What is a Neutron Star and What Are They Made Of?

The death of a star in a supernova is a catastrophic event. It leaves behind an unimaginably dense object called a neutron star.

Their nuclei are very small compared to the size of the atom as a whole, measured out to the orbit of electrons.

Now imagine squeezing a lump of gold, uranium or lead so hard that the nuclei come close together to the point they touch, and you'll estimate the space. You would have something incredibly dense, similar to a giant atomic nucleus. That is what neutron stars are. The only force up to the task of squeezing come to this point is gravity.

A neutron star forms when a star reaches the end of its life and no longer has enough fuel to keep it from collapsing. The core contracts, causing the nuclei to fuse violently and touch. This fusion results in a bounce back; the entire core recoils and drives off the outer layers, producing a brilliant supernova. What remains is a rapidly spinning cinder core - the densest thing you can have without becoming a black hole. It is so dense that a teaspoon of the material on Earth would weigh around one billion tonnes.

Contemplate the complexity of matter at such incredibly high densities. Do the nuclei of each atom stay separate? Do the atoms become an indistinguishable mix of protons, neutrons and electrons floating around each other? Or does it go still further? Do the protons and electrons merge so we end up with a soup of neutrons? Do they merge into a giant soup of quarks and gluons? Or does it go further than that?

The nuclear physics community doesn't know what happens at these densities. The question refers to the 'f' state, a term in nuclear physics that describes the nature of matter at a fundamental level, specifically the state where density is related to pressure.

If the star has a hardcore - a stiff equation of state - you would have a larger star with a bigger radius. If you have a soft equation of state, where the neutrons give up their identities and form a soup of quarks, a type of elementary particle, you would have a soft, squishy core and a smaller radius. So, if we can measure the star's radius, we will know what the interior composition must be. But we cannot measure directly. These stars are only thought to be between 10km and 15km in radius - the size of a city - far smaller than Hubble could ever imagine. So, measurements must be made in indirect ways.

The best method is to infer the radius by looking at X-ray emissions; to do that, we have to observe neutron stars from space. Neutron stars that flash rapidly as they turn their glowing magnetic poles towards us like cosmic lighthouses are called pulsars. These pulsars emit X-rays that we can detect and analyze to estimate the star's radius, providing us with valuable indirect measurements.

Through meticulous measurements, we can unravel the mysteries of neutron stars. We can learn how they warp space-time from the shapes of these pulses. How sharply do they rise and fall? Is there a residual glow? These characteristics carry further information about the star and should enable us to determine the mass and radius of several neutron stars, and so deduce what lies within.