Astroparticle Physics

100 6 Primary Cosmic Rays6.2.3 Supernova Neutrinosdiscovery of SN 1987AThe brightest supernova since the observation of Kepler inthe year 1604 was discovered by Ian Shelton at the Las Campanasobservatory in Chile on February 23, 1987 (see Fig.6.22). The region of the sky in the Tarantula Nebula in theLarge Magellanic Cloud (distance 170 000 light-years), inwhich the supernova exploded, was routinely photographedby Robert McNaught in Australia already 20 hours earlier.However, McNaught developed and analyzed the photographicplate only the following day. Ian Shelton was struckby the brightness of the supernova which was visible to thenaked eye. For the first time a progenitor star of the supernovaexplosion could be located. Using earlier exposures ofthe Tarantula Nebula, a bright blue supergiant, Sanduleak,was found to have exploded. Sanduleak was an inconspic-uous star of 10-fold solar mass with a surface temperatureof 15 000 K. During hydrogen burning Sanduleak increasedits brightness reaching a luminosity 70 000 higher than thesolar luminosity. After the hydrogen supply was exhausted,the star expanded to become a red supergiant. In this processits central temperature and pressure rose to such valuesthat He burning became possible. In a relatively shorttime (600 000 years) the helium supply was also exhausted.Helium burning was followed by a gravitational contractionin which the nucleus of the star reached a temperature of740 million Kelvin and a central density of 240 kg/cm 3 .These conditions enabled carbon to ignite. In a similar fashioncontraction and fusion phases occurred leading via oxygen,neon, silicon, and sulphur finally to iron, the elementwith the highest binding energy per nucleon.The pace of these successive contraction and fusionphases got faster and faster until finally iron was reached.Once the star has reached such a state, there is no way togain further energy by fusion processes. Therefore, the stabilityof Sanduleak could no longer be maintained. The starcollapsed under its own gravity. During this process the elec-trons of the star were forced into the protons and a neutronstar of approximately 20 km diameter was produced. In thecourse of this deleptonization a neutrino burst of immenseintensity was created,Fig. 6.22Supernova 1987A in the TarantulaNebula {6}fusion cycles of a stargravitational collapseproduction of a neutron stardeleptonization→ neutrino burste − + p → n + ν e . (6.38)