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.

Everything we can touch is made of atoms. Subatomic particles called protons and neutrons make up the nucleus, and electrons orbit around them. But at this atomic level, even the densest materials in our world, such as gold, lead and uranium, are mainly made up of empty space.

Their nuclei are very, 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 empty 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. Loosely speaking, the core contracts until the nuclei have fused rather violently and are touching. At that point there is a bounce back; the entire core recoils and drives off the outer layers, producing a brilliant supernova. Just a rapidly spinning cinder core remains - the densest thing you can have without it becoming a black hole. It is so dense that a teaspoon of the material on Earth would weigh around one billion tonnes.

How does matter behave at such incredibly high densities? Do the nucleus 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 that 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 boils down to something called the equation "f" state, a term that describes the nature of matter at a fundamental level that relates density 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, you would have a soft, squishy core and a smaller radius. So if we can just measure the radius of the star 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 - which is far smaller than Hubble could ever hope to imagine. So measurements must be made in indirect ways.

We believe that the best method is to infer the radius by looking at X-Ray emissions and 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.

From accurate measurements, we will be able to learn how neutron stars 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.