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The neutron is a subatomic particle that does not have an electric charge and is slightly more massive than a proton. Nearly every atomic nucleus contains protons and neutrons; the only exception being hydrogen, so they are referred to as nucleons. The number of neutrons(neutron number) determines the isotope of an element. When neutrons are bound in a stable nucleus they are stable, but when a neutron is free it is unstable and will undergo beta decay in about 15 minutes. Free neutrons are usually produced by nuclear fission and fusion. They are not considered to be a chemical element, but are sometimes called neutronium.
Since a neutron consists of three quarks(fundamental part of matter), the only way that it can decay without a baryon number change is for one of the quarks to change flavor(quantum number of particle in global symmetry) through weak interaction. The neutron consists of two down quarks and one up quark, and the decay of one of the down quarks into a lighter up quark can be achieved by the emission of a W boson. This means that the neutron decays into a proton, an electron, and an antineutrino.
At extremely high pressures and temperatures, nucleons and electrons are believed to collapse into bulk neutronium. Many scientists think that this is what happens in neutron stars. A neutron star is a stellar remenant that can result from the gravitational collapse of a massive star during a type II, Ib, or Ic supernova. These stars are composed almost entirely of neutrons. Neutron stars are very hot and are supported against further collapse because of the Pauli exclusion principle that states that no two neutrons can occupy the same place and quantum state at the same time. A typical neutron star is between 1.35 and about 2.1 solar masses and has a radius of about 12 km. Neutron stars are smaller in radius than our Sun, but the are far more massive and dense.
In just a few paragraphs we have gone from the tiny neutron to the a neutron star. Interesting how physics and astronomy go hand in hand, isn’t it?
If you’d like more info on the Atom, check out NASA’s Article on Analyzing Tiny Samples, and here’s a link to NASA’s Article about Atoms, Elements, and Isotopes.
We’ve also recorded an entire episode of Astronomy Cast all about the Atom. Listen here, Episode 164: Inside the Atom.