June 17, 2022

Behold the Magnetar, nature’s ultimate superweapon

Artist's conception of a magnetar.
Enlarge / Artist’s conception of a magnetar.
Mark Garlick/Science Photo Library

If you think black holes are the scariest things in the Universe, I have something to share with you.

There are balls of dead matter no bigger than a city yet shining a hundred times brighter than the Sun that send out flares of X-rays visible across the galaxy. Their interiors are made of superfluid subatomic particles, and they have cores of exotic and unknown states of matter. Their lifetime is only a few thousand years.

And here’s the best part: They have the strongest magnetic fields ever observed, so strong they can melt you—literally dissociate you down to the atomic level—from a thousand kilometers away.

These are the magnetars, perhaps the most fearsome entities ever known.

Little green men

The best discoveries in science happen by accident, and to get to magnetars, we have to trace them through two unexpected observations.

The first came in 1967, when graduate student Jocelyn Bell (now Dame Jocelyn Bell Burnell) was working with her advisor, Antony Hewish, on the newly constructed and very fancy Interplanetary Scintillation Array at the Mullard Radio Astronomy Observatory in Cambridge, United Kingdom. While pouring over one night’s data, she found “a bit of scruff” (her words). It was a strikingly regular pattern, a flash of radio emission repeating every 1.33 seconds. Further observations showed that the signal came from the same point in the sky night after night, ruling out a terrestrial source.

At first, Bell and Hewish didn’t know what to make of it. It was so regular and predictable that they half-jokingly called the source “LGM-1,” wondering if “little green men” (i.e., aliens) might be responsible for the mysterious signal.

The Vela Pulsar, a neutron star corpse left from a titanic stellar supernova explosion, shoots through space, powered by a jet emitted from one of the neutron star's rotational poles.
Enlarge / The Vela Pulsar, a neutron star corpse left from a titanic stellar supernova explosion, shoots through space, powered by a jet emitted from one of the neutron star’s rotational poles.

Ever the wet blanket, astrophysicists came up with another explanation: It was a neutron star, the leftover core of a giant star that underwent a supernova catastrophe long ago. Physicists had hypothesized the existence of neutron stars decades before but had assumed that their tiny size would make them essentially unobservable. To their surprise, there the neutron stars were, revealing their presence in regular flashing beams of radio.

These objects came to be known as pulsars, a situation of pure coincidence. The rotating neutron star can emit beams of radiation that sweep out in circles like a lighthouse. When they flash over the Earth, we see them as a repeating pattern.

(In an unfortunate bit of history, Hewish won the Nobel prize for the discovery, but the committee excluded Bell.)

Around the same time, the United States Department of Defense launched a series of satellites, known as the Vela satellites, to monitor the Soviet Union for any naughtiness—specifically, any signs of violations of the nuclear test ban treaty. If the Soviets tested a nuclear bomb, it would release a flood of gamma rays that the Vela satellites could see from space.

The Vela satellites did indeed see a lot of flashes of gamma rays—but they came from the wrong direction. For years, the satellites monitored flash after flash coming from deep space and cataloged the mysterious events.

In 1973, the satellites finally let the astronomers in on the secret, and gamma-ray astronomy was born. After decades of study, astronomers realized that there were many different kinds of gamma-ray signals, with one in particular, the soft gamma repeaters, doing exactly what the name suggests: repeating.

To generate gamma rays (even the “soft” ones that are still incredibly powerful), you need a lot of energy, especially in the form of electromagnetic fields. And to make those emissions regular, you need something to be rotating. Astrophysicists realized that the best explanation for the origins of these soft gamma ray bursts was that they were beefed-up versions of pulsars, which would mean a highly magnetized neutron star.

In the 1990s, the concept of the magnetar was born.

Magnetic machine

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

Artist's conception of a magnetar and its magnetic field lines.
Enlarge / Artist’s conception of a magnetar and its magnetic field lines.

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.

If you get within approximately 1,000 kilometers of a magnetar, you die. Instantly. Leaving aside the copious amount of X-ray radiation constantly pouring out of these objects (we’ll get to that), the magnetic fields make life literally impossible. The problem is that atoms are made of positively charged protons and negatively charged electrons. In weak magnetic fields, this doesn’t make a bit of difference. But in strong fields, the electrons and protons respond differently. Atoms lose their traditional shape, and the electron orbitals become elongated along the direction of the magnetic field lines.

If you somehow made it to the surface of a magnetar, your individual atoms would only be 1 percent as wide as they are long. With atoms turning into needles, atomic physics as we know it breaks down. As does all the bonds that atoms use to glue themselves together into complex molecules.

In other words, the static magnetic field of a magnetar is strong enough to simply… dissociate you. All the molecules that you’re made of simply come apart into oddly shaped atoms.

These insanely strong magnetic fields also affect the vacuum of space-time and the quantum foam, the seething froth of particles that constantly appear and disappear at subatomic scales. Many of those particles are electrically charged, and at these field strengths, the particles gyrate around the magnetic field lines at nearly the speed of light. This produces something called a birefringence in the vacuum itself. Like ordinary cellophane, the birefringence can split light into separate directions, leading to weird optical illusions, distortions, and magnification—all from the simple presence of the magnetic field.

The heart of a killer

Like all neutron stars, magnetars aren’t very large. A typical neutron star will have a diameter of just around 20 kilometers. But within that small volume, they will hold up to twice the mass of the Sun, making them the densest known objects in the cosmos, one step shy of black holes themselves (which aren’t objects in any traditional sense). A single teaspoon of neutron star material weighs somewhere around 100 million tons. To support themselves against catastrophic gravitational collapse, neutron stars don’t rely on the release of energy from nuclear fusion but rather an exotic quantum phenomenon known as degeneracy pressure.

At densities comparable to that of an atomic nucleus, the neutrons that make up the bulk of these objects aren’t able to occupy the same energy states at the same time. This puts a limit on the densities they can reach. Another way to look at degeneracy pressure is to remember Heisenberg’s uncertainty principle: You can’t ever know both a particle’s position and velocity accurately at the same time. By cramming the neutrons in so tightly against each other, you know their positions extremely well. But this causes their velocities to skyrocket, vibrating them like angry trapped bees. This buzzing velocity provides a source of pressure against further collapse.

Magnetars are around a hundred times more luminous than the Sun, but they generate all that radiation from a volume roughly the size of Manhattan.

What happens inside a magnetar is a matter of pure speculation. Physicists think that the surface of a magnetar is covered in a shell of heavy atomic nuclei and free electrons. Because of the intense gravity, these surfaces are incredibly smooth; the highest “mountains” will only be a couple of centimeters tall. But don’t think of them as trivial. If you were to fall off one of those mountains, by the time you reached the bottom, you would already be traveling at half the speed of light.

Deeper into the magnetar, the atomic nuclei eventually dissociate in a sea of neutrons. Due to the enormous stresses, the neutrons compress and compact into odd shapes: lumps, tubes, and other tangled knots known affectionately as nuclear pasta. The cores of magnetars are beyond the realm of known physics. It could just be a superfluid of neutrons or other strange states of matter (as in, literally made of a soup of strange quarks).

Fully armed and operational

In normal, everyday neutron stars, the power to generate radiation comes from the initial heat of their formation and the loss of rotational energy as they slow down. With magnetars, the energy contained in the magnetic field completely swamps any other source. If you were to convert its energy density into mass via E/c2, the magnetic field would be 10,000 more dense than lead.

By itself, a magnetar is hot enough to generate enormous amounts of X-ray radiation due to its surface temperature of nearly 20 million Kelvin. But with magnetars, there’s more. The magnetic field of a magnetar whips particles around it at a healthy fraction of the speed of light. These high-energy particles then slam into any photons that wander nearby, energizing them through a process called Compton scattering and turning them into more X-rays. The same magnetic fields funnel charged particles directly into the crust like a roided-out version of our own aurorae, generating even more X-rays.

Artist's conception of a supernova and associated gamma-ray burst driven by a magnetar.
Enlarge / Artist’s conception of a supernova and associated gamma-ray burst driven by a magnetar.

The result is that magnetars are around a hundred times more luminous than the Sun, but they generate all that radiation from a volume roughly the size of Manhattan. Plus, magnetars radiate almost exclusively in X-rays, which is significantly less pleasant than the gentle warmth of our own star.

Sometimes, the extreme magnetic field pins down on the charged particles of the crust with almost overwhelming force. To relieve itself of the enormous pressure, the crust of a magnetar suddenly shifts, rearranging itself to find a new equilibrium that sustains the heavy magnetic load. This process triggers the release of an enormous amount of energy stored in the magnetic field (imagine pressing on one corner of a table until the leg underneath buckles—you’re going to lose some energy as you fall to the ground). The extreme energies lost by the magnetic field produce a flood of electrons and positrons, which then recombine to form a storm of gamma rays.

After the release, the magnetar settles back into its normal X-ray-producing mode. But after enough time, the pressures build back up, and the only relief is a new arrangement of the crust, triggering another round of gamma ray emission. This is what causes those soft gamma repeaters discovered by the Department of Defense.

The rearranging of a magnetar’s field may also be responsible for yet other mysterious cosmic flashes. Astronomers recently discovered a fast radio burst—a blaze of radio energies that lasts for only a fraction of a second—going off in our own galaxy and found that its origins coincided with a known magnetar. Magnetars may also be responsible for superluminous supernovae, in which a flaring magnetar can reenergize the remnants of the supernova that birthed it, causing both to suddenly and dramatically rise in brightness.

Alas, like all good things, the party must come to an end. The extreme magnetic fields act as a drag, slowing down the spin of the magnetar and providing an avenue for energy to escape. Within about 10,000 years, a magnetar will turn into just another normal neutron star—still exotic but without that sharp magnetic edge.

Astronomers know only about 24 magnetars, almost all of them found within our galaxy. Their short lifetimes mean that we will only ever see a minority of the potential ones in the time that we’ve been capable of observing them. But given what we know, astronomers estimate that there are around 30 million dead magnetars within the Milky Way galaxy alone.

It was fun while it lasted.

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