Neutron Stars and Pulsars

## Neutron Stars: Extreme Physics

When a massive star (8-25 solar masses) exhausts its nuclear fuel, its iron core collapses in milliseconds. Protons and electrons are crushed together to form neutrons, creating a neutron star — an object so dense that a teaspoon of its material would weigh about 6 billion tons on Earth.

### Physical Properties

| Property | Value |
|----------|-------|
| Mass | 1.1 - 2.3 solar masses |
| Radius | ~10-12 km |
| Density | ~10^17 kg/m3 |
| Surface gravity | ~2 x 10^11 m/s2 (200 billion g) |
| Surface temperature | 600,000 - 1,000,000 K (young) |
| Magnetic field | 10^8 - 10^15 gauss |
| Escape velocity | ~100,000 km/s (1/3 speed of light) |

The interior of a neutron star contains matter in states not reproducible in any laboratory. The outer crust is a lattice of iron nuclei. Below that, increasingly neutron-rich nuclei exist. The inner core may contain a superfluid of free neutrons, quark matter, or exotic particles.

### Pulsars: Cosmic Lighthouses

Jocelyn Bell Burnell discovered the first pulsar in 1967 — a radio source pulsing every 1.337 seconds (PSR B1919+21). Initially nicknamed "LGM-1" (Little Green Men), it was soon understood as a rapidly rotating neutron star with misaligned magnetic poles.

The lighthouse model: a neutron star's magnetic axis is tilted relative to its rotation axis. Charged particles accelerated along magnetic field lines emit radiation in narrow beams. As the star rotates, these beams sweep past Earth, creating regular pulses.

### Types of Pulsars

| Type | Period | Example | Notes |
|------|--------|---------|-------|
| Normal | 0.1-5 s | Crab Pulsar (33 ms) | Young, slowing down |
| Millisecond | 1-10 ms | PSR B1937+21 (1.56 ms) | Spun up by accretion |
| Magnetar | 2-12 s | SGR 1806-20 | Extreme B field (~10^15 G) |
| X-ray | Variable | Cen X-3 | Accreting in binary system |

The **Crab Pulsar** (PSR B0531+21) rotates 30 times per second and powers the Crab Nebula — the supernova remnant from the 1054 CE explosion. It is slowing at a rate of 36.4 nanoseconds per day, converting rotational energy into the nebula's glow.

### Millisecond Pulsars

Old pulsars in binary systems can be "recycled" — spun up to millisecond periods by accreting matter from a companion star. The fastest known, PSR J1748-2446ad, rotates 716 times per second — its equator moves at 24% the speed of light.

### Gravitational Wave Sources

Binary neutron star mergers, first detected by LIGO on August 17, 2017 (GW170817), are sources of gravitational waves. The merger was simultaneously observed in gamma rays, optical light, and radio — confirming that such events produce heavy elements like gold and platinum through the r-process.