for a few days to several weeks.
- The explosion ejects the accumulated material into space, forming a shell of gas and dust around the white dwarf.
- After the nova event, the white dwarf gradually returns to its normal state and starts accumulating material again for future eruptions.

Conclusion:
In a binary system, a nova event on the surface of a white dwarf is triggered by the accumulation of material from a companion star. The transferred material forms an accretion disk, which eventually leads to a thermonuclear runaway reaction and an explosive eruption on the surface of the white dwarf.