Neutron stars are the dead remnants of massive stars that aren’t massive enough to explode as supernovae but are massive enough that when they are no longer able to produce energy through fusion, will collapse down and crush their atoms into exotic forms of matter.

Neutron stars are roughly the size of Manhatten Island and can contain a bit more than two times the Sun’s mass in stuff. Theories suggest that the outer skin can be fairly normal matter, but inside the dead star, pressure forces protons and electrons to combine into a dense degenerate gas of neutrons. What we didn’t know was if the super-dense core was just neutrons or some more compact form of matter, like a quark soup. 

New research from the ISS’s NICER X-ray Telescope looks at one of the largest mass neutron stars out there, pulsar J0740+6620, to see if its size can reveal its inner nature. They also looked at the average-massed J0030+0451 for comparison. Early results indicate the higher mass pulsar is bigger than predicted by quark-soup-center theories and smaller than predicted by theories with just neutrons and protons. This was not expected and hints at new states of matter. According to the team lead, Cole Miller: NICER is not only rewriting the textbooks on neutron stars but also revolutionizing our confidence in our measurements of objects that are both very distant and very small. 

According to study co-author Anna Watts: Our new measurements of J0740 show that even though it’s almost 50% more massive than J0030, it’s essentially the same size. That challenges some of the more squeezable models of neutron star cores, including versions where the interior is just a sea of quarks. J0740’s size and mass also pose problems for some less squeezable models containing only neutrons and protons. 

Understanding what is inside neutron stars may help us understand black holes and allow us to get at new, fundamental physics that tells the story of the earliest moments of our universe.

More Information

NASA press release

APS press release

“Constraining the Neutron Star Mass–Radius Relation and Dense Matter Equation of State with NICER. III. Model Description and Verification of Parameter Estimation Codes,” Slavko Bogdanov et al., submitted to the Astrophysical Journal Letters (preprint on arxiv.org)