Binary Stars and Multiple Star Systems

## Binary and Multiple Stars

The majority of stars in the Milky Way are not solitary — over 50% of Sun-like stars and up to 80% of massive O-type stars exist in gravitationally bound systems of two or more stars.

### Types of Binary Systems

| Type | Detection Method | Example |
|------|------------------|---------|
| Visual binary | Resolved in telescope | Albireo, Castor |
| Spectroscopic binary | Doppler shifts in spectra | Mizar A, Spica |
| Eclipsing binary | Periodic brightness dips | Algol, Beta Lyrae |
| Astrometric binary | Wobble in position | Sirius (historically) |

### Visual Binaries

Stars resolvable as two separate points in a telescope. The orbital period depends on separation and total mass — from decades (close pairs) to millennia (wide pairs).

Famous visual binaries:

| System | Separation | Period | Colors |
|--------|------------|--------|--------|
| Albireo (Beta Cyg) | 34" | ~100,000 yr | Gold + blue |
| Alpha Centauri A/B | 4-22" | 79.9 yr | Yellow + orange |
| Castor | 5" | 467 yr | White + white |
| Almach (Gamma And) | 10" | ~5,000 yr | Gold + blue-green |

### Eclipsing Binaries

**Algol** (Beta Persei) is the prototype eclipsing binary. Every 2.867 days, a dimmer K-type subgiant passes in front of a brighter B8 main-sequence star, causing the system's brightness to drop from magnitude +2.1 to +3.4 for about 10 hours.

Algol is also a semi-detached system — the less massive star is larger because it has transferred mass to its companion. This "Algol paradox" (the less massive star appears more evolved) is explained by mass transfer altering the stars' evolution.

### Mass Transfer and Evolution

In close binaries, stars can exchange material through Roche lobe overflow:

1. **Detached**: Both stars within their Roche lobes — no interaction
2. **Semi-detached**: One star fills its Roche lobe, matter streams to companion (Algol-type)
3. **Contact**: Both stars overflow, sharing a common envelope (W UMa-type)

Mass transfer creates exotic objects:
- **Cataclysmic variables**: White dwarf accreting from a companion → nova eruptions
- **X-ray binaries**: Neutron star or black hole accreting → X-ray emission
- **Type Ia supernovae**: White dwarf exceeds Chandrasekhar limit (1.4 M_sun) via accretion → thermonuclear explosion

### Hierarchical Multiples

Systems with 3+ stars are usually hierarchical — a close pair orbited by a distant companion. Castor is actually a sextuple system: three spectroscopic binaries arranged in a hierarchy. Alpha Centauri is a triple (A-B close pair + distant Proxima).

The current record-holder for most confirmed components is the septuple system Nu Scorpii, with seven stars in a nested hierarchy.