Pulsars: Cosmic Lighthouses

When a large star collapses to neutron-star size, conservation of angular momentum spins it up dramatically. A star turning once a month can become a neutron star spinning hundreds of times per second.

If one of those beams sweeps past Earth, we pick up a pulse of radio waves with every rotation. The fastest known pulsar spins 716 times per second, so its surface moves at nearly a quarter of the speed of light. Pulsars are the most precise natural clocks we know of.