White Dwarfs and Planetary Nebulae

## White Dwarfs and Planetary Nebulae

Stars between about 0.5 and 8 solar masses end their lives as white dwarfs — the exposed cores of dead stars. The transition produces some of the most beautiful objects in the sky: planetary nebulae.

### The Death of a Sun-Like Star

1. **Asymptotic Giant Branch (AGB)**: After core helium burning, the star expands to 200+ solar radii with thermal pulses
2. **Mass loss**: Strong stellar winds strip the outer layers at rates up to 10^-4 M_sun/year
3. **Planetary nebula**: The hot core ionizes the ejected shell, creating a glowing nebula lasting ~20,000 years
4. **White dwarf**: The remaining core (0.5-1.0 M_sun) slowly cools over billions of years

### White Dwarf Properties

| Property | Typical Value |
|----------|---------------|
| Mass | 0.5-1.0 M_sun (peak at 0.6) |
| Radius | ~6,000 km (Earth-sized) |
| Density | ~10^9 kg/m3 (1 ton per cm3) |
| Surface temperature | 8,000-180,000 K (hot, then cooling) |
| Composition | Carbon-oxygen (most), helium, or oxygen-neon |
| Support mechanism | Electron degeneracy pressure |

The **Chandrasekhar limit** (1.44 M_sun) is the maximum mass a white dwarf can sustain. Above this, electron degeneracy pressure fails and the star collapses further (to a neutron star or triggers a Type Ia supernova).

### Famous White Dwarfs

| Star | Mass | Temperature | Distance |
|------|------|-------------|----------|
| Sirius B | 1.02 M_sun | 25,200 K | 8.6 ly |
| Procyon B | 0.60 M_sun | 7,740 K | 11.5 ly |
| 40 Eridani B | 0.57 M_sun | 16,500 K | 16.3 ly |
| Van Maanen's Star | 0.68 M_sun | 6,130 K | 14.1 ly |

**Sirius B** was the first white dwarf discovered (1862). It orbits Sirius A every 50 years — despite having nearly the mass of the Sun, it is roughly the size of Earth.

### Spectacular Planetary Nebulae

| Nebula | Catalogue | Distance | Angular Size | Central Star Temp |
|--------|-----------|----------|-------------|-------------------|
| Ring Nebula | M57 | 2,570 ly | 1.4' x 1.0' | 120,000 K |
| Dumbbell Nebula | M27 | 1,360 ly | 8.0' x 5.7' | 85,000 K |
| Cat's Eye Nebula | NGC 6543 | 3,300 ly | 22" | 48,000 K |
| Helix Nebula | NGC 7293 | 655 ly | 25' x 12' | 120,000 K |
| Eskimo Nebula | NGC 2392 | 5,000 ly | 48" | 40,000 K |

The term "planetary nebula" is a misnomer from William Herschel (1785), who noted their round, planet-like appearance in small telescopes. They have nothing to do with planets.

### Cooling Timescale

White dwarfs cool predictably, serving as cosmic clocks. The oldest white dwarfs in the Milky Way disk (~11 billion years old) have cooled to about 4,000 K — the coolest are nearly invisible, with luminosities below 0.0001 L_sun.