Supernova Explosions and Remnants stellar structure For a 25 solar mass star, the duration of each stage is stellar corpses: the core When the central iron core continues to grow and approaches Mch , two processes begin: nuclear photodisintegration and neutronization. Nuclear photodisintegration: The temperature is high enough for energetic photons to be abundant and get absorbed in the endothermic reaction: with an energy consumption of 124 MeV. The Helium nuclei are further unbound: consuming 28.3 MeV(the binding energy of a He nucleus). The total energy of the star is reduced per nucleon by With about 10 57 protons in a Chandrasekhar mass, this corresponds to a total energy loss of stellar corpses: the core Neutronization: The large densities in the core lead to a large increase in the rates of processes such as This neutronization depletes the core of electrons and their supporting degeneracy pressure, as well as energy, which is carried off by neutrinos. The two processes lead, in principle, to an almost total loss of thermal pressure support and to an unrestrained collapse of the core of a star on a free-fall timescale: In practice, at these high densities, the mean free path for neutrino scattering becomes of order the core radius. This slows down the energy loss, and hence the collapse time to a few seconds. core collapse supernovae As the collapse proceeds and the density and the temperature increase, the reaction becomes common, and is infrequently offset by leading to a equilibrium ratio of densities Thus most nucleons become neutrons review article type Ia supernovae. If it were not for radioactive heating, adiabatic radius of ϳ109 km; however, it was dimmer than a typical type II expansion of the debris would cool it to near invisibility in less than and early relied on 56Ni to power its muted optical light curve. Yet an hour. Type Ia supernovae are about ten times less prevalent than there is no reason to suspect that the explosion itself was not of the core-collapse supernovae, but yield about ten times as much iron, common core-collapse variety. The light curve and spectrum of a are often more than ten times brighter at peak light, and are supernova reflect more its progenitor’s radius, chemical makeup, spectacular sources of nuclear g-ray lines and continuum8. It is and expansion velocities than the mechanism by which it exploded. with these bright supernovae that observers are now obtaining the To the theorist, the achievement of the critical Chandrasekhar mass best and, perhaps, the most provocative information about the unites the types; the supernova mechanism is either by implosion to geometry of the Universe. nuclear densities and subsequent hydrodynamic ejection, or by Astronomers use observational, not theoretical, criteria to type thermonuclear runaway and explosive incineration. supernovae. A type I supernova (such as a type Ia) is one with no There is approximately one supernova explosion in the Universe hydrogen in its spectrum, while the spectrum of a type II supernova every second. In our galaxy, there is one supernova every ϳ30–50 has prominent hydrogen lines. The epochal supernova in the Large years and one type Ia supernova every ϳ300 years. Supernova Magellanic Cloud (LMC), SN1987A, was a core-collapse supernova, hunters, peering deeply with only modest-aperture telescopes, can because it exploded as a ϳ15–20M᭪ blue supergiant with a radius of now capture a dozen or so extragalactic supernovae per night, ϳ4 ϫ 107 km (ref. 9) and not as the canonical red supergiant with a mostly the bright type Ias. Approximately 200 supernova remnant shells are known in the Milky Way and these are radio, optical, and X-ray echoes of only the most recent galactic supernova explosions. review article Within the last millennium, humans have witnessed and recorded six supernovae in our galaxy (Table 1). nito oge r St 9 type Ia supernovae. If it were not for radioactive heating, adiabatPicr radiusaor f ϳ10 km; however, it was dimmer than a typical type II expansion of the debris would cool it to near invisibility in less than and early relied on 56Ni to power itsSmuupteedrnoopvtiaceal flrigohmt cmuraves.sYievte stars an hour. Type Ia supernovae are about ten times less prevalent than there is no reason to suspect that theAexsptlaors’isonfiirtsstelfthwearsmnootnoufctlheear stage is the fusion of hydrogen into core-collapse supernovae, but yield about ten times a8s much ironH, FceomO m- Soi n core-collapse variety. The light curve and spectrum of a 6x10 km He helium in its hot core. With the exhaustion of core hydrogen, most are often more than ten times brighter at peak light, and are supernova reflect more its progenitostra’srsratdhiuesn, cphreomcieceadl mtoakesuhpe,ll hydrogen burning, and then to core spectacular sources of nuclear g-ray lines and continuum8. It is and expansion velocities than the mechanism by which it exploded. with these bright supernovae that observers are now obtaining the To the theorist, the achievement of thheecliruitmicalbCuhrannindrga.seTkhhearamshaess of the latter are predominantly carbon best and, perhaps, the most provocative information about the unites the types; the supernova mechanidsmoxisyegitehnerabnydimlopwlo-smionastso stars do not proceed beyond this stage. C geometry of the Universe. Ironnuocrleear densities and subsequent hHydorwodeyvnear,msitcaresjewctiitohnm, oarssbeys from ϳ8M᭪ to ϳ60–100M᭪ (the upper review article Astronomers use observational, not theoretical, criteria to type thermonuclear runaway and explosivlieminitcidneepraetniodni.ng upon the heavy-element fraction at birth) proceed supernovae. A type I supernova (such as a type Ia) is one with no There is approximately one supernova explosion in the Universe 3 to carbon burning, with mostly oxygen, neon, and magnesium as hydrogen in its spectrum, while the spectrum of a typ6xe1I0I skumpernova every second. In our galaxy, there is one s1u,2pernova every ϳ30–50 9 ashes . For stars more massive than ϳ9–10M᭪, the ashes of carbon type Ia supernovae. If it were not for rhaadsiporaocmtiivneenhtehaytdinrogg,eandliianbesa.tTiche eproadchiualssuopfeϳrn1o0va kinmth; ehLoawrgeever,yietarws aasndimonme etryptehaIan sauptyerpniocaval teyvpeeryIIϳ300 years. Supernova 56 burning achieve sufficient temperatures to ignite and they burn expansion of the debris would cool it tMo angeeallraninicvCisliobuildit(yLMinCl)e,sSsNth19a8n7A, wanasdaecaorrley-croellaiepdseosunperNnoivtao, pohwunerteirtss, pmeeurtinedg doepetpilcyawl iltihghotnclyumrvoed.eYset-taperture telescopes, can predominantly to silicon, sulphur, calcium, and argon. Finally, these an hour. Type Ia supernovae are aboutbteecnautsiemiteesxlpelsosdpedreavsaalϳen1t5t–h2a0nM᭪ btlhuerseupiserngioanrtewasiothnatroadsiuuspoefct tnhoawt tchaepteuxrpe loa sdioonzenitsoerlf swoaesxntroatgaolafctthice supernovae per night, 7 ~ products ignite to produce iron and its congener isotopes near the core-collapse supernovae, but yield abϳo4uϫt t1e0n ktimm(ersefa.s9)manudchnoitroasnt,he cacnoomnimcaol nredcosurpe-ecrgoilalnatpwseithvaariet1 my.oTsthlyethliegbhrtigchutrtvyepeanIads. sAppepcrtorxuimatoeflya200 supernova remnant s Collapse of Core e peak of the nuclear binding energy curve. Fusion is exothermic only c shells(~1a.r5eMkn) own in the Milky Way and these are radio, optical, and are often more than ten times brighter at peak light, and are supernova reflect more its p. rogenitor’s radius, chemical makeup, spectacular sources of nuclear g-ray lines and continuum8. It is and expansion velocities thanXt-hreaymecehcoheasnoifsomnlbyythwehmicohst irteceexnptlfgooadrlaetcdhti.ec sauspsermnobvlayeoxfplloigsihotnesr. elements into elements up to the iron SuWpeirtnhin the last millennium, humansgrhoauvep,wnitonetsbseedyoanndd.reHcoerndcede, at the end of a massive star’s thermo- with these bright supernovae that observers are now obtaining the To the theorist, the achievrleyment oovf the critical Chandrasekhar mass Ea a 00 kmsi/xs supernovae in our galaxy (Table 1). ,0 Bo nuclear life, it has an ‘onion-skin’ structure in which an iron or best and, perhaps, the most provocative information about the guennitiotreSs the types; the supe60rnova meuchanism is either by implosion to o t a - n Pr r c 0 e oxygen–neon–magnesium core is nested within shells comprised of 0 geometry of the Universe. nuclear densities and0 subseqSuupenertnohS vyaderofrdoymnammasicsiveejescttaiorsn, or by , 3 h 0 o elements of progressively lower atomic weight at progressively 2x10 km 3 Astronomers use observational, not theoretical, criteria to type thermonuclear runaway and Aexsptlaor’ssivc fiersitntchienremraotniuocnle.ar stage is the fusion of hydrogen into 8 H Fe O - Si k supernovae. A type I supernova (such as a type Ia) is o6xn1e0 wkmith no He There is approximately onehesluiupmerinnoivtsaheoxtpcloorsei.oWnitihntthheeexUhnaluiovswetireosrne odfecnosrietiheysdraongedn,tmemospt eratures. The outer zone consists of hydrogen in its spectrum, while the spectrum of a type II supernova every second. In our galaxy, tshtaerrsethisenonperoscuepedertnoosvhaelelvheyrdyroϳge3un0n–b5uu0rrnninedg, haynddrothgeenntoancdor‘eprimordial’ helium. A typical nesting is has prominent hydrogen lines. The epochal supernova in the Large years and one type Ia supehrneloiuvma beuvrenriyng. T3h0e0asyheeasros.f tShueplaetFrtneroavraeSipredoOminaHntely carHbo. nThe oxygen in the ‘oxygen’ zone is the ν ν ϳ → → → → M and oxygen and low-mass stars do nmotajporrocseoeudrbceyoofnodxtyhgisenstaigne.the Universe, for little oxygen survives in Magellanic