The neutron star's strong gravity pulls gas from a nearby companion star. The gas forms a disk as it flows on to the neutron star, like water pouring down a drain (red area). Since a neutron star has about the mass of the Sun compressed into a sphere only about 16 km across, it is incredibly dense. This gives the star tremendous gravity, about 300 000 times greater than Earth's surface gravity, which compresses the gas as it builds up on the surface of the neutron star.

Eventually, pressure and heat in the gas on the surface becomes so high that the gas detonates in a tremendous nuclear explosion. The explosion distorts and illuminates the gas disk.

Credits: NASA/ Dana Berry

The neutron star (blue sphere) is part of a binary star system, and its neighbouring star (yellow-red sphere) supplies the fuel for the thermonuclear bursts. During solar outbursts or when the orbit brings the stars closer together, gas from the companion star flows toward the neutron star, attracted by its strong gravity. The flow of gas forms a swirling disk around the neutron star, called an accretion disk (multi-coloured swirl around the blue sphere).

Thermonuclear bursts arise as gas moving at close to the speed of light crashes onto the neutron star surface. The gas, pinned to the neutron star by gravity, spreads across the surface. As more and more gas rains down, pressure builds and temperature climbs until there is enough energy for nuclear fusion. This ignites a chain reaction that engulfs the entire neutron star within a second. Bursts last for one to two minutes and can occur several times per hour.

Credits: NASA