Neutron stars are created in the fiery hearts of supernova explosions. When a giant star nears the end of its life, it crushes its core of carbon and oxygen with such incredibly high pressures that electrons and protons merge in a process called “inverse beta decay,” which creates a solid mass of almost pure neutrons. This proto-neutron star is capable of (briefly) resisting the collapse of the star, triggering the onset of the supernova. Sometimes, the mass of the neutron star collapses into a black hole, but other times, it survives.
When the neutron star first emerges out of the wreckage of the supernova, it will have a temperature of 10 to 20 million Kelvin. This high temperature forces the neutrons to circulate, creating rapidly moving circular convection cells. These carry heat from the interior to the surface, where it can radiate away into space. At those high temperatures, the neutrons behave like a fluid, allowing any remaining electrons and protons to wander freely.
If the neutron star is rotating fast enough (which can occur if its parent star was also rapidly spinning), the combination of fast spin, convection currents, and freely moving charges sets up a dynamo mechanism: the circulating electric charges generate a weak magnetic field. Then the motion of the convention cells causes the magnetic field to fold in over itself, which amplifies it. With every rotation, the magnetic field grows stronger.
Similar mechanisms happen inside the Earth’s core to generate our magnetic field, just at much lower energies. With the energies involved in neutron stars, things can quickly spiral out of control.
In as little as 10 seconds, a newborn neutron star can generate the strongest magnetic fields in the known Universe. In that same amount of time, the frenetic convection and spinning cool the neutron star off, shutting off the dynamo mechanism. Normally, this would cause the magnetic field to disappear (if the Earth’s core cooled off, that’s what would happen to ours). But because of the strange physics of neutron stars, the protons and electrons become a superfluid and can maintain their motion without any electrical resistance. This allows the magnetic field to lock in, remaining long after the neutron star has cooled off.
Oddly, if the newborn neutron star is spinning too fast, it won’t generate a strong magnetic field because the convection will cool off the neutron star before it has a chance to build up the dynamo mechanism. So only some neutron stars, roughly one in 10, can become magnetars.
Walk softly but carry a big magnet
I’m not kidding when I say that magnetars have the strongest magnetic fields in the Universe. To illustrate, let’s start with something you’re familiar with, the Earth’s magnetic field, and work up from there.
Measured at the North Pole, the Earth has a magnetic field strength of around half a Gauss. At its strongest, our planet can roughly double that number. That’s pretty impressive—it’s the most powerful magnetic field among the rocky planets of the Solar System—and enough to nudge a compass needle around for handy navigation.
The kind of magnet you stick on your fridge is about 100—200 times stronger than that and can easily counteract the gravitational might of the entire planet.
Moving off the Earth, sunspots reach magnetic field strengths of around 4,000 Gauss, the strongest in the Solar System.
Humans are capable of making some seriously powerful magnets. The most powerful sustained electromagnets reach a few tens of thousands of Gauss. If you’ve ever had an MRI, you have personally experienced around 10,000 Gauss with no ill effects (if you remembered to take off your jewelry). It’s difficult for us to make stronger sustained magnetic fields because they tend to destroy the devices we use to make them. That said, inside of focused explosions, we can make magnetic fields that reach 10 million Gauss for a few microseconds.
If you somehow made it to the surface of a magnetar, your individual atoms would be 1 percent as wide as they are long.
A typical magnetar has a surface magnetic field strength of 1014 to 1015 Gauss, with interior strengths 10 times stronger.
That is not a typo. Magnetars have magnetic fields about a quadrillion times stronger than the Earth’s and a billion times stronger than the best that humanity can achieve.