The most massive neutron star ever recorded has been discovered by astronomers 4,600 light years from Earth.
The star is more than twice the mass of the sun but just 15 miles in diameter, making it the most dense object in the universe except for black holes. It is so dense a single sugar-cube worth of neutron-star material would weigh the same as the entire human population of Earth (100 million tons).
Neutron stars are objects formed from the collapsed cores of large stars following a supernova explosion. They are also known as pulsars due to the pulses of radiation they emit as they rotate at high speeds.
Named J0740+6620, the star is 2.17 times the mass of the sun and 333,000 times the mass of the Earth, according to the paper published in Nature Astronomy. Scientists say this star is approaching the limits of how compact a single object can become without crushing in on itself.
“Neutron stars are as mysterious as they are fascinating,” sad lead researcher Thankful Cromartie, a pre-doctoral fellow at Virginia University.
“These city-sized objects are essentially ginormous atomic nuclei. They are so massive that their interiors take on weird properties. Finding the maximum mass that physics and nature will allow can teach us a great deal about this otherwise inaccessible realm in astrophysics.”
The neutron star was identified by the Green Bank Telescope (GBT) in West Virginia which is so sensitive it can pick up radio waves emitted milliseconds after the birth of the universe.
“These stars are very exotic. We don’t know what they’re made of and one really important question is, ‘How massive can you make one of these stars?’ It has implications for very exotic material that we simply can’t create in a laboratory on Earth,” said Maura McLaughlin, astrophysics professor at West Virginia University.
The neutron star is a pulsar that emits beams of radio waves like a lighthouse as it spins. Pulsars get their name because of twin beams of radio waves they emit from their magnetic poles as they rotate hundreds of times each second.
Astronomers measure these radio waves to work out the mass of stellar objects. They can do this thanks to its orbiting companion star.
As the white dwarf passes in front of the pulsar star there is a subtle delay in the arrival time of the radio waves. This phenomenon – known as “Shapiro Delay” – is because the gravity from the white dwarf star slightly warps the space surrounding it.
This warping means the pulses from the rotating neutron star have to travel a little bit further. Astronomers measure that delay to calculate the mass of the white dwarf, which in turn provides a measurement of the neutron star.
Once the mass of one of the co-orbiting bodies is known they can accurately determine the mass of the other.
“The orientation of this binary star system created a fantastic cosmic laboratory,” said Scott Ransom, an astronomer at National Radio Astronomy Observatory (NRAO) and co-author on the paper.
“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 mind boggling densities.”
Pulsars spin with such speed and regularity that astronomers can use them to study the nature of space-time, the mass of stellar objects and our understanding of general relativity.