(CNN) - Astronomers have detected the most massive neutron star ever, and it almost shouldn't even exist.
Neutron stars are the smallest in the universe, with a diameter comparable to the size of a city like Chicago or Atlanta. They are the leftover remnants of supernovae. But they are incredibly dense, with masses bigger than that of our sun. So think of the sun, compressed into a major city.
In the case of the newly detected neutron star, dubbed J0740+6620, it's 333,000 times the mass of the Earth and 2.17 times the mass of the sun. But the star is only about 15 miles across. It's 4,600 light-years from Earth.
This is close to the limit of how much mass a compact object can contain before it crushes itself into a black hole.
The neutron star itself is rapidly spinning, which is called a pulsar because they send out beams of radio waves from each magnetic pole. The beams mimic the sweeping motion of a lighthouse beam, hence the name "pulsar."
Pulsars act like atomic clocks because they regularly pulse, so astronomers can use them to study space and time.
The star was detected by the Green Bank Telescope in West Virginia. Researchers published their findings about the star Monday in the journal Nature Astronomy.
The researchers weren't necessarily looking for this neutron star. Rather, it was happenstance as they searched for gravitational waves.
"At Green Bank, we're trying to detect gravitational waves from pulsars," said Maura McLaughlin, study author and Eberly Distinguished Professor of physics and astronomy at West Virginia University. "In order to do that, we need to observe lots of millisecond pulsars, which are rapidly rotating neutron stars. This is not a gravitational wave detection paper but one of many important results which have arisen from our observations."
They were able to measure its mass because of a white dwarf companion star that warped the space around both stars. This warping acted as a way to accelerate the pulsar's pulses through space. They could learn the mass of the white dwarf and neutron star this way through the "Shapiro Delay."
"Neutron stars are as mysterious as they are fascinating," said Thankful Cromartie, a graduate student at the University of Virginia and Grote Reber pre-doctoral fellow at the National Radio Astronomy Observatory in Charlottesville, Virginia. "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."
Due to their mysterious nature, astronomers still want to answer more questions about neutron stars and this unique one could help them. They want to know if crushed neutrons become a type of "superfluid," what the breakdown looks like and where the actual tipping point lies when gravity takes over.
"The orientation of this binary star system created a fantastic cosmic laboratory," said Scott Ransom, study co-author and an astronomer at the National Radio Astronomy Observatory. "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."