When a massive star runs out of nuclear fuel, it forms an iron core that collapses into neutronium, a state of matter where electrons and protons fuse into neutrons. This collapse creates a supernova that results in a neutron star, which is incredibly dense and can emit radio or X-ray waves. Some neutron stars, called magnetars, have an extremely strong magnetic field.
A neutron star is the gravitationally collapsed core of a massive star. When large stars use up all their nuclear fuel, they form an iron core the size of the planet Jupiter, containing about 1.44 solar masses of material. Because fusion of iron nuclei requires more energy input than is produced, nuclear fusion no longer produces the core pressure needed to keep the star from collapsing on itself.
During the final moments of collapse, the phase of the giant star’s iron core transforms into neutronium, a state of matter in which all the electrons and protons in the iron atoms are fused together to produce nothing but neutrons. Because neutrons are neutral, they don’t repel each other the way negatively charged electron clouds do in conventional matter. Being pushed together by enormous gravitational energy, neutronium has a density similar to an atomic nucleus, and indeed the entire nucleus can be seen as one large atomic nucleus. Its source of light and heat cut off, the star’s outer layers fall inward, then bounce back after slamming into the nearly incompressible neutronium. The result is a supernova, a process that lasts from days to months.
The final result is a supernova remnant, a neutron star between 1.35 and 2.1 solar masses, with a radius between 20 and 10 km. It is a mass greater than the Sun condensed into space the size of a small city. The neutron star is so dense that a single teaspoon of its material weighs a billion tons (over 1.1 billion tons).
Depending on the neutron star’s mass, it could quickly collapse into a black hole or continue to exist practically forever. Different neutron stars include radio pulsars, X-ray pulsars, and magnetars, which are a subcategory of radio pulsars. Most neutron stars are called pulsars because they emit regular pulses of radio waves, through some not fully understood physical mechanism, slowly absorbing energy from their own angular momentum.
Some neutron stars do not emit visible radiation. This is probably because radio pulses are emitted from their poles and some neutron stars’ poles do not face Earth.
X-ray pulsars emit X-rays rather than radio waves and are powered by extremely hot input matter rather than their own spin. If enough matter falls into a neutron star, it could collapse into a black hole.
The most intense variety of neutron stars are those that come from a very rapidly rotating parent star. If the star rotates fast enough, the rotational speed matches the internal convective currents and creates a natural dynamo, pumping the collapsing star’s magnetic field to enormous levels. The star is then called a magnetar. A magnetar has a magnetic field similar to that of a trillion stars of high-power neodymium magnets that overlap in the same spot.
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