Binary Stars and Exotic Phenomena

A binary contains two stars bound by gravity and orbiting their common center of mass. Two points of light that merely appear close together on the sky need not be a physical pair. Triple and higher-order systems add further companions.

Companions are especially common among massive stars, while many low-mass stellar systems are single. Any quoted “binary fraction” needs a definition: counting systems with companions differs from counting individual stars that belong to multiple systems.[1]

Many binaries form when star-forming gas fragments, sometimes within a disk. Later encounters can rearrange systems. Two initially unbound stars cannot become a lasting pair through an isolated, purely gravitational two-body encounter alone: energy must be removed or exchanged with gas, tides, or additional bodies.[1]

Distance determines whether companionship becomes interaction

A wide pair may exchange little material. In a closer pair, a star’s expansion can bring its outer layers to a critical boundary called its Roche lobe. Surveys of massive O-type stars show that interaction is a central part of their evolution, rather than an exceptional ending.[2]

These terms describe the stars’ geometry and interaction. A system can move between configurations as the stars and orbit evolve.[3], [4]

In the usual circular, synchronized model, a Roche lobe is a three-dimensional boundary defined by the effective potential in the frame rotating with the orbit. This description combines gravity with the centrifugal term. The boundary is not a solid shell around the star.[5]

The two lobes meet at L₁, the inner Lagrange point: a saddle point in that potential. When the donor’s outer layers reach this region, gas can flow toward the companion. Which star overflows depends on its radius relative to its own lobe, not simply on which star looks bigger.[5]

Flowing gas also carries angular momentum. It may orbit the accretor in a disk, strike the star directly, or be guided by a strong magnetic field. A disk is therefore common but not automatic. Some material can escape from the system instead of becoming part of the companion.[6], [7]

Envelope loss can expose a hot stellar core. Accretion can spin up the receiving star and alter its subsequent evolution. Whether it also mixes fresh hydrogen into the core—and how much it appears to regain youth—depends on the star’s internal response.[8]

Mass loss changes the donor’s size, while changes in mass and orbital angular momentum reshape its Roche lobe. If these responses allow the donor to settle, transfer can continue in a regulated way. If overflow grows faster instead, the interaction can become unstable.[9]

A companion can become engulfed

In a common-envelope episode, the companion and the donor’s core orbit inside an extended gaseous envelope. Drag and gravitational torques transfer orbital energy and angular momentum to the gas, and the embedded pair can spiral closer together.[10]

How efficiently an envelope is ejected remains a major uncertainty. A contact binary with connected outer layers need not be undergoing this rapid engulfment and inspiral.[10], [3]

A merger can be caught in the light

Before its 2008 eruption, V1309 Scorpii showed the changing brightness of a contact binary with a shortening orbital period. The observations provide strong evidence that its red outburst accompanied a stellar merger. Such luminous red novae are physically distinct from the white-dwarf surface explosions called classical novae.[11]

A classical nova begins when a white dwarf accumulates fresh, usually hydrogen-rich gas from a companion. Compression and heating at the base of the accreted layer can ignite runaway nuclear burning. The heated envelope expands and material is expelled; the white dwarf itself survives.[12]

If accretion resumes, another nova can eventually occur. Recurrent novae are systems in which more than one such eruption has been observed. The recurrence time depends on the white dwarf’s mass, its thermal state, and the supply of fuel. A nova seen only once need not be a one-time event.[12]

A dwarf nova brightens in a different place

In a dwarf nova, an accretion disk switches between cooler and hotter states associated with hydrogen ionization. The disk’s rate of transporting and releasing accretion energy rises. This outburst does not require a thermonuclear explosion on the white dwarf.[7]

Many interacting white-dwarf binaries are called cataclysmic variables. Their diversity includes disk outbursts and strongly magnetic systems in which the flow is channeled toward magnetic poles.[7]

A normal Type Ia supernova is a thermonuclear explosion of a carbon–oxygen white dwarf. Runaway burning rapidly releases enough energy to disrupt the star. Its spectrum near maximum light typically lacks hydrogen and shows a prominent feature from singly ionized silicon. “Type Ia” is more specific than the broader hydrogen-poor category “Type I.”[13]

There is more than one route to ignition

A donor may be a nondegenerate star—the single-degenerate family—or another white dwarf, the double-degenerate family. These describe the companion, whereas “double detonation” describes ignition. The categories overlap. A pair of white dwarfs does not automatically explode merely because their combined mass exceeds a limit.[13]

Why these explosions matter far beyond their home galaxy

Type Ia supernovae eject iron-group nuclei, and radioactive decay helps power their changing light.[14] Their peak luminosities are not identical. A relationship between peak brightness and the rate of fading, together with color and other corrections, makes them standardizable candles for estimating distances.[17]

Comparisons between distant supernovae and nearby examples supplied the 1998–1999 evidence that cosmic expansion is accelerating. Their usefulness depends on careful calibration and control of dust, population differences, and observational biases.[18], [19]

When the receiver is a neutron star or black hole, infalling matter can release large amounts of gravitational energy. A companion may feed this accretion through Roche-lobe overflow or a stellar wind. The resulting X-ray binary lets us study a remnant through its surroundings.[20]

A neutron star has a material surface on which fuel can accumulate. A black hole has an event horizon; radiation and escaping jets come from processes outside it. Similar-looking brightening therefore need not imply the same engine.[21], [20]

Orbiting compact objects emit gravitational waves, carrying away orbital energy and angular momentum. A sufficiently tight pair spirals inward, eventually producing a signal whose frequency rises toward merger. A wide pair may take much longer than the present age of the universe to reach that stage.[23]

There are several ways to assemble the pair

Two stars born together can exchange matter and survive the mass loss and kicks associated with remnant formation. Encounters in dense clusters can also bring remnants together or exchange their partners. A third body can alter an inner binary’s eccentricity and help it merge. The contributions of these channels remain under investigation.[24]

The previous article, Nucleosynthesis: Elements Heavier than Iron, follows what happens to that newly assembled matter.

Most binaries cannot be watched as two clearly separated stars exchanging gas. Researchers combine changing brightness, spectra, positions, and precise timing to reconstruct the system. No single observing method supplies every property.

Combining eclipses with the radial velocities of both stars can yield especially precise stellar masses and radii. Such measurements test the stellar models used throughout this chapter.[31], [7], [12]

Population patterns test the unseen stages

A short-lived common envelope is hard to observe directly, but its possible descendants remain. Researchers compare populations of close binaries and mergers with models that vary transfer efficiency, envelope ejection, and remnant kicks. Several histories can produce similar endpoints, so matching one system does not establish a universal route.[10], [24]

LISA is designed to extend gravitational-wave astronomy into the millihertz band, including many Galactic compact binaries. Observing systems while they are still orbiting, and combining their waves with light, will help connect surviving binaries with the mergers and explosions they may eventually produce.[28], [32]

Companions also change what stars return to space. Stripping, winds, novae, thermonuclear supernovae, and neutron-rich merger ejecta contribute different material on different timescales. Binary evolution links the fate of individual stars to the chemical history of galaxies.[13], [12], [26]