Pulsars & Neutron Stars
Compact stellar remnants — clocks more precise than your watch, spinning ~1000× per second.
A massive ($\gtrsim 8 M_\odot$) star ends life in a core-collapse supernova; if the remnant core mass is below ~$3 M_\odot$, it forms a neutron star: $M \sim 1.4 M_\odot$ packed into $R \sim 10$ km. Density at the core exceeds nuclear matter density.
Conservation of angular momentum and magnetic flux during collapse produces enormous angular velocities (Crab pulsar: $30$ Hz; millisecond pulsars: $\sim 700$ Hz) and surface magnetic fields $\sim 10^{12}$ G (10¹⁵ for magnetars).
If the magnetic dipole axis is misaligned with the rotation axis, radio/X-ray beams from the magnetic poles sweep past Earth like a lighthouse — the pulsar. Period stability of millisecond pulsars rivals atomic clocks ($\sim 10^{-15}$ fractional).
Hulse–Taylor binary pulsar (PSR B1913+16, 1974) showed orbital period decay matching GR's gravitational-wave prediction to <0.2% — first indirect detection of gravitational waves (Nobel 1993).