Neutron star

A neutron star is the gravitationally collapsed core of a massive supergiant star. These objects are the second densest and second smallest known class of stellar objects. Only black holes are known to be denser than a neutron star. They typically have a mass of about 1.4 solar masses. However, their radius is incredibly small, often around 12 kilometers. This combination of massive weight and tiny size creates extreme physical conditions.

The formation of a neutron star is a violent, multi-step process. It begins with a main-sequence star having an initial mass greater than eight solar masses. As the star evolves, stellar nucleosynthesis creates an iron-rich core. When the nuclear fuel is exhausted, the core must be supported by degeneracy pressure. Eventually, the core exceeds the Chandrasekhar limit. This causes the core to collapse, raising temperatures above 10 billion kelvin. At these temperatures, photodisintegration breaks iron nuclei into alpha particles.

As the collapse continues, a process called electron capture occurs. During this stage, electrons and protons combine to form neutrons. This reaction releases a massive flood of neutrinos. The core continues to shrink until it reaches nuclear density. At this point, neutron degeneracy pressure and strong force repulsion halt the contraction. The outer envelope of the star is then rapidly flung outward by the neutrino flux. This results in a supernova explosion, leaving the neutron star behind.

Neutron stars exhibit several distinct types and behaviors. Many are detected as pulsars, which are stars that emit beams of electromagnetic radiation. These beams make the star appear to pulse as it rotates. Some neutron stars exist in binary systems with a companion star. In these systems, the neutron star can pull gas from its companion through accretion. This process is called accretion.

Other neutron stars may be part of systems that eventually merge. The merger of two neutron stars can produce gravitational waves and a kilonova.

History shows how our understanding of these objects has grown. In 1967, Jocelyn Bell Burnell and Antony Hewish discovered pulsars. This was the first observational evidence that neutron stars actually existed. Later, scientists studied the Hulse–Taylor pulsar to find indirect evidence of gravitational waves. A major milestone occurred in 2017. The LIGO and Virgo interferometers detected GW170817. This was the first direct detection of gravitational waves from a neutron star merger.

The physical properties of neutron stars are truly extreme. Their surface gravity is billions of times stronger than Earth's gravity. This gravity is so powerful that the escape velocity exceeds half the speed of light. Infalling matter is accelerated to tremendous speeds, and tidal forces can cause spaghettification. The density is also staggering. A matchbox-sized piece of neutron star material would weigh about 3 billion tonnes.

Rotation speeds also vary wildly, from 1.4 milliseconds to 30 seconds. The fastest known, PSR J1748−2446ad, rotates 716 times per second.

Studying neutron stars is central to modern gravitational wave astronomy. They serve as natural laboratories for probing fundamental physics. Because we cannot replicate their density on Earth, we must study them from space. Scientists use them to study the equation of state, which describes how matter behaves under pressure. This research helps us understand quantum chromodynamics and the strong interaction. By observing these stars, we learn how the most extreme matter in the universe is structured.