Sheer Magnetism: What’s a Magnetar?

Article August 16, 2025
An artist’s image of a magnetar. Credit: ESO/L. Calçada
An artist’s image of a magnetar. Credit: ESO/L. Calçada

This past week’s edition of Spacing Out included a brief mention of magnetars in the blurb in which I gushed about the whatever-the-heck-it-is in the galaxy NGC 4945 that seems to have both an extremely organized magnetic field and be bright at millimeter wavelengths. Then I got a text from my dad, a fairly science-literate guy with a strong interest in space science, saying he’d never heard of a magnetar before.

That’s not surprising because magnetars, (the word is formed by smooshing the words “magnet” and “star” in an astoundingly unimpressive fit of creativity) may sound like some sort of niche electric guitar brand, but they’re actually very weird dead stars doing weird magnetic things in weird ways. They’re not exactly high up there in the public consciousness. But if you read any Spacing Out stuff you know I love weird things in space, so thanks Dad for the inspiration for this week’s blog post! Let’s talk about those magnetic minions of the cosmos, magnetars.

Spoiler alert: if you’re looking for a bunch of answers, you might be disappointed. When it comes to magnetars, we’re pretty good on the “whats” but the “hows” and “whys” are a different story.

 

Big Star Go Boom

Before we talk about magnetars we have to talk about neutron stars which means we have to talk about stars blowing up (poor us). When a star with more than eight times the mass of the Sun reaches the point where it can no longer maintain stellar fusion in its core, it goes and blows itself up. This is a supernova.

A typical neutron star next to the island of Manhattan for scale. Credit: NASA/Goddard Space Flight Center
A typical neutron star next to the island of Manhattan for scale. Credit: NASA/Goddard Space Flight Center

Being more specific, the outer layers of the star explode outward, but the core collapses inward. Depending on the mass the star began with, this collapsing core can go a couple of different ways. For the most massive of stars (like…reaaally massive, 20 solar masses or more), the core collapse will continue all the way down into a black hole.

For stars that go supernova but don’t have the core mass to make a black hole, the core collapse stops when the pressure in the core has started shoving electrons into protons to make a giant ball of neutrons. The neutrons push on each other with what is known as neutron degeneracy pressure, and this push counteracts the continuing yank of gravity that wants to continue the collapse, and the core stabilizes as an object only a few miles across but with more mass than the Sun. This is a neutron star.

All neutron stars are weird. I mean, how can they help it, being tiny angry balls of neutron soup so dense that just a teaspoonful would weigh more than a mountain? But some are especially weird. Some of them are magnetars.

 

How to Make a Magnetar

Why some collapsing stellar cores form magnetars instead of normally weird neutron stars isn’t entirely known. There are theories though. It may be that certain conditions during the collapse can trigger a feedback process that causes the resulting neutron star to go absolutely crazy bananas with its magnetic field.

It’s also possible that it’s something about the progenitor star that causes its core to make a magnetar. A 2014 study suggests that they can form when the progenitor star is in a binary system with an even more massive star. In this scenario when the other star begins to swell into a red giant in its pre-supernova phase (being the bigger star, it would reach this stage first), the magnetar progenitor steals a bunch of its outer materials, causing the progenitor star to spin faster and faster, and that something about this rapid rotation is what triggers the formation of the magnetar once the progenitor star dies.

One way magnetars might form is through the merger of two normal neutrons tars. Credit: NASA/ESA/D. Player
One way magnetars might form is through the merger of two normal neutrons tars. Credit: NASA/ESA/D. Player

Or it’s possible, despite everything you’ve read here so far, that the death of a star does not directly make a magnetar. A study from earlier this year tracked what was thought to be a fairly young magnetar (only 20,000 years old because we space-y folks have our brains wired differently from most folks when it comes to thinking about time) and found no close enough supernova remnants for this magnetar to have been a part of a supernova that happened in that vicinity.

The thinking in this case is that this magnetar actually formed from a merger, perhaps when two neutron stars orbiting each other collided and smooshed. Or perhaps it formed when a white dwarf, the remnants of a star not massive enough to supernova, stole a bunch of stuff from a binary companion. Normally this leads to a Type 1a supernova when the white dwarf steals enough matter to trigger an explosion, but it’s possible in this case it led to the white dwarf triggering the collapse process that forms a neutron star—only this time it made a magnetar.

Part of the reason we just don’t quite know where magnetars come from is that we just don’t know about that many. We think it’s possible that, through one means or another, only 10% of neutron stars are magnetars, and it’s possible those impressive magnetic fields are a temporary feature that fades with time. We’ve seen a few dozen scattered throughout the Milky Way, and that’s it. We just don’t have a big sample size to work from.

 

Sheer Magnetism

This artist’s impression of a magnetar includes yellow lines to represent the field lines of the strong magnetic field. Credit: NASA/Goddard Space Flight Center
This artist’s impression of a magnetar includes yellow lines to represent the field lines of the strong magnetic field. Credit: NASA/Goddard Space Flight Center
This ring of dust and gas stretches seven light years across and contains the magnetar SGR 1900+14 at its center. Credit: NASA/JPL-Caltech/S. Wachter
This ring of dust and gas stretches seven light years across and contains the magnetar SGR 1900+14 at its center. Credit: NASA/JPL-Caltech/S. Wachter

Obviously the defining characteristic of a magnetar is the magnetic field. It’s right in the name. So how powerful a field are we talking about? Only the most powerful in the universe, no big deal! Let’s toss some numbers out.

The standard unit of measurement for magnetic field strength is a tesla (T). Your average microwave’s magnetic field, if you were a foot away from it, could be measured in microteslas (10-6 T). Refrigerator magnets are more like milliteslas (10-3 T). MRI machines measure over 1 T. An average neutron star’s magnetic field can be measured in megateslas (106 T), so that’s already insanely powerful.

The magnetic field of a magnetar at the weaker end will measure over a billion teslas. That’s powerful enough that, to quote a favorite example amongst magnetar enthusiasts, if you were to put a magnetar halfway between the Earth and the Moon, every credit card on Earth would be wiped. If a human body got within 1,000 km (600 miles) of a magnetar, the magnetic field would start ripping electrons off the atoms in your body, leaving you as a (no doubt impressed) very dead cloud of atomic nuclei.

So…yeah. They’re powerful.

 

Big Magnet Go Boom?

There’s a kind of thing that happens out in space called a Fast Radio Burst (FRB). Honestly the name doesn’t do them justice, because these are very fast, very insane blasts from the cosmos. Within a few milliseconds an FRB will put out as much energy as the Sun does in several days. Sometimes they’re one-offs and sometimes they repeat, but they’re always intense.

That being said, it took us a long time to find them. The first FRB was detected only in 2007, because they happen quite far away from us and by the time all that energy reaches us it’s pretty weak. Given it’s a fairly young field in astronomy, it doesn’t feel like too big a letdown to say that the origin of FRBs is, to this day, a mystery. That said, we have at least one very strong suspect.

In April 2020 an FRB was detected originating from within our own Milky Way for the first time (all others detected had been in other galaxies). Being a mere 30,000 light years away allowed astronomers to inspect the origin site much more closely than usual. In fact, this FRB originated at or very, very close to an object called SGR 1935+2154—a known magnetar.

An artist’s image of an eruption arising from a crack in a magnetar’s crust, creating a Fast Radio Burst. Credit: NASA Goddard Visualization Studio
An artist’s image of an eruption arising from a crack in a magnetar’s crust, creating a Fast Radio Burst. Credit: NASA Goddard Visualization Studio

It sure seems like some FRBs, at the very least, are coming from magnetars. Why? How? Great questions! We don’t know. I personally like the idea that the interaction between a magnetar’s magnetic and gravitational field can cause little tiny starquakes on its surface. Wee little things that would shift the neutron crust just a teeeeeeensy bit—and in the process create an outburst so powerful that it can be detected across the universe. 

I warned you magnetars are weird, even weirder than the normal weird of neutron stars. But  sweet Carl Sagan you gotta respect something with a magnetic field a trillion times the size of the Earth that can set off observatories billions of light years away just by fidgeting a little. I mean come on…that’s just cool.