Supernova remnants represent the residual effects of massive stars which have reached the end of their life-cycle, including a massive stellar explosion as a grand finale, and which leave behind them spectacular gas clouds and stellar remnants (neutrinos) which cover multiple full moons in width across the sky. Regrettably, for residents of the northern hemisphere, only four supernova remnants (SNR) are visible and, more specifically, the Crab Nebula (M1) in Taurus, the massive Veil complex (NGC 6960, 6974, 6979, 6992, 6995) in Cygnus, the Jellyfish Nebula (IC 433) in Gemini and Simeis 147 (aka Shajn 147, Sh 2-240) also in Taurus. The most recognized supernova remnant is perhaps the Crab nebula in Taurus which is believed to have exploded in 1054 AD as documented by Chinese astronomers of the time whereas Simeis 147 is especially dim and represents one of the faintest objects in the sky.

Note: The Crab Nebula depicted below represents the first entry in Charles Messier's catalog of deep-sky entries and which he observed in 1758 while searching for a return of comet Halley. This confusion led Messier to pursue the cataloguing of such entities which could be confused for comets and when in reality they represent deep-sky objects. Since the nebula is expanding at an estimated 1500 km/sec and given its current size of 11 light-years in diameter, reverse mathematics suggests that the supernova itself occurred in mid-1054, a date which coincides with Chinese and Arabic records of a bright star which shone brightly enough to be visible during the day for nearly a month and during the evening for almost two years (with a magnitude estimated to lie between -4.5 and -7). The Crab Nebula is unique in that the progenitor star responsible for the supernova is a pulsar neutron star with an initial mass between eight and twelve solar masses. The current nebula is estimated to have a mass of 2-3 solar masses whereas the pulsar star is between 1.4 and 2 solar masses; there is no satisfactory explanation at the moment to account for the remaining mass which has yet to be accounted for.

Note: For an excellent article on observing supernova remnants, see Astronomy Magazine (Sept/2006: 64-67).