Magnetars: The Strongest Magnets in the Universe
A magnetar is a type of neutron star with the most powerful magnetic field known in the universe — up to a thousand trillion times stronger than Earth's. These rare, extreme stellar corpses pack more mass than the Sun into a sphere the size of a city, and their magnetism is so intense it would be lethal from thousands of kilometres away and can distort the very structure of atoms.
From dying star to cosmic magnet
Magnetars are born, like ordinary neutron stars, when a massive star exhausts its fuel and its core collapses in a supernova. What sets a magnetar apart is the strength of its magnetic field, which can reach 1015 gauss — compared with a few gauss for a refrigerator magnet. Exactly how such fields arise is still debated, but a leading idea is a "dynamo" mechanism in which violent, rapid churning of the newborn neutron star's fluid interior amplifies the field to colossal values in the first seconds of its life.
Starquakes and giant flares
A magnetar's magnetic field is so strong that it stresses and cracks the star's rigid crust, triggering starquakes. These sudden ruptures release stupendous bursts of X-rays and gamma rays. The most violent events, called giant flares, can briefly outshine entire galaxies in gamma rays. A 2004 giant flare from a magnetar 50,000 light-years away was so powerful it measurably disturbed Earth's upper atmosphere. In their quieter moments, magnetars are observed as soft gamma repeaters and anomalous X-ray pulsars — two classes of object now understood to be the same kind of star.
Why they matter
Magnetars are natural laboratories for physics that is impossible to recreate on Earth: matter at nuclear density, and electromagnetic fields near the quantum limit where the vacuum itself behaves strangely. They have also been linked to some fast radio bursts — millisecond flashes of radio waves from across the cosmos — after a burst was traced to a magnetar in our own galaxy in 2020, tying these mysterious signals to extreme neutron-star physics.
A common misconception
A magnetar's power output is not driven by nuclear fusion, the way a normal star shines. A magnetar has stopped fusing; its flares and steady glow are powered by the decay and rearrangement of its magnetic field. It is, quite literally, a star running on magnetism.
Active research and further reading
Neutron-star magnetism is an active research area, and recent work has studied magnetism in strongly frustrated and exotic magnetic systems relevant to extreme-field physics. For the established physics behind this article:
- Duncan, R. C. & Thompson, C. "Formation of very strongly magnetized neutron stars." The Astrophysical Journal Letters 392, L9 (1992).
- Carroll, B. W. & Ostlie, D. A. An Introduction to Modern Astrophysics, 2nd ed. Cambridge University Press, 2017.
- Shapiro, S. L. & Teukolsky, S. A. Black Holes, White Dwarfs, and Neutron Stars. Wiley, 1983.