Magnetars: The Strongest Magnets in the Universe

Beyond Ordinary Magnetism

Take the strongest magnet ever built in a laboratory — roughly 45 tesla, enough to levitate a frog — and multiply it by a billion. Then multiply again by a thousand. The result is the magnetic field of a magnetar: 10⁸ to 10¹¹ tesla, the most intense magnetic fields known to exist in the universe.

Magnetars are a rare subclass of neutron stars — stellar corpses only 20 kilometres across but containing 1.4 to 2 solar masses. What distinguishes them from ordinary neutron stars and pulsars is their magnetic field, which is so strong that it dominates the star’s behaviour, cracks its crust, twists the surrounding spacetime, and produces bursts of radiation visible across the galaxy.

About 30 magnetars are currently known. They are among the most extreme objects in astrophysics, and they are the most powerful magnets in the known universe.

The Scale of the Field

To appreciate how extraordinary a magnetar’s magnetic field is, consider a hierarchy of magnetic field strengths. Earth’s field is about 5 × 10⁻⁵ T — strong enough to deflect a compass needle. A neodymium refrigerator magnet produces about 0.3 T. A hospital MRI machine operates at 1.5–3 T. The strongest continuous laboratory magnets reach 45 T. The strongest pulsed magnets, which last only milliseconds before destroying themselves, reach about 1,200 T.

A typical magnetar field is 10⁸ to 10¹¹ T — at least a hundred million times stronger than the strongest laboratory magnet, and about a quadrillion times stronger than Earth’s field. At these field strengths, the physics becomes extraordinary.

Extreme Magnetic Physics

At 10⁹ T and above, the magnetic field fundamentally alters the behaviour of matter and light.

Atomic structure breaks down. In a strong magnetic field, atomic electron orbitals — normally spherical — are compressed into narrow cylinders aligned with the field. At magnetar-level fields, the magnetic force on an electron exceeds the electrostatic force binding it to the nucleus. Atoms become elongated, cigar-shaped structures, and chemistry as we know it ceases to exist. Hydrogen atoms, for example, shrink in the transverse direction to a fraction of their normal size while stretching along the field lines.

Vacuum birefringence. Quantum electrodynamics predicts that sufficiently strong magnetic fields make the vacuum itself birefringent — light polarised parallel to the field travels at a different speed than light polarised perpendicular to it. This effect, predicted in the 1930s by Heisenberg and Euler, is too small to detect in terrestrial laboratories but has been observed in X-ray emissions from the magnetar RX J1856.5−3754 by the IXPE (Imaging X-ray Polarimetry Explorer) mission — a direct confirmation of quantum vacuum effects in nature.

Photon splitting. At the strongest magnetar fields, individual photons can spontaneously split into two lower-energy photons — a process forbidden in field-free vacuum by energy-momentum conservation but permitted in the presence of an intense magnetic field. This exotic quantum process affects the X-ray emission spectrum of magnetars and has no terrestrial analogue.

Magnetohydrodynamic stress. The magnetic field exerts pressure. In a magnetar, magnetic pressure can exceed the pressure of the degenerate matter that makes up the star’s crust. When the field rearranges, it can fracture the crystalline crust — triggering a starquake.

Formation: Birth of a Monster

Magnetars form in core-collapse supernovae — the explosive deaths of massive stars (roughly 8–25 solar masses). When the core of such a star exhausts its nuclear fuel, it collapses under gravity from a radius of thousands of kilometres to about 10 km in less than a second. This is the birth of a neutron star.

Conservation of magnetic flux during collapse amplifies the progenitor star’s magnetic field enormously. A typical massive star has a surface field of about 0.01 T. When the core radius decreases by a factor of 10⁵, the field (proportional to 1/r² for flux conservation) increases by a factor of 10¹⁰, reaching about 10⁸ T — the low end of the magnetar range.

But flux conservation alone may not explain the strongest magnetar fields. The leading hypothesis invokes a convective dynamo in the proto-neutron star during the first 10–30 seconds after collapse. If the newborn neutron star rotates with a period of 1–3 milliseconds (plausible for some progenitors), vigorous convection driven by neutrino heating can generate large-scale currents that amplify the field to 10¹⁰–10¹¹ T through a dynamo mechanism analogous to — but vastly more powerful than — the process that generates Earth’s magnetic field.

Not all neutron stars become magnetars. The process requires a specific window of rotation rate and convective vigour. Most neutron stars are born as ordinary pulsars with fields of 10⁴–10⁹ T — impressive, but orders of magnitude below magnetar strength.

Starquakes and Giant Flares

The most dramatic manifestation of magnetar activity is the giant flare — a catastrophic event in which the star releases as much energy as the Sun emits in 100,000 years, compressed into a fraction of a second.

The mechanism begins with stress in the magnetar’s crust. The interior magnetic field slowly evolves, twisting and deforming the rigid crystalline crust (composed of iron-group nuclei arranged in a lattice structure, supported by electron degeneracy pressure). When the accumulated magnetic stress exceeds the crust’s yield strength — typically after years to decades — the crust fractures in a starquake.

The fracture releases a burst of magnetic energy into the magnetosphere. The magnetic field lines, previously twisted and stressed, snap and reconfigure in a process analogous to solar flares but incomparably more violent. The released energy heats trapped plasma to billions of kelvin, producing an intense flash of gamma rays and X-rays.

Three giant flares have been observed from magnetars in and near our galaxy. The most powerful — from SGR 1806−20 on 27 December 2004 — released approximately 2 × 10³⁹ joules in the initial spike (lasting about 0.2 seconds). Despite occurring 50,000 light-years away, the gamma-ray flash was bright enough to saturate most orbiting detectors and measurably ionised Earth’s upper atmosphere. Had SGR 1806−20 been at a distance of 10 light-years, the effects on Earth’s biosphere would have been severe.

Soft Gamma Repeaters and Anomalous X-ray Pulsars

Magnetars were historically identified in two observational categories before being recognised as the same type of object.

Soft Gamma Repeaters (SGRs) are sources that emit repeated bursts of soft (lower-energy) gamma rays, typically lasting 0.1–1 seconds. These bursts are powered by fractures in the magnetar crust and rearrangements of the magnetic field. Unlike gamma-ray bursts from distant galaxies (which are produced by jets from collapsing stars or merging neutron stars), SGR bursts repeat from the same source.

Anomalous X-ray Pulsars (AXPs) are X-ray sources that pulse with periods of 2–12 seconds but whose X-ray luminosity far exceeds what could be powered by rotational energy loss alone. The excess energy comes from the decay of the magnetic field itself — a magnetar’s field is so strong that its dissipation releases enough energy to power persistent X-ray emission for thousands of years.

Both SGRs and AXPs are now understood to be magnetars observed under different conditions. The unifying model — the magnetar model, proposed by Robert Duncan and Christopher Thompson in 1992 — explains both phenomena as consequences of an ultrastrong magnetic field.

Magnetar Lifetimes and the Magnetic Graveyard

Magnetar activity is short-lived on astronomical timescales. The ultrastrong magnetic field decays through ohmic dissipation and ambipolar diffusion in the neutron star interior, with a characteristic timescale of about 10,000 years. After this time, the field weakens to ordinary pulsar levels, and the magnetar becomes indistinguishable from a normal, slowly rotating neutron star.

This short active lifetime explains why magnetars are rare: at any given time, only a small fraction of neutron stars are in the magnetar phase. The Milky Way is estimated to contain about a billion neutron stars, but only about 30 active magnetars are known. Most former magnetars have joined the magnetic graveyard — invisible, quiet, their extraordinary fields long since decayed.

Why Magnetars Matter

Magnetars are natural laboratories for extreme physics. They test quantum electrodynamics in field regimes billions of times beyond any terrestrial experiment. They probe the equation of state of ultra-dense matter through their oscillation modes. They contribute to our understanding of gamma-ray bursts, fast radio bursts (the magnetar SGR 1935+2154 was the first galactic source of a fast radio burst, in 2020), and the origin of heavy elements through their potential role in r-process nucleosynthesis.

They are also a reminder that the universe routinely produces conditions that exceed our most ambitious engineering by factors of billions — and that the fundamental forces of nature, pushed to their extremes, produce phenomena that challenge our physical intuition at every turn.