This image shows the illustration of a massive neutron star, along with the distorted gravitational effects an observer might see if they had the capability of viewing this neutron star at such a close distance. While neutron stars are famous for pulsing, not every neutron star is a pulsar. They all are stable, however, and don’t appear to have their neutrons decay over timescales of at least hundreds of millions of years, despite the free neutron having a mean lifetime of only ~15 minutes. (Credit: Daniel Molybdenum/flickr and raphael.concorde/Wikimedia Commons)

Neutrons can be stable when bound into an atomic nucleus, but free neutrons decay away in mere minutes. So how are neutron stars stable?

Starts With A Bang!

One of the most exotic naturally occurring objects in the Universe is the neutron star: achieving some of the highest densities, temperatures, and energies found anywhere outside of a black hole or separate from the Big Bang. Often containing more than a solar masses worth of matter compressed into a sphere just 10–20 kilometers across, neutron stars generate the strongest magnetic fields in the cosmos, and some neutron stars may even contain more exotic states of matter — like a quark-gluon plasma, possibly including not just up-and-down quarks but heavier species as well — towards their innermost cores.

But neutron stars, as far as we can tell, remain stable over very long timescales. Based on observations of millisecond pulsars, the oldest of the known neutron stars, they must persist for at least hundreds of millions of years, and potentially much, much longer. And yet, if you had just a single free neutron in your possession, it would decay after a mean lifetime of about ~15 minutes. This bothers reader Gary Camp, who wants to know:

“If neutrons decay when not in an atom, why does a neutron star not decay? Is it really a giant atom?”

A neutron star is like a giant atomic nucleus in some ways, in the sense that there is an intense amount of binding energy at play. But unlike the heart of an atom, it isn’t the strong nuclear force doing the heavy lifting here, but gravitation. Here’s the science of why.

This illustration shows 5 of the main types of radioactive decays: alpha decay, where a nucleus emits an alpha particle (2 protons and 2 neutrons), beta decay, where a nucleus emits an electron, gamma decay, where a nucleus emits a photon, positron emission (also known as beta-plus decay), where a nucleus emits a positron, and electron capture (also known as inverse beta decay), where a nucleus absorbs an electron. These decays can change the atomic and/or mass number of the nucleus, but certain overall conservation laws, like energy, momentum, and charge conservation, must still be obeyed. Beta decay always involves a neutron, whether free or within a nucleus, decaying into a proton, electron, and electron antineutrino. (Credit: CNX Chemistry, OpenStax/Wikimedia Commons)

Free neutrons

One of the more surprising properties of matter is that of the three main constituents of atoms — protons, neutrons, and electrons — only two of them are inherently stable. The electron, the lightest charged particle in the Standard Model, is perfectly stable to the best of our knowledge, as there are no particles it can decay into (i.e., with less rest mass) and still conserve all of the quantum properties (like electric…

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