A white dwarf will become a nova if a. the original star was more than \(1.4 \mathrm{M}_{\odot}\) b. it accretes an additional \(1.4 M_{\circ}\) from a companion. c. even a small amount of mass falls on it from a companion. d. enough mass accretes from a companion to give the white dwarf a total mass of \(1.4 M_{\odot}.\)

A white dwarf is a stellar remnant composed mainly of electron-degenerate matter. These stars are incredibly dense and have a mass limit known as the Chandrasekhar limit. This limit is approximately 1.4 times the mass of our Sun, denoted as 1.4 \( \mathrm{M}_{\odot} \).

The Chandrasekhar limit represents the maximum mass that a white dwarf can have while being supported against gravitational collapse by electron degeneracy pressure. If the mass exceeds this limit, the white dwarf cannot support itself and will collapse, potentially leading to a supernova explosion.

This concept is critical in understanding the life cycle of stars and stellar remnants. It explains why white dwarfs have a uniform upper mass and why they can become explosive under certain conditions.

A nova is a spectacular astronomical event involving a sudden, dramatic increase in brightness. This phenomenon occurs in a binary star system where a white dwarf accretes mass from a companion star.

Here’s how a nova happens:

• The white dwarf pulls in hydrogen-rich material from its companion.
• As this material accumulates on the white dwarf's surface, it heats up due to gravitational compression.
• Once the temperature and pressure are high enough, nuclear fusion ignites in the accumulated layer.
• This ignition causes a sudden, explosive release of energy, resulting in a brightening called a nova.

Note that a nova is distinct from a supernova, which involves the white dwarf reaching the Chandrasekhar limit and collapsing. In contrast, a nova can repeat several times as long as the white dwarf continues to accrete material from its companion.

Mass accretion is the process by which a white dwarf accumulates additional matter from a companion star in a binary system. This material often comes from the companion star's outer layers, which are pulled in by the white dwarf's intense gravitational field.

The process involves:

• Roche Lobe Overflow: The companion star expands or is close enough that its outer envelope spills over into the gravitational influence of the white dwarf.
• Accretion Disk Formation: The material forms an accretion disk as it spirals inwards, heating up and emitting light due to friction.
• Surface Accumulation: The accreted material eventually lands on the surface of the white dwarf.

Mass accretion is crucial in the evolution of white dwarf systems. It can lead to critical mass accumulation, causing events like novae or possibly supernovae if the white dwarf reaches the Chandrasekhar limit. Understanding this process helps explain the dynamic and often explosive behavior seen in binary star systems.