Neutron Star Structure Diagram (IMAGE) Royal Astronomical Society Caption Neutron stars are very small and dense stars, where roughly half a million earth masses of material are squeezed into a city-sized volume with a radius of 10 km. They are mostly composed of neutrons and the interior is thought to contain layers of superfluid, although the composition of the innermost core is still unknown. The NICER mission will probe these stars to reveal the nature of matter under these extreme physical conditions. Credit Credit: K.C. Gendreau et al. (2012), SPIE, 8443, 13 Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.