NASA’s NICER Tests Matter’s Limits

Matter in the hearts of neutron stars – dense remnants of exploded massive stars – takes the most extreme form we can measure. Now, thanks to data from NASA’s Neutron star Interior Composition Explorer (NICER), an X-ray telescope on the International Space Station, scientists have discovered that this mysterious matter is less squeezable than some physicists predicted. The finding is based on NICER’s observations of PSR J0740 6620 (J0740 for short), the most massive known neutron star, which lies over 3,600 light-years away in the northern constellation Camelopardalis. Previous observations place the neutron star’s mass at about 2.1 times the Sun’s. At a neutron star’s surface, an atmosphere of hydrogen or helium rests on an iron crust. A mile or so down is the outer core, where atoms brake down into their building blocks: neutrons, protons, and electrons. Here, the immense pressure has crushed together protons and electrons to form a sea of mostly neutrons – packed together at up to twice the