Young Collapsed Supernova Remnants: Similarities and Differences in   Neutron Stars, Black Holes, and More Exotic Objects
  Type Ia supernovae are thought to explode completely, leaving no condensed remnant, only an expanding shell. Other types of supernovae are thought to involve core collapse and are expected to leave a condensed remnant, which could be either a neutron star or a black hole, or just possibly, something more exotic, such as a quark orstrange star, a naked singularity, a frozen star, a wormhole or a red hole. It has proven surprisingly difficult to determine which type of condensed remnant has been formed in those cases where the diagnostic highly regular pulsar signature of a neutron star is absent. We consider possible observational differences between the two standard candidates, as well as the more speculative alternatives.   We classify condensed remnants according to whether they do or do not possess three major features: 1)a hard surface, 2)an event horizon, and 3)a singularity. Black holes and neutron stars differ on all three criteria. Some of the less frequently considered alternatives are "intermediate," in the sense that they possess some of the traits of a black hole and some of the traits of a neutron star. This possibility makes distinguishing the various possibilities even more difficult. 
James S. Graber