Stellar Physics
Binary Stars
Two stars orbiting each other — most stars in galaxy are in multiples
Binary stars are systems of two stars orbiting their common center of mass. ~50% of all stars are in binaries (or higher multiples — triple, quadruple). Many famous: Sirius (A + B = white dwarf), Algol (eclipsing), Cygnus X-1 (BH + companion), neutron star binaries. Eclipsing binaries (orbit edge-on) reveal stellar masses, radii. Mass-luminosity relation calibrated by binaries. Orbital period from days to millions of years. Dynamical effects: mass transfer (cataclysmic variables, X-ray binaries), explosions (Type Ia SN from white dwarf in binary).
- Fraction of stars~50%+ in binaries or higher
- Period rangeHours (close) to Myr (wide)
- Famous binarySirius A + B (8.6 ly); 50-year orbit
- Mass measurementDirect from orbit (Kepler's third law)
- Eclipsing binariesEdge-on; periodic dimming
- ImportantType Ia SN; X-ray binaries; mass measurement
Interactive visualization
Press play, or step through manually. The visualization is yours to drive — try it before reading on.
Watch the 60-second explainer
A condensed visual walkthrough — narrated, captioned, under a minute.
Why binaries matter
- Mass measurement. Direct from orbits.
- Stellar evolution. Mass transfer alters paths.
- End-state physics. Type Ia, kilonovae.
- X-ray binaries. Accretion physics.
- Habitable zones. Special considerations.
- Star formation. Many stars start as binaries.
- Test pulsars. Binary pulsars test GR.
Common misconceptions
- All stars are single. ~50% in binaries.
- Binaries unstable. Most stable for billions of years.
- Binaries have to merge. Most don't.
- Binaries make poor habitable conditions. Some are habitable.
- Eclipsing binaries are exact alignment. Need approximately edge-on.
- Mass transfer always disruptive. Sometimes; often gradual.
Frequently asked questions
How are binaries detected?
(1) Visual binary — both stars resolved by telescope. (2) Astrometric — wobble in star's position from gravitational pull (now superseded by Gaia). (3) Spectroscopic — Doppler shifts in spectra (lines split or shift). (4) Eclipsing — periodic dimming. (5) X-ray — binary with compact companion accreting matter. Most binaries detected by combination of methods.
How are masses determined?
Kepler's third law: P² = (4π²/GM) a³, where M is total mass. From orbital period P and semi-major axis a: total mass. Mass ratio from velocity ratio (RV measurement) or angular separation. Result: individual masses. Eclipsing binaries: also gives radii. Best stellar mass-radius-luminosity relations from these systems.
What's an eclipsing binary?
Binary with orbit edge-on (i ≈ 90°). Periodic dimming as one star passes in front of the other. Light curve shape reveals: orbital period, mass ratio, radii, inclination. Famous: Algol (Beta Persei) — period 2.87 days. Most studied eclipsing binary: TY CrB. Eclipsing binaries are gold standard for stellar parameter measurement.
What about mass transfer?
Close binaries: mass can transfer between stars. Driver: Roche lobe overflow. Effects: stellar evolution altered. Cataclysmic variables (white dwarf + companion): novae, recurrent novae. X-ray binaries (NS or BH + companion): X-ray emission from accretion. Type Ia SN: WD reaches Chandrasekhar via mass transfer.
What's a contact binary?
Stars touching — share envelope. Same temperature. Common envelope. W UMa stars. Common in close orbits. Mass and angular momentum exchange complex. Some merger candidates eventually become merging stars. Different from non-contact close binaries.
Are most stars binary?
~50% confirmed; trend toward higher fraction at higher masses. O stars: ~80% in binaries. K dwarfs: ~30%. M dwarfs: ~25%. Higher-mass stars more often in binaries (formation conditions). Sun is single — somewhat unusual for G dwarfs, but not extreme.
Why is binary important?
(1) Mass measurement (gold standard). (2) Stellar evolution (mass transfer). (3) End states (Type Ia SN, neutron star mergers). (4) Habitable zone — complications from two stars. (5) Star formation — many young stars in binaries. (6) Stellar dynamics — angular momentum from binary fragments.