What’s Inside Neutron Stars? Posted on Size measurements of two neutron stars are narrowing down what kinds of exotic matter might exist in their extremely dense cores. Neutron stars are the tricksters of the celestial sphere. Their age, their temperature, even their size is not always what it first appears to be. But with the Neutron star Interior Composition Explorer (NICER) aboard the International Space Station, astronomers are finally beginning to make some headway measuring these stars’ actual size — and with that, some insight into their strange interiors. Members of the NICER team presented two independent size measurements of the most massive neutron star known at the recent virtual meeting of the American Physical Society. These studies, now undergoing scientific review, suggest that nuclear physicists might need to rethink what happens in the stars’ ultra-dense cores.