23.5 The Evolution of Binary Star Systems

Binary Star System Evolution and Supernovae

Binary star systems contain two stars orbiting a shared center of mass. When these stars are close enough, they can exchange material, leading to some of the most dramatic events in astronomy: novae and supernovae. Understanding how binaries evolve is essential because these systems produce Type Ia supernovae, which serve as key tools for measuring cosmic distances.

Characteristics of Nova-Producing Binaries

A nova occurs in a close binary system where a white dwarf orbits near a companion star (typically a main sequence or giant star). The key ingredient is mass transfer: material flows from the companion onto the white dwarf.

Here's how the nova cycle works:

1. The companion star expands or fills its Roche lobe (the region around a star where material is gravitationally bound to it). Gas spills over toward the white dwarf.
2. The transferred material, mostly hydrogen, doesn't fall straight onto the white dwarf. Instead, it spirals inward and forms an accretion disk around it.
3. Hydrogen-rich material accumulates on the white dwarf's surface, where gravity compresses and heats it.
4. When the temperature and pressure at the base of this layer get high enough, thermonuclear fusion ignites and rapidly spreads across the surface.
5. The resulting explosion ejects the accumulated material and causes a sudden brightening of the system. This is the nova.
6. After the outburst, the white dwarf remains intact, and the cycle can repeat. Some systems, like RS Ophiuchi and T Pyxidis, are recurrent novae that have been observed erupting multiple times.

Note that Sirius and Procyon are binary systems containing white dwarfs, but they are not close enough for active mass transfer. They're useful examples of white-dwarf binaries, not nova-producing systems.

Conditions for Type Ia Supernovae

A Type Ia supernova is far more destructive than a nova. Instead of a surface explosion, the entire white dwarf is destroyed. This happens through the following process:

1. A carbon-oxygen white dwarf in a binary system accretes material from its companion (which can be a main sequence star, giant, or even another white dwarf).
2. As mass piles on, the white dwarf's total mass climbs toward the Chandrasekhar limit of approximately $1.4$ solar masses. This is the maximum mass a white dwarf can support against gravitational collapse through electron degeneracy pressure.
3. As the white dwarf approaches this limit, conditions in its core become extreme. Temperature and pressure rise to the point where carbon fusion ignites.
4. Unlike a nova (which is a surface event), this carbon ignition triggers a thermonuclear runaway that rips through the entire white dwarf in seconds.
5. The white dwarf is completely disrupted. No compact remnant (no neutron star, no black hole) is left behind.

Because the explosion always occurs near the same mass ($\approx 1.4$ solar masses), Type Ia supernovae release a very consistent amount of energy and reach a predictable peak luminosity. This makes them excellent standard candles for measuring distances across the universe. Historical examples include SN 1572 (Tycho's supernova) and SN 1006.

Type Ia vs. Type II Supernovae

These two types of supernovae have fundamentally different origins:

Type Ia supernovae:

1. Originate from carbon-oxygen white dwarfs in binary systems accreting mass from a companion
2. Triggered when the white dwarf reaches the Chandrasekhar limit and carbon ignites
3. Leave no compact remnant behind
4. Have consistent peak luminosity (making them reliable standard candles)
5. Lack hydrogen lines in their spectra (the white dwarf's hydrogen was long gone)

Type II supernovae:

1. Originate from the core collapse of massive stars (greater than $\sim 8$ solar masses) at the end of their lives
2. Triggered when the iron core becomes too massive to support itself against gravity
3. Leave behind a compact remnant (neutron star or black hole)
4. Have varying peak luminosity depending on the progenitor star's mass and structure
5. Show hydrogen lines in their spectra (the massive star still had its hydrogen envelope)

Advanced Binary Evolution Processes

Two additional processes shape how binary systems evolve:

• Common envelope phase: When one star in a binary expands enough to engulf its companion, both stars end up orbiting inside a shared gas envelope. Friction causes the orbit to shrink rapidly. The stars may spiral closer together or even merge. This phase can produce the very tight binary systems that later give rise to novae or Type Ia supernovae.
• Gravitational waves: Close binary systems (especially those with compact objects like white dwarfs or neutron stars) emit gravitational waves, which carry energy away from the system. Over time, this causes the orbit to slowly shrink, potentially bringing the two objects close enough to merge.