A pulsar is a highly magnetized, rotating neutron star that emits beams of electromagnetic radiation from its magnetic poles. These beams sweep across the sky like a lighthouse as the star rotates, which can be detected as regular pulses of radiation when they point toward Earth.
Pulsars form when massive stars (8-20 solar masses) explode as supernovae. The core collapses under gravity, compressing protons and electrons into neutrons. This creates an incredibly dense neutron star, where a teaspoon of material would weigh billions of tons on Earth.
The first pulsar was discovered in 1967 by Jocelyn Bell Burnell and Antony Hewish. Initially, the regular pulses were so precise that they were nicknamed "LGM-1" (Little Green Men) as scientists briefly considered they might be signals from an alien civilization.
Pulsars rotate extremely rapidly, with periods ranging from milliseconds to seconds. This rapid rotation comes from the conservation of angular momentum during the collapse of the progenitor star. The fastest known pulsar, PSR J1748-2446ad, rotates at 716 times per second!
Pulsars have incredibly strong magnetic fields, typically around 10⁸ to 10¹⁵ times stronger than Earth's. The magnetic field is not aligned with the rotation axis, which is why the radiation beams sweep through space as the star rotates.
Despite having masses of about 1.4 solar masses, pulsars are only about 20 kilometers in diameter. Their density is comparable to that of an atomic nucleus, making them among the densest objects known in the universe.
The most common type, powered by the loss of rotational energy. They gradually slow down over time as they convert rotational energy into radiation. The Crab Pulsar is a famous example, formed in a supernova observed in 1054 CE.
These pulsars rotate hundreds of times per second. They are thought to be "recycled" pulsars that have been spun up by accreting matter from a companion star. They are extremely stable rotators, making them excellent cosmic clocks.
Pulsars with extraordinarily strong magnetic fields, up to 1000 times stronger than ordinary pulsars. They can produce powerful gamma-ray flares and are associated with soft gamma repeaters and anomalous X-ray pulsars.
The binary pulsar system PSR B1913+16 (Hulse-Taylor binary) provided the first indirect evidence for gravitational waves, as predicted by Einstein's theory of general relativity. The discoverers, Russell Hulse and Joseph Taylor, were awarded the Nobel Prize in Physics in 1993 for this work.
Pulsars can serve as cosmic lighthouses for spacecraft navigation. NASA's NICER mission is developing a pulsar-based navigation system that could one day guide spacecraft through the solar system with unprecedented accuracy.