Supernovae: The Death of Massive Stars

## Supernovae: Cosmic Cataclysms

A supernova releases more energy in seconds than the Sun will produce in its entire 10-billion-year lifetime. These explosions create neutron stars and black holes, forge elements heavier than iron, and trigger new rounds of star formation.

### Types of Supernovae

| Type | Mechanism | Progenitor | Peak Luminosity |
|------|-----------|------------|-----------------|
| Type Ia | Thermonuclear | White dwarf exceeding 1.44 M_sun | -19.3 abs mag (standard candle) |
| Type II | Core collapse | Red supergiant (8-25 M_sun) | -17 to -18 abs mag |
| Type Ib | Core collapse | Wolf-Rayet (no hydrogen envelope) | -17 to -18 abs mag |
| Type Ic | Core collapse | Wolf-Rayet (no hydrogen or helium) | -17 to -18 abs mag |

### Core-Collapse Supernovae (Type II)

When a massive star builds an iron core exceeding ~1.4 M_sun:

1. **Core collapse**: The iron core collapses from ~6,000 km to ~20 km in less than a second
2. **Neutron star formation**: Protons and electrons combine into neutrons; the core rebounds
3. **Shock wave**: The bounce launches a shock wave outward through the star's layers
4. **Neutrino burst**: 99% of the gravitational energy (~3 x 10^46 joules) escapes as neutrinos in ~10 seconds
5. **Explosion**: The shock, revived by neutrino heating, disrupts the star — ejecting 5-15 M_sun of material at 10,000-30,000 km/s

### Historical Supernovae

| Year | Designation | Galaxy | Peak Magnitude | Remnant |
|------|-------------|--------|----------------|---------|
| 185 CE | SN 185 | Milky Way | -8 (est.) | RCW 86 |
| 1006 | SN 1006 | Milky Way | -7.5 | PKS 1459-41 |
| 1054 | SN 1054 | Milky Way | -6 | Crab Nebula (M1) |
| 1572 | SN 1572 (Tycho) | Milky Way | -4 | Tycho SNR |
| 1604 | SN 1604 (Kepler) | Milky Way | -2.5 | Kepler SNR |
| 1987 | SN 1987A | LMC (168,000 ly) | +2.9 | Ring structures |

**SN 1987A** in the Large Magellanic Cloud was the nearest supernova since 1604 and the first detected by its neutrino burst (24 neutrinos captured by three detectors worldwide).

### Nucleosynthesis

Elements up to iron are created by stellar fusion. Elements heavier than iron — gold, platinum, uranium — are primarily created in the extreme conditions of supernovae and neutron star mergers through rapid neutron capture (r-process).

Every atom of calcium in your bones, iron in your blood, and oxygen you breathe was forged in a star that exploded billions of years ago.

### Supernova Remnants

The expanding debris cloud interacts with the interstellar medium for thousands of years:
- **Young** (< 1,000 yr): Crab Nebula — powered by a central pulsar
- **Middle-aged** (1,000-10,000 yr): Vela SNR — shock waves visible
- **Old** (> 10,000 yr): Cygnus Loop — thin filaments of cooling gas