Neutron Stars

Neutron stars are the remnants of massive stars that have undergone gravitational collapse, and are one of the two evolutionary endpoints of most massive stars, the other being a black hole.

Formation

As a star (of mass nearly 10 and 20 times that of the Sun) nears its death, their cores become mostly comprised of Iron. But since the core is so massive, the gravitational pressure is so strong that it causes the core to collapse in on itself. The core collapses so quickly that the protons and electrons are forced together to form neutrons. The outer layers of the star are then thrown off in a massive explosion called a supernova. The remaining core is then a neutron star.

Some massive cores are at this point saved from further collpase by a quantum phenomenon called neutron degeneracy pressure, which occurs when such a density is reached that neutrons can no longer be packed any closer together. The core then becomes a neutron star.

“With neutron stars, we’re seeing a combination of strong gravity, powerful magnetic and electric fields, and high velocities. They are laboratories for extreme physics and conditions that we cannot reproduce here on Earth.”

Properties

A neutron star isn’t really a star. It’s more like a giant atomic nucleus 8 - 16 \(km\) across, made entirely of neutrons and can have masses of about twice that of the Sun, giving them an average density of about 10\(^14 g/cm^3\). For reference, that about a trillion times that of the density of water. The total mass of a neutron star is around 1.1 to 2.2 times that of the Sun.

Since so much mass is stuffed into such a small sphere, a neutron star is very dense. A sugar cube-size lump would weigh about a billion tons, more than 10,000 aircraft carriers.

On top of this, since the original star might be having a spin, it would need to conserve its angular momentum. Consequently, since the core implodes forming a neutron star, its radius decreases rapidly and to a fraction of its original. This leads to a massive gain in the spin of neutron stars.

The fastest rotating neutron star rotates at a rate of 43,000 RPM. This gives it a linear (tangential) speed at the surface to be equal to nearly a quarter the speed of light.

The temperature inside a newly formed neutron star can be very high as well, reaching around 10\(^{11}\) to 10\(^{12} K\). But due to the huge number of neutrons that it emits, it looses so much of its energy that the temperature falls to 10\(^6 K\) within a few years. At such a low temperature, the majority of the light generated by a neutron star falls in the X-ray region.