What Is a Magnetar — and Did Scientists Just Watch One Being Born?

In one-tenth of a second, a magnetar called SGR 1806-20 released more energy than the Sun has emitted over the past 100,000 years.

I have been writing about extreme space objects for a long time. I still cannot read that sentence without stopping.

What a Magnetar Actually Is

To understand a magnetar, you first need a neutron star. When a massive star — somewhere between 10 and 25 times the mass of our Sun — reaches the end of its life and explodes in a supernova, what gets left behind is not a black hole and not nothing. It is a neutron star: a collapsed stellar core so dense that a single tablespoon of its material would weigh more than 100 million tons on Earth.

A magnetar is what happens when one of those neutron stars forms with an unusually intense magnetic field. Not just stronger than normal — about 1,000 times stronger than a typical neutron star pulsar, and roughly a trillion times more powerful than the field surrounding Earth. The magnetic field of a magnetar sits between 10⁹ and 10¹¹ teslas. Earth’s magnetic field, by comparison, measures 30 to 60 microteslas.

The numbers do not really translate into anything you can hold in your head. That is not a failure of imagination. The scale is just genuinely too large.

In physical terms: a magnetar about 20 kilometers across — roughly the size of a city — crammed with the mass of something 1.4 times heavier than the Sun. Rotating once every two to ten seconds. Radiating X-rays and gamma rays continuously as its magnetic field slowly decays. And capable, when it has a bad day, of releasing more energy in a fraction of a second than our star produces in a hundred millennia.

The Physics Behind the Extremes — What That Field Actually Does

A field this powerful does not just attract iron filings. It distorts reality in a fairly literal sense.

At around 10⁵ teslas, atomic orbitals begin to deform into elongated rods. At 10¹⁰ teslas — well within a magnetar’s range — a hydrogen atom becomes 200 times narrower than its normal diameter, according to a 2003 Scientific American analysis of magnetar physics. X-ray photons split in two or merge. The vacuum itself becomes polarized, acting like a crystal rather than empty space.

There is also a more visceral way to understand the scale. From a distance of 1,000 kilometers — where you would already be well past any safe zone near such an object — a magnetar’s field would disrupt the electron clouds around the atoms in your body, making the chemistry that keeps you alive impossible. The field does not heat you. It does not crush you. It simply undoes the physical relationships that constitute matter as we know it.

At the distance halfway between Earth and the Moon — about 192,000 kilometers — a magnetar could erase the magnetic data on every credit card on the planet simultaneously, as noted by NASA’s archived analyses of magnetar SGR 1806-20.

You will not find a magnetar anywhere near here. The closest known one is thousands of light-years away. But understanding what the field does at range helps calibrate just how extreme these objects are up close.

How Magnetars Form — and Why They Are So Rare

The process that creates a magnetar begins with a dying star. When a massive star’s core collapses, it can form a neutron star in a fraction of a second — going from something the size of our Sun to a ball roughly 20 kilometers across. During that collapse, conservation of magnetic flux means that whatever magnetic field the original star had gets compressed and amplified dramatically.

For a magnetar to form, that process has to go a step further. The leading model, proposed by Robert Duncan and Christopher Thompson in 1992, involves a turbulent magnetohydrodynamic dynamo in the collapsing star’s interior — a churning of hot, dense, electrically conducting fluid that converts heat and rotational energy into magnetic energy. If the newly formed neutron star spins fast enough and the conditions align correctly, the result is a field amplified beyond the normal neutron star range by orders of magnitude.

About one in ten supernova explosions is estimated to produce a magnetar rather than a more standard neutron star, according to astrophysical population modeling. That already makes them rare. Combine that with the fact that a magnetar’s strong magnetic field decays after roughly 10,000 years — geologically brief — and the number of actively observable magnetars drops further. Scientists estimate there may be 30 million inactive magnetars in the Milky Way alone, all of them long since quiet.

As of 2021, only 24 had been confirmed. Six more are candidates.

What Magnetars Do: Flares, Bursts, and a Connection to One of Astronomy’s Biggest Puzzles

Magnetars are not just exotic curiosities. They are active, violent objects — at least during their relatively brief window of magnetic intensity.

Starquakes on a magnetar’s surface — caused by the enormous stress the magnetic field places on the rigid crust — can release gamma-ray flares of stunning power. On March 5, 1979, a burst from a magnetar in the Large Magellanic Cloud hit multiple spacecraft simultaneously: Venera 11, Venera 12, Helios 2, the Pioneer Venus Orbiter, three Vela satellites, the Soviet Prognoz 7, and the Einstein Observatory. It went from 100 counts per second to over 200,000 counts per second in a fraction of a millisecond. At the time, it was the most powerful extra-solar gamma event ever detected by a factor of more than 100.

More recently, magnetars have been implicated in one of astronomy’s longest-running open questions: the source of fast radio bursts (FRBs). These are milliseconds-long pulses of radio energy that, for decades, seemed to arrive randomly from distant galaxies with no clear explanation. In 2020, a magnetar in our own galaxy — SGR 1935+2154 — produced a burst that matched the profile of an FRB, strongly suggesting that magnetars are at least one source of the phenomenon. The detection, made with multiple telescopes including the Australian Square Kilometre Array Pathfinder, significantly narrowed the field of possible explanations, as reported in Nature that year.

The connection to the cosmic structure of the universe and large-scale phenomena remains an active area of research — and magnetars, it turns out, may leave fingerprints well beyond their immediate neighborhood.

For the First Time, Astronomers Watched a Magnetar Being Born

In December 2024, a superluminous supernova — a class of explosion 10 or more times brighter than ordinary supernovae — appeared about a billion light-years away. It was designated SN 2024afav. What made it unusual was not its peak brightness. It was what came after.

Instead of fading smoothly, the supernova’s light curve showed four distinct bumps, with the intervals between them getting progressively shorter. Graduate student Joseph Farah at UC Santa Barbara, working with the Las Cumbres Observatory — a global network of 27 telescopes — observed the event for over 200 days and noticed that the pattern was accelerating. He called it a chirp, a term borrowed from the gravitational wave community, where the same shape of signal appears as two black holes spiral inward and merge.

The explanation Farah and his colleagues arrived at, published in Nature on March 11, 2026, is this: some debris from the explosion fell back toward the newly formed magnetar and formed an accretion disk. That disk was misaligned with the magnetar’s spin axis. Because a rapidly rotating massive object drags space-time with it — the Lense-Thirring effect, a prediction of general relativity — the disk began to wobble. As the disk spiraled inward, the wobble sped up, periodically blocking and reflecting the magnetar’s light. Four bumps. Increasing frequency. The signature of a real engine deep inside.

“We tested several ideas, including purely Newtonian effects and precession driven by the magnetar’s magnetic fields, but only Lense-Thirring precession matched the timing perfectly,” Farah said. “It is the first time general relativity has been needed to describe the mechanics of a supernova.”

The estimated parameters: a spin period of 4.2 milliseconds (roughly 238 rotations per second in the hours after birth) and a magnetic field about 300 trillion times the strength of Earth’s. Classic magnetar signatures, both of them — and now confirmed not by inference but by direct observation of a process playing out across 200 days of data.

This also confirms a theory first proposed in 2010 by UC Berkeley theoretical astrophysicist Dan Kasen, who argued that a newborn magnetar’s rotational energy — transferred into the surrounding debris — was the power source behind superluminous supernovae. Sixteen years of what Kasen himself called “a theorist’s magic trick” finally had a physical signal to point to.

What Comes Next — and Why This Changes How We Will Find Them

Think about what that actually means for the field. Before SN 2024afav, the magnetar-powering-superluminous-supernovae theory had circumstantial support and no smoking gun. Now it has a specific, physically explained observational signature — a chirp in the light curve — that future surveys can actively look for.

Farah expects that as the Vera C. Rubin Observatory comes fully online, the sheer volume of its sky survey will turn up dozens more of these chirping supernovae. Rubin’s Legacy Survey of Space and Time (LSST) will image the entire available sky every few nights, making it effectively a continuous motion picture of the transient universe. Superluminous supernovae that would previously have been caught at peak brightness and then lost as resources moved on will now be tracked through their full decay curves, bumps and all.

It is worth noting, as UC Berkeley astronomer Alex Filippenko cautioned in the original paper, that this does not mean all superluminous supernovae are magnetar-powered. Some fraction are likely explained by shock interaction with surrounding material, and possibly by black hole formation producing a similar accretion disk geometry. The question is what fraction — and that, now, is actually answerable.

Magnetars sit at the intersection of neutron star physics, general relativity, and some of the most energetic events the universe produces. For an object the size of a city, they leave a disproportionate mark on the sky. If you have been following the search for life beyond Earth, it is worth knowing that a magnetar within a few thousand light-years could, in principle, irradiate an entire stellar neighborhood with a single burst. The universe produces them quietly, one in ten supernovae, mostly unannounced.

Now, for the first time, we watched one arrive.