Neutron Stars

A neutron star is one example of a stellar remnant: something left behind by a star at the end of itslifetime. Stars are what they are because they burn hydrogen gas. In reality, they fuse the hydrogen nuclei (the cores of the atom) together into helium nuclei. This nuclear reaction releases much more energy than any kind of ordinary fire. In addition to making a star a source of light and heat, the nuclear reaction also generates a great deal of pressure. For most of its life, the star exists due to a balance between this outward pressure and the contracting force of its own gravity.

What happens when the hydrogen runs out? The balance of pressure and gravity is destroyed and the star collapses. Between that point, and the very end of the story, several interesting things may happen. There may be brief periods (only a million years or so!) when other types of fusion occur. The star may shed some of its matter, making a kind of nebula. The color and nature of the star may vary wildly. But if the star has the right mass (medium large, but not too large), the final collapse results in a big explosion when everything crunches into the center and rebounds. This is one type of supernova. If conditions are just right, there will be a compact object left behind, made of very different material than normal stuff and a product of the enormous forces of the supernova explosion. What is this stuff? To answer that, we have to switch the scale of our thinking from huge objects, like stars, and think about things smaller than atoms. Inside an atom, the nucleus, or core, is essentially made up of particles called protons and neutrons. The protons have a positive electric charge, and the neutrons are electrically neutral. Together, they account for most of the mass of an atom. Buzzing aroundoutside the nucleus are the electrons. They are much lighter (about 2000 times lighter), and have anegative charge. In between the nucleus and this “cloud” of electrons is a lot of empty space. In fact, a normal atom is composed mostly of empty space! Without getting into the details, powerful nuclear forces keep the electrons separate from the core, maintaining the atom’s structure.