What is a neutron star? One of the densest objects in space, explained

A neutron star begins as a massive star, at least about eight times the mass of the Sun. For most of its life, nuclear fusion in the star's core produces energy and outward pressure that balances the inward pull of gravity.

When the core runs out of fuel, that pressure fails and the core collapses in a fraction of a second. Protons and electrons are squeezed together into neutrons, and the star's outer layers are blasted away in a supernova explosion. What remains of the core is a neutron star. If the collapsing core is too massive, nothing can stop the collapse and it becomes a black hole instead.

Extraordinarily. NASA puts it this way: one sugar cube of neutron-star material would weigh about a billion tonnes on Earth, about as much as a mountain. Another way to picture it: if the whole Earth were squeezed to the density of a neutron star, it would fit into a ball about 250 metres wide.

Such material cannot actually be scooped out and brought to Earth, because it only stays this dense under the crushing pressure inside the star. That is exactly why physicists value neutron stars: they are natural laboratories for matter in conditions no experiment on Earth can create.

The name suggests a simple ball of neutrons, but scientists think the structure is layered:

• a very thin atmosphere
• a solid outer crust
• an inner crust of extremely dense matter
• a core whose exact make-up is still unknown

What happens to matter in that core is one of the open questions in physics.

A pulsar is a spinning neutron star that sends out beams of radio waves or other radiation from its magnetic poles. If a beam sweeps across Earth as the star turns, telescopes pick up a regular pulse, like the flash of a lighthouse. The first pulsar was found in 1967 by Jocelyn Bell Burnell, then a graduate student, whose signal was so regular it was jokingly labelled LGM-1, for "little green men".

Some pulsars spin hundreds of times a second; the fastest known turns 716 times a second. Their pulses are so regular that the best of them rival atomic clocks, and the very first planets ever found beyond the solar system, in 1992, orbit a pulsar (see exoplanet).

A magnetar is a neutron star with an exceptionally strong magnetic field. An ordinary neutron star's field is already trillions of times stronger than Earth's; a magnetar's is about a thousand times stronger again, the strongest magnetic fields known in the universe. Magnetars can release sudden bursts of X-rays and gamma rays.

Some neutron stars orbit each other in pairs. Over millions of years they slowly spiral together and finally collide. On 17 August 2017, the LIGO and Virgo detectors caught the gravitational waves from such a collision, called GW170817, and telescopes around the world then saw the explosion it caused. Observations showed that the collision produced heavy elements such as gold and platinum. India has approved its own gravitational-wave detector, LIGO-India, to be built in Maharashtra.

Whether a dying star leaves a neutron star or a black hole depends mainly on how much mass its collapsing core contains.

The Giant Metrewave Radio Telescope (GMRT), 30 large dishes near Pune run by the National Centre for Radio Astrophysics, is one of the world's leading telescopes for studying pulsars. Indian astronomers using it, as part of the Indian Pulsar Timing Array, contributed to the 2023 international evidence for a background of very low-frequency gravitational waves. In space, ISRO's AstroSat and XPoSat observatories study the X-rays from neutron stars and their surroundings.

How stars live and die, from red giants to white dwarfs and beyond, is lesson 10 of Astronomy 201, the WhyCosmos course that follows Astronomy 101.