Neutron star is the smallest and densest type of star known. Neutron stars measure only about 12 miles (20 kilometers) across, but they have a mass between 1.4 times and 3 times that of the sun.
A neutron star can form when a fairly large star—roughly 8 to 11 times as massive as the sun or larger—runs out of fuel to burn. The star’s intense gravitational force causes it to collapse. Gravity squeezes together the protons and electrons in the core of the star. These protons and electrons combine to form neutrons. The neutrons may, in turn, break down into elementary particles called quarks. The star then explodes as a supernova, leaving behind its spinning core—a neutron star.
Scientists believe that the surface of a neutron star consists of a solid crust of atomic nuclei and electrons. Beneath the crust, the neutron star’s core contains mainly neutrons. The center of the neutron star may contain quarks. An extremely thin atmosphere of plasma(charged particles) may surround the crust.
A neutron star has a magnetic field billions of times stronger than the most powerful magnets on Earth. This magnetic field creates an electric field that rips electrons and protons from the star’s surface. In some cases, these particles produce a beam of radio waves, X rays, or other radiation that flows from the star.
Physicists predicted the existence of neutron stars in 1938. The prediction remained a theory until 1967. In that year, radio telescopes in the United Kingdom picked up regular bursts of radio waves from an object in space. Scientists later concluded that such objects, called pulsars, are actually neutron stars. In 1979, scientists first observed high-energy radiation from a type of neutron star similar to a typical pulsar, but with a magnetic field 100 to 1000 times as strong. These rare neutron stars were later called magnetars.
Some neutron stars orbit with a companion star. The intense gravitational field of the neutron star pulls material from its companion. This process is called accretion. The material falls into the neutron star, releasing a large amount of energy in the form of X rays. The process of accretion can cause a neutron star to spin at nearly 600 rotations each second. When accretion stops, these neutron stars can become special types of pulsars called millisecond radio pulsars.
In August 2017, astronomers observed an event called a kilonova (an explosion caused by the collision of two neutron stars orbiting each other). The collision released an incredible amount of energy in the form of radio waves, X rays, gamma rays, visible light, and even gravitational waves, detected by observatories on Earth. Scientists determined that neutrons released at high speed by the kilonova collided with atoms in the surrounding space to form new chemical elements heavier than zirconium. Scientists think that most atoms of such heavy elements in the universe, including gold, platinum, and uranium, formed in such kilonova events.
