The progressive slowdown of the rotation of neutron stars leads to their contraction and thus to a chemical imbalance between the neutrons, protons, and other particles in their core. This imbalance is adjusted by beta decays that release energy into the stellar interior, keeping it warm. Hubble Space Telescope observations show far ultraviolet emission from a few very old neutron stars, which may well be due to this process. This can be used to probe the state of very dense matter, as well as the possible time-variation of Newton’s constant of gravitation.

Andreas Reisenegger received his BS and MS from Universidad de Chile and his PhD from the California Institute of Technology, where his thesis advisor was Prof. Peter Goldreich. After two years as a postdoctoral member of the Institute for Advanced Study, he joined the faculty of Pontificia Universidad Católica de Chile. His main subject of research has been neutron stars, particularly their thermal and magnetic evolution, oscillation modes, and gravitational wave emission. He has also worked on topics of extragalactic astrophysics, such as structure formation, clusters, and superclusters of galaxies.