Astronomers say a rapidly rotating millisecond pulsar called J0740+6620 is the most massive neutron star ever measured, packing 2.17 times the mass of the Sun into a sphere only 20-30 kilometres across.

Artist impression of the pulse from a massive neutron star being delayed by the passage of a white dwarf star between the neutron star and Earth – a phenomenon known as the Shapiro Delay. Credit: BSaxton, NRao/AUI/NSF

And this, they suggest, approaches the limits of how massive and compact a single object can become without crushing itself down into a black hole.

The discovery was made by members of the NANOGrav Physics Frontier Centre, using the Green Bank Telescope in West Virginia, US.

“These city-sized objects are essentially ginormous atomic nuclei; they are so massive that their interiors take on weird properties,” says Thankful Cromartie, principal author of a paper published in the journal Nature Astronomy.

“Finding the maximum mass that physics and nature will allow can teach us a great deal about this otherwise inaccessible realm in astrophysics.”

Pulsars emit twin beams of radio waves from their magnetic poles, and these sweep across space. Some rotate hundreds of times each second.

As pulsars spin with phenomenal speed and regularity, they can be used for very precise timekeeping.

In the case of this binary system, which is nearly edge-on in relation to Earth, this cosmic precision provided a pathway for astronomers to calculate the mass of the two stars thanks to what is known as the Shapiro Delay.

Gravity from a white dwarf companion star warps the space surrounding it, in accordance with Einstein’s general theory of relativity. This makes the pulses from the pulsar travel a little further as they move through the distorted spacetime around white dwarf.

This delay tells researchers the mass of the white dwarf, which in turn provides a mass measurement of the neutron star.

“The orientation of this binary star system created a fantastic cosmic laboratory,” says co-author Scott Ransom.

“Neutron stars have this tipping point where their interior densities get so extreme that the force of gravity overwhelms even the ability of neutrons to resist further collapse.

“Each ‘most massive’ neutron star we find brings us closer to identifying that tipping point and helping us to understand the physics of matter at these mindboggling densities.”