Variable Stars: Why Some Stars Change Brightness

## Variable Stars

A variable star is any star whose apparent brightness changes over time. Over 500,000 variable stars are catalogued, with mechanisms ranging from geometric eclipses to thermonuclear explosions.

### Classification

| Category | Mechanism | Example | Period |
|----------|-----------|---------|--------|
| Eclipsing | Orbital geometry | Algol | 2.867 days |
| Pulsating (Cepheid) | Radial oscillation | Delta Cephei | 5.366 days |
| Pulsating (RR Lyrae) | Radial oscillation | RR Lyrae | 0.567 days |
| Pulsating (Mira) | Radial oscillation | Mira (Omicron Ceti) | 332 days |
| Eruptive | Flares, outbursts | UV Ceti, T Tauri | Irregular |
| Cataclysmic | Accretion/explosion | SS Cygni (dwarf nova) | ~50 days |
| Rotating | Starspots, obliquity | BY Draconis, Alpha2 CVn | Days |

### Cepheid Variables: Cosmic Yardsticks

Cepheid variables are supergiant stars that pulsate radially with periods of 1-100 days. In 1908, Henrietta Swan Leavitt discovered that brighter Cepheids pulsate more slowly — the **period-luminosity relation**:

M_V = -2.43 x (log P - 1) - 4.05

where P is the period in days and M_V is the absolute visual magnitude.

This relationship is fundamental to measuring cosmic distances. By observing a Cepheid's period (easy) and apparent brightness, astronomers calculate its distance. Edwin Hubble used Cepheids in M31 to prove that galaxies exist beyond the Milky Way (1924) and to discover the expansion of the universe (1929).

### RR Lyrae Stars

Shorter-period cousins of Cepheids, RR Lyrae stars are old, low-mass horizontal-branch stars with periods of 0.2-1 day. They are found in globular clusters and the galactic halo, serving as standard candles for distances within the Milky Way.

| Type | Luminosity | Period | Population | Use |
|------|-----------|--------|------------|-----|
| Classical Cepheid | 300-30,000 L_sun | 1-100 days | Young (disk) | Extragalactic distances |
| RR Lyrae | ~50 L_sun | 0.2-1 day | Old (halo) | Milky Way distances |
| Mira | 1,000-10,000 L_sun | 80-1,000 days | AGB stars | Distance, mass loss |

### Mira Variables

**Mira** (Omicron Ceti) is the prototype long-period variable. It changes from magnitude +2.0 (easily visible) to +10.1 (needs a telescope) and back over 332 days. Mira variables are AGB stars undergoing thermal pulses and heavy mass loss — they are in the final stages of evolution before becoming planetary nebulae.

### Amateur Observations

Variable star observation is one of the few areas where amateur astronomers make scientifically valuable contributions. The American Association of Variable Star Observers (AAVSO) coordinates a global network that has collected over 50 million brightness estimates since 1911.