Guest Stars: Historical Supernovae and Remnants

Supernova explosions occur in massive stars in their final stages of stellar evolution. The explosion leaves behind a collapsed object, usually a neutron star observable as a pulsar, and an expanding outer envelope of the star observable as a supernova remnant (SNR). Because of the large amount of energy in the range of 1050 −1052 ergs released in the explosion, the energy radiated by the exploded star is equivalent to that of millions of normal stars. Thus a "new star" seems to appear and is easily visible to the naked eye for several months. Astronomers have estimated that in our galaxy such supernova explosions occur once about every 50 years. In ancient times, the occurrence of supernova might have been noticed as appearance of a bright new star which faded away into obscruity after a few months. Thus the ‘guest stars’ in the ancient records are indeed possible occurrences of supernova explosions in the galaxy. The guest star of 1054 A.D., well recorded by Chinese and Japanese astronomers is the best known historic supernova (Brecher et al. 1983; and references therein); today its remnants the Crab Nebula and the pulsar are observable over the entire electromagnetic spectrum from radio to γ-rays. Of the 150 SNRs known in our galaxy, only about 10 have been identified with guest stars of historical supernovae.