A magnetar is a type of neutron star with an extraordinarily powerful magnetic field, roughly a thousand times stronger than an ordinary neutron star and trillions of times stronger than Earth’s. These stellar remnants pack more mass than our Sun into a sphere only about 20 kilometers across, making them some of the most extreme objects in the known universe. Only 29 magnetars have been cataloged in our Milky Way galaxy so far.
How Magnetars Compare to Other Neutron Stars
All neutron stars begin the same way: a massive star runs out of fuel, its core collapses, and the resulting supernova leaves behind an incredibly dense remnant made almost entirely of neutrons. That remnant typically packs more than the mass of the Sun into a ball roughly ten kilometers in radius. What separates magnetars from other neutron stars is the sheer strength of their magnetic fields.
Ordinary neutron stars already have powerful magnetic fields. Some spin rapidly and shoot beams of radiation from their poles, appearing to “pulse” as they rotate. These are called pulsars. Magnetars take things further. Their magnetic field strength reaches around 10 trillion to 1 quadrillion gauss, about a thousand times stronger than a typical neutron star. For perspective, Earth’s surface magnetic field measures between 0.25 and 0.65 gauss, and magnetars are more than 100 million times stronger than the most powerful magnets humans have ever built.
Magnetars also rotate more slowly than most pulsars, completing one revolution every 5 to 12 seconds rather than hundreds of times per second. This slow spin, combined with the intense magnetic field, gives magnetars their distinctive behavior.
What Creates Such an Extreme Magnetic Field
The leading explanation involves what happens in the first few seconds after a supernova’s core collapse. The newborn neutron star, called a protoneutron star, is incredibly hot and turbulent. If it’s spinning fast enough, violent convection currents inside the star act like a dynamo, converting the energy of that rotation and turbulence into magnetic energy. Three-dimensional simulations published in Science Advances showed that this dynamo process can generate the required magnetic field strength when the young neutron star spins with a period shorter than about 10 milliseconds.
The speed of rotation is the critical ingredient. Very fast rotation produces an immediate, powerful dynamo that generates the strongest fields. Somewhat slower rotation leads to a delayed buildup, which may still produce a magnetar but with a weaker field. Stars that don’t spin fast enough at the moment of collapse simply become ordinary neutron stars or pulsars instead. In 2023, astronomers identified a star called HD 45166 with a magnetic field of 43 kilogauss that is expected to collapse in several million years, concentrating that field into a magnetar with a strength around 100 trillion gauss.
Inside a Magnetar
A magnetar’s structure is layered and bizarre by everyday standards. The outermost layer is a solid crust, thought to be composed largely of iron nuclei arranged in a crystalline lattice. Deeper into the crust, above a certain density threshold, neutrons begin to “drip” out of atomic nuclei and form a superfluid, a state of matter with zero friction that flows without resistance. This happens at temperatures below roughly 5 billion degrees, which counts as “cool” for a neutron star.
The liquid core is even stranger. Neutrons exist in a superfluid state while protons become superconducting, meaning they carry electric current with no resistance. The magnetic field in the outer core threads through the superconducting protons in tiny bundles called fluxtubes, each carrying a single quantum of magnetic flux. Electrons fill in the gaps, creating a three-fluid system unlike anything found on Earth. This exotic interior is what allows the magnetic field to be so strong and to evolve over time in ways that produce dramatic outbursts.
Starquakes and Giant Flares
The magnetic field inside a magnetar is so powerful that it actually stresses and deforms the solid crust. Over time, this stress builds until the crust fractures in an event called a starquake. When the crust cracks, it releases enormous amounts of energy in the form of X-rays and gamma rays. These events are why magnetars were originally detected as “soft gamma repeaters,” objects that repeatedly emitted bursts of lower-energy gamma radiation.
Most magnetar activity consists of relatively modest X-ray bursts. But on rare occasions, a magnetar produces a giant flare. The most spectacular one ever recorded came from a magnetar called SGR 1806-20 on December 27, 2004. Despite being located about 28,000 light-years away, this single burst was so powerful that it caused measurable changes in Earth’s upper atmosphere. Its energy release was roughly 100 times greater than the only two other giant flares ever observed. The expanding debris from the explosion moved at about 70% the speed of light.
Of the 29 magnetars cataloged in our galaxy, only two have produced giant flares. Astronomers have also identified giant flares from magnetars in other galaxies, including one in the spiral galaxy NGC 253 (about 11.4 million light-years away), and candidates near M81, the Andromeda galaxy, and M83.
The Link to Fast Radio Bursts
One of the most exciting discoveries in recent astrophysics connected magnetars to fast radio bursts (FRBs), mysterious millisecond-long flashes of radio energy that had puzzled scientists since their discovery in 2007. On April 28, 2020, the Galactic magnetar SGR 1935+2154 emitted a millisecond-duration radio burst that was detected by the STARE2 radio array. This burst, designated FRB 200428, released 4,000 times more energy in radio waves than any pulse ever recorded from the Crab pulsar, which had previously held the record for the brightest radio bursts in our galaxy.
FRB 200428 was only about 30 times less energetic than the weakest fast radio burst ever detected from outside our galaxy, placing it squarely in the same population. The burst arrived at the same time as an X-ray burst from the magnetar, supporting theoretical models that describe FRBs as being powered by magnetar flares. This discovery implies that active magnetars can produce FRBs bright enough to be seen at extragalactic distances, potentially solving one of modern astronomy’s biggest mysteries.
How Long Magnetars Last
A magnetar’s extreme magnetic field doesn’t last forever. The field decays over time, and it’s actually this decay that powers most of the dramatic activity magnetars are known for. The strongest magnetars, those with fields around a quadrillion gauss, show evidence of their dipole field decaying on a timescale of roughly 1,000 years. As the field weakens, the magnetar’s spin period barely changes, effectively freezing at a terminal value while the magnetic energy continues to drain away.
Magnetars with the strongest initial fields burn brightest but fade fastest. Objects born with fields in the range of a few hundred trillion gauss may remain active for tens of thousands of years before quieting down. A class of older, cooler neutron stars appears to represent magnetars that are 100,000 to 600,000 years old, their original fields having significantly decayed. At that point, they no longer produce dramatic bursts and become essentially invisible compared to their younger selves. The active, flaring phase of a magnetar’s life is a brief chapter in cosmic terms, lasting a few thousand to perhaps a hundred thousand years before the star settles into a quiet retirement as an ordinary, slowly spinning neutron star.