Astronomers Discover A Near-Impossibly Massive Neutron Star


The Green Bank Telescope (GBT) has detected the densest neutron star yet recorded, approaching the theoretical density limit for such stars. The star, J0740+6620, has 2.17 times the mass of the Sun. But if you ran a marathon, you'd have gone more than the 30-kilometer diameter of this neutron star.

“Neutron stars are as mysterious as they are fascinating,” said Thankful Cromartie, the main author of the paper documenting the new star. “These city-sized objects are essentially ginormous atomic nuclei. They are so massive that their interiors take on weird properties.”

The ultimate state of a star relies on its mass. To comprehend how neutron stars arise from dead stars, we must first understand white dwarfs. Because of a cosmic stop sign, most stars (97%) will eventually become white dwarfs, the second densest kind of star after neutron stars. White dwarves are so dense that their atomic connections have shattered, turning them into a plasma of nuclei and electrons. However, electrons do not want to be in the same state as one another and will fight being squeezed to this point. It's called electron degeneracy pressure.

Stars with fewer than 10 solar masses tend to become white dwarfs, which have a mass of 1.44 solar masses. But a denser star, 10 to 29 solar masses, may form a neutron star. The star's density now exceeds the pressure of electron degeneracy. Because electrons do not wish to inhabit the same state, they must join with protons, generating neutrons and releasing neutrinos. Thus, neutron stars are virtually completely made up of neutrons.

Similar to how electron degeneracy keeps up white dwarfs, neutron degeneracy pressure holds up neutron stars. Like white dwarfs, neutron stars have a maximum pressure they can withstand.

“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,” said Scott Ransom, a co-author of the paper. That's why J0740+6620 seems to be a 2.17 solar mass neutron star. J0740+6620 would have become a black hole if it had greater mass.

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

The Milky Way contains an estimated 100 million neutron stars, most of which are old and cold, making detection challenging. Surprisingly, J0740+6620 was a pulsar, a form of rapidly rotating neutron star that blasts radio waves and other electromagnetic radiation out beyond the magnetic poles. From our vantage point, these rays appear to “pulse” with astonishing regularity. Neutron stars are hard to observe, but pulsars are easy to spot and analyze as their radio waves sweep over Earth.

J0740+6620 was a fortuitous find for researchers for another reason. The star was actually a white dwarf in a binary system. These two characteristics allowed the researchers to measure the new star's mass using the Shapiro Delay.

As J0740+6620’s white dwarf partner crossed in front of the neutron star’s beam of radio waves, astronomers on Earth could identify a brief delay in the inbound radio waves. Because the white dwarf's gravity bent space around it, passing radio waves had to go a little further. Astronomers might compute the white dwarf's mass by measuring this. Knowing the mass of one planet in a binary system simplifies the calculation of the mass of the companion; consequently, J0740+6620 was revealed to be the most massive neutron star ever detected.

Reference(s): 

Arvix.org


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