Neutron Stars & Magnetars

When a star between about 8 and 25 solar masses exhausts its nuclear fuel, its core collapses in a fraction of a second. The outer layers are blasted away as a supernova. What remains is a neutron star - an object roughly 20 kilometres across, yet containing more mass than the Sun.

The density is beyond intuitive comprehension: a single teaspoon of neutron star material weighs approximately a billion tonnes. The entire star is essentially a single atomic nucleus, 20 kilometres wide, held together by gravity and supported against further collapse by neutron degeneracy pressure - the quantum mechanical resistance of neutrons to being compressed further.

A neutron star's magnetic field is one of the most powerful forces in nature. During collapse, the original star's magnetic field is compressed from a volume millions of kilometres across into a sphere just 20 kilometres wide. Conservation of magnetic flux means the field strength increases by a factor of roughly 10 billion.

The result is a magnetic dipole field - the same shape as a bar magnet, with field lines arcing from one magnetic pole to the other. This is what the visualisation renders: particles tracing the dipole field structure, concentrated near the poles where the field is strongest. The magnetic axis is typically tilted relative to the rotation axis, which is what produces the lighthouse effect in pulsars.

The field equations follow the dipole formula: B_r = (2m cos θ) / r³ and B_θ = (m sin θ) / r³, where m is the magnetic moment, θ is the colatitude, and r is the distance from the centre. Field strength falls off as the cube of the distance - it drops by a factor of eight every time you double your distance.

Quiet neutron star. Most neutron stars are invisible. They are born extremely hot (around a trillion degrees) but cool rapidly, emitting thermal X-rays that fade over millions of years. Without active emission, they are nearly undetectable. The visualisation shows a sparse, slowly rotating field structure with no beam activity.

Pulsar. If the magnetic field is strong enough and the rotation fast enough, charged particles are accelerated along the open field lines near the magnetic poles, producing beams of coherent radio emission. The beam sweeps through space as the star rotates. The visualisation shows the intensified particle flow along the polar field lines and the characteristic twin-beam structure.

Magnetar starquake. Magnetars have magnetic fields a thousand times stronger than ordinary pulsars - up to 10¹¹ Tesla. The immense magnetic stress can fracture the neutron star's crystalline crust. When the crust cracks, the sudden rearrangement of the magnetic field releases enormous energy in milliseconds. The visualisation shows this as an explosive perturbation of the particle system, with the field structure distorting and particles scattering before gradually settling.

A neutron star packs 1.4 solar masses into a sphere 10 kilometres in radius. Its density of 3.7 × 10¹&sup7; kg/m³ means that a cubic centimetre weighs as much as a mountain. The escape velocity is about 100,000 km/s - a third of the speed of light. Light leaving the surface is gravitationally redshifted by about 20%.

The surface gravity is roughly 2 × 10¹² m/s² - about 200 billion times Earth's gravity. A human standing on the surface would be compressed into a film less than an atom thick. The crust is a rigid lattice of iron nuclei, about a kilometre thick, stronger than any material on Earth by a factor of roughly ten billion.

The fastest known pulsar, PSR J1748-2446ad, rotates 716 times per second. Its equator moves at 24% of the speed of light. The strongest known magnetic field belongs to magnetar SGR 1806-20, measured at approximately 10¹¹ Tesla - strong enough that at a distance of half the Earth-Moon separation, it would erase every credit card on Earth.