An average neutron star is 1.4 times the mass of the Sun — but it’s no bigger than a city in size

A neutron star is the densest object astronomers can observe directly, crushing half a million times Earth's mass into a sphere about 22 kilometers across, according to the new resultsNASA Goddard Space Flight Center/AEI Hannover
  • An average neutron star packs the density of nearly half-a-million Earths into a circle that’s only 11 kilometres wide.
  • A new study published in Nature Astronomy observed the merger of binary star system GW170817 to determine the most accurate measurement of neutron stars to date.
  • The study also concludes that if a neutron star were ever to collide into a black hole, it would likely be swallowed up whole.
Neutron stars are one of the densest objects in the universe, second only to black holes. Using observations from the merger of binary star system GW170817, astronomers believe that the mass of nearly half-a-million Earths is packed into an area that’s only 11 kilometres-wide .

"GW170817 was caused by the collision of two city-sized objects 120 million years ago when dinosaurs were walking around here on Earth. This happened in a galaxy a billion trillion kilometres away," said Collin Capano, lead author of the study published in Nature Astronomy.

On average, a neutron star has 1.4 times the density of the Sun. However, the Sun is more than a million kilometres wide. The typical neutron star, on the other hand, is smaller than the average city.


"Our results limit the radius to likely be somewhere between 10.4 and 11.9 kilometres. This is a factor of two more stringent than previous results," said Badri Krishnan, co-author and head of the research team at the Albert Einstein Institute ( AEI).

How stars die
When a massive star explodes, one or two events can occur. The supernova will either be so huge that the star turns into a black hole. But on the off chance that it doesn’t have enough energy to go kaput — it turns into a neutron star.

"Neutron stars contain the densest matter in the observable universe. In fact, they are so dense and compact, that you can think of the entire star as a single atomic nucleus, scaled up to the size of a city," said Collin Capano, lead author of the study.


Numerical relativity simulation of two inspiraling and merging neutron stars. Higher densities are shown in orange, lower densities are shown in blueT. Dietrich (Max Planck Institute for Gravitational Physics)

Although this study pegs the mass of an average neuron star at 1.4 solar masses, other discoveries ascertain that neutron stars can be as dense as 2.3 solar masses. Any denser, and it would enter black hole territory.

The study also found that if a neutron star were ever to collide into a black hole, it would likely be swallowed up whole. The only case where the neutron star would somehow disrupt is if the black hole were somehow smaller or rotating fast enough.

"Only when the black hole is very small or rapidly spinning, can it disrupt the neutron star before swallowing it; and only then can we expect to see anything besides gravitational waves," said Capano.

See also:
'Jekyll and Hyde' binary stars discovered by Indian-origin scientist

Black hole or a neutron star — NASA puzzled over strange bright lights in the 'Fireworks' galaxy

‘Zombie’ stars are coming back to life and scientists are on the hunt for more