By Emily Levesque, University of Washington
Neutron stars are only about 12 miles across, about the size of a small city. One could drive all the way around a neutron star in under an hour. They’re also the densest objects that we know of in the universe; if one was to scoop up a single teaspoon of material from a neutron star, it would weigh more than the Mount Everest. Staggeringly, the entire neutron star can be up to twice as massive as our own Sun—that mass is just crammed into an extremely small package.
Neutron stars Form from Supernova
Neutron stars are created during the deaths of massive stars. A star is considered massive if it has at least eight times the mass of our own Sun, and because of these large masses the stars lead relatively short lives, dying as supernovae after only about 10 million years. In those supernova deaths, newly formed iron cores lose the star’s lifelong battle between the inward press of gravity and the outward push of energy produced by nuclear fusion.
When this happens, the star’s core implodes and the its outer layers bounce off the imploded core and produce a spectacular rebound shock. The bounce and shock give us the supernova explosions, but that collapsed core remnant left behind at the heart of the supernova is what produces a neutron star.
A Hot Neutron Soup
Neutrons are subatomic particles with a neutral charge, joining positively charged protons and negatively charged electrons as the tiny ingredients that make up individual atoms. Normally, neutrons are bound up with protons in the nucleus at the heart of an atom. However, at the incredibly high temperatures achieved when a massive star dies, those atoms are broken apart.
The neutrons become free particles, and the protons and electrons combine to form even more neutrons. The result is a hot neutron soup.
This soup forms a neutron star thanks to the Pauli exclusion principle of quantum physics. This principle states that subatomic particles like neutrons can’t occupy the same quantum state within a system. In other words, two neutrons with identical physical properties can’t occupy identical locations in space; if one tries to squeeze them together in a tiny space, they’ll push back, exerting an outward pressure.
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A Tiny, Dense Remnant
In the incredibly dense interior of a neutron star, further collapse would start shoving neutrons too close together with their neighbors, putting them at risk of violating this principle. To avoid this, the neutrons create an outward pressure so powerful that it actually halts further gravitational collapse.
For the size scales we’re talking about, the result is an unbelievably tiny and dense remnant of the original star’s core—what we know as a neutron star.
Rotating Stellar Core
The surface of a neutron star is also very hot, over 1 million degrees Fahrenheit—that’s 100 times hotter than the surface of our Sun!
These neutron stars also rotate at incredible speed. This is because the mass of an entire rotating stellar core has been just compressed down into an area the size of New York City. One can understand this as being similar to what happens when a spinning figure skater pulls in their arms. The same principle applies here—by bringing their own mass closer to their axis of rotation, they’re able to spin faster and faster. Now imagine doing this on the scale of a massive star. The fastest rotating neutron stars will complete several hundred rotations in a single second.
Finally, we think that the surface of a neutron star is composed of a thin crust of ionized atoms and electrons. But as we go deeper, we mainly find neutrons. The environment deep inside these stars is so extreme that we still know very little about how to mathematically describe and model this sort of matter.
Common Questions about Understanding a Neutron Star
A star is considered massive if it has at least eight times the mass of our own Sun, and because of these large masses the stars lead relatively short lives, dying as supernovae after only about 10 million years.
Neutron stars also rotate at incredible speed. This is because the mass of an entire rotating stellar core has been just compressed down into an area the size of New York City. One can understand this as being similar to what happens when a spinning figure skater pulls in their arms. The same principle applies here—by bringing their own mass closer to their axis of rotation, they’re able to spin faster and faster.
Pauli exclusion principle of quantum physics states that subatomic particles like neutrons can’t occupy the same quantum state within a system. In other words, two neutrons with identical physical properties can’t occupy identical locations in space; if one tries to squeeze them together in a tiny space, they’ll push back, exerting an outward pressure.
