1. Supernovae and Their Remnants

The Crab Nebula is expanding and its pulsar is slowing down, both on time scales that indicate some very interesting real-time stellar evolution must have occurred about 1000 years ago, as indeed we know from Chinese and other ancient records. The Crab was, in fact, the first SNR to be confidently identified as such and to have its expansion rate measured and the first pulsar to have its first and second time derivatives measured. Among the interesting related details are (a) the current expansion is a bit faster than the average since the 1054 explosion, indicating outward pressure from the relativistic electrons and magnetic field responsible for the synchrotron emission, (b) some of the central synchrotron emission features wiggle and oscillate around at much larger speeds than the expansion but in a non-secular way, (c) the braking index of the pulsar is about 2.5 rather than 3.0 (as predicted for a pure magnetic dipole emitter), (d) energy is conserved (it wasn’t obviously so before 1968), in that the pulsar is losing rotational kinetic energy fast enough to keep up the supply of optical and X-ray emitting electrons, and (e) glitches in the slowing of the pulsar (for which data go back to a 1964 balloon flight) indicate complex coupling of crust to interior, presumably a magnetic one.