millisecond pulsar

Some pulsars spin so fast they defy belief — hundreds of times every second. A city-sized ball of neutron-star matter, heavier than the Sun, whirling around so quickly that its equator is moving at a sizeable fraction of the speed of light. These are millisecond pulsars, named because the interval between pulses is just a few thousandths of a second. The fastest known turns about 716 times per second; if it spun much faster it would start to fly apart.

Here is the puzzle: a young pulsar slows down over time as it radiates energy, so by the time a neutron star is old it should be spinning lazily, not faster than a kitchen blender. The resolution is a process called 'recycling'. An old, slow neutron star in a binary system pulls gas from its companion, and that infalling gas carries angular momentum — it spins the neutron star up, like water from a hose speeding up a water wheel. Over millions of years the dead pulsar is spun back to life as a blazingly fast millisecond pulsar.

Millisecond pulsars are the most precise natural clocks known, some stable to better than a microsecond over decades — which is exactly why they are the backbone of pulsar timing arrays hunting for gravitational waves. They also tell a story of stellar partnership: finding one almost always means there is, or was, a companion star that fed it. Some 'black widow' and 'redback' systems even show the millisecond pulsar now evaporating the very companion that spun it up — a stellar tale of being brought back to life, then devouring your benefactor.