The universe has a reputation for producing bizarre phenomena, and neutron stars are among its absolute weirdest creations. Formed when huge stars explode and collapse into small city-scale spheres, these objects are the most dense stellar remnants that we know of aside from black holes.
A team of astronomers has now successfully identified the most massive neutron star on record, according to a paper published on Monday in Nature Astronomy. Called J0740+6620, the star is 2.14 times as massive as the Sun, and measures about 15 miles in diameter.
This beats out previous contenders, such as J1614−2230 and J0348+0432, and pushes the known upper limit of neutron star masses well beyond two times the mass of the Sun.
The discovery is important because scientists have yet to pinpoint the maximum possible mass for neutron stars. J0740+6620 likely sits near this threshold, which means it can shed light on the mysterious interior dynamics of neutron stars and yield insights into the deaths and afterlives of massive stars.
The new mass detection “is interesting because it informs our understanding of how supernovae form neutron stars (and how massive the progenitor stars must be),” said lead author Thankful Cromartie, a graduate student in astronomy at the University of Virginia, in an email.
“In order to account for neutron stars that are born extremely massive, we need to refine our models of stellar evolution and supernovae explosions,” she added.
J0740+6620 is a pulsar, a special type of neutron star that emits luminous beams of radiation out of its magnetic poles. Because these poles happen to be oriented toward Earth, scientists can clearly observe it regularly blinking like a cosmic lighthouse, despite the fact that it is located 4,600 light years away.
The pulsar has a companion star that scientists think is a white dwarf, another type of stellar corpse that is not quite so dense as neutron stars or black holes. Cromartie and her team observed this binary system from 2014 to 2019 using the Green Bank Telescope in West Virginia.
The researchers were able to calculate the pulsar’s mass due to its interactions with its companion white dwarf. As the two objects orbit each other, their immense gravity warps space surrounding them, which distorts the radiant pulses emitted by J0740+6620.
Light from the pulsar travels slightly farther as a result of this distortion, which is a phenomenon called the “Shapiro Delay,” after astrophysicist Irwin Shapiro, who first described it in 1964.
The team used this subtle time delay to estimate the mass of the white dwarf, which in turn enabled them to calculate the mass of the pulsar. The research revealed that J0740+6620 is the biggest neutron star known to scientists.
The results “serve as a strong validation of the existence of high-mass neutron stars,” the team concluded in the paper. “Even small increases in the measured mass of the most massive neutron stars force a reconsideration of the fundamental physics at play in their interiors.”
Correction: An earlier version of this paper stated that J0740+6620 was measured at 2.17 solar masses. That was based on a preprint version of the Nature Astronomy paper. A new version of the paper had a measurement of 2.14 solar masses. The article has been updated to reflect this number.