From the December 2017 issue

Why do neutron stars have magnetism? Is the magnetism of a neutron star the same as a Child’s toy magnet? If a neutron star emits photons, where would they come from, and would they not experience the Zeeman effect? Why are neutron stars called stars?

Bruce Ameismeier Chicago, Illinois
By | Published: December 4, 2017 | Last updated on May 18, 2023
This artist’s impression shows a magnetar — a neutron star with an incredibly strong magnetic field — in the Westerlund 1 star cluster. The curved lines depict the star’s magnetic field, which can average a thousand trillion times stronger than the magnetic field of Earth and 1,000 times stronger than an ordinary neutron star.
ESO/L. Calçada
Good questions! Neutron stars are historically called stars because they are round, massive, and compact — no better reason, and that convention doesn’t hold for white dwarfs. Neutron stars are magnetic because their interiors contain powerful electrical currents. In that sense, they have more in common with electromagnets, which are associated with electric fields, than with toy magnets, which are permanent magnets and require no electric field to incite their magnetic properties.

The next level of “why,” however, is tougher to answer. Why do neutron stars have magnetic fields whose strengths typically range between a billion and a quadrillion times Earth’s magnetic field? An easy, and wrong, answer is that if you were to contract a star like the Sun to the size of a neutron star (maybe 15 miles [24 kilometers] in diameter) and conserve magnetic flux, you’d get about the right value. This is wrong, though, because the Sun won’t become a neutron star; instead, a neutron star originates from the core of a more massive star, and for such cores, the flux conservation argument gives far too low a magnetic field. So, this question isn’t resolved yet.

The Zeeman effect is a splitting of atomic lines due to magnetic fields. Neutron stars, however, have such huge magnetic fields that the structures of the atoms on the surface are altered. Rather than being basically spheres, the atoms become narrow (and short) cylinders aligned along the magnetic field. This should produce huge changes in the lines, rather than the small perturbations that we’re used to from the Zeeman effect.

M. Coleman Miller
Professor of Astronomy and Center for Theory & Computation Director, University of Maryland, College Park, Maryland