A supernova is an extremely energetic stellar explosion or explosive stellar event that can produce an enormous increase in brightness and eject material into surrounding space. Supernovae are not one single physical phenomenon but a broad category containing several different mechanisms. Two major pathways are core collapse supernovae, which occur when sufficiently massive stars develop unstable cores and can no longer support themselves against gravity, and thermonuclear supernovae, most famously Type Ia events, which involve runaway nuclear burning in a white dwarf under particular circumstances.
During a supernova, enormous amounts of energy can be released, with much of the energy in some core collapse events carried away by neutrinos. The explosion can synthesize and distribute chemical elements, generate shock waves and profoundly alter the surrounding interstellar medium. Supernovae therefore play major roles in stellar evolution, galactic chemical enrichment and the dynamics of interstellar gas.
Core collapse supernovae originate from stars that have developed massive cores after successive stages of nuclear burning. In sufficiently massive stars, hydrogen burning is followed by helium burning and then increasingly advanced burning stages involving carbon, neon, oxygen and silicon. These processes ultimately produce an iron group dominated core. Fusion beyond the iron peak cannot provide the same net energy support because the binding energy per nucleon no longer increases in the required direction.
Once the core can no longer generate enough pressure through exothermic nuclear burning, gravitational collapse begins. Electron captures and photodisintegration alter the core composition and pressure, while the collapse proceeds extremely rapidly. The central region can reach nuclear densities, leading to the formation of a proto neutron star or, if the core is sufficiently massive and conditions permit, a black hole.
During core collapse, the inner core can stiffen dramatically when nuclear densities are reached. The collapse launches a shock wave, although the exact mechanism by which this shock is revived sufficiently to produce a successful explosion is complex. Neutrinos carry away an enormous amount of gravitational binding energy from the newly formed compact object. Interactions involving neutrinos, matter and hydrodynamic instabilities contribute to the explosion dynamics.
The shock propagates through the outer stellar layers, heating and accelerating them and potentially triggering explosive nuclear reactions. Multi dimensional turbulence, convection, rotation, magnetic fields and the detailed structure of the progenitor star can all influence the outcome. Not every massive star necessarily produces an ordinary visible supernova in exactly the same way, and some may undergo fallback or direct collapse into black holes with relatively weak or failed explosions.
Thermonuclear supernovae have a different origin. Type Ia supernovae are associated with carbon oxygen white dwarfs that undergo runaway thermonuclear burning under particular binary or merger conditions. A white dwarf is normally supported by electron degeneracy pressure, so increasing its temperature does not cause the same stabilising expansion response found in an ordinary gas supported star. If carbon ignition occurs under sufficiently degenerate conditions, nuclear burning can accelerate rapidly.
The resulting flame can transform much of the white dwarf into heavier elements, including intermediate mass and iron group nuclei, and can disrupt the star completely. Type Ia supernovae generally leave no neutron star remnant from the exploding white dwarf. Their detailed progenitor pathways can include accretion from a companion or mergers between white dwarfs, and the relative importance of these channels remains an area of ongoing research.
Supernova explosions are important sites of nucleosynthesis. Core collapse events produce and disperse many elements created during the progenitor stars lifetime and during the explosion itself. Explosive burning can create elements such as silicon, sulphur, calcium and iron group nuclei, while neutron rich environments associated with some explosions can contribute to the production of heavier elements.
Type Ia supernovae are especially important sources of iron group elements, including much of the iron that later becomes incorporated into stars, planets and living systems. However, not every heavy element is produced primarily in supernovae. Neutron star mergers and other stellar processes contribute significantly to the origin of certain heavy nuclei. Supernovae are therefore one component of the broader cosmic chemical evolution process through which successive generations of stars become enriched in elements heavier than hydrogen and helium.
The brightness of a supernova comes from several energy sources and processes. Shock heating can produce intense radiation, while radioactive decay of unstable nuclei synthesized during the explosion can power the light curve over longer periods. In many supernovae, radioactive isotopes such as nickel 56 decay through chains that ultimately produce energy deposited in the expanding ejecta. The luminosity changes over time according to the expansion, opacity, radioactive inventory and energy transport within the ejecta.
Some supernovae display characteristic spectral features that allow astronomers to classify them according to their physical mechanisms and composition. Type Ia events lack hydrogen lines in their spectra and have distinctive silicon features, while core collapse supernovae can retain evidence of hydrogen or helium depending on how much of the progenitor envelope remains at the time of explosion. Spectroscopy therefore provides a major tool for identifying supernova types and understanding their origins.
Supernova remnants persist long after the initial explosion. The expanding ejecta collide with surrounding interstellar gas, generating shock waves that heat and accelerate material. Magnetic fields can be amplified in these shocks, and charged particles can be accelerated to very high energies. Over thousands of years, the remnant can develop shells, filaments and complex structures visible at radio, optical, X ray and other wavelengths.
Supernova remnants contribute energy and heavy elements to the interstellar medium and can trigger or suppress later star formation depending on the surrounding conditions. Their expanding shocks can also contribute to cosmic ray acceleration. The remnant eventually mixes with the surrounding gas, returning processed stellar material to the galactic environment and influencing the chemical composition and thermal structure of future star forming regions.
Supernovae are also important astronomical distance and time probes. Certain Type Ia supernovae have relatively standardised relationships between their observed light curves and intrinsic luminosities, allowing astronomers to estimate distances after appropriate calibration and corrections. Observations of distant Type Ia supernovae contributed to the discovery that the expansion of the Universe is accelerating, leading to the modern concept of dark energy. Core collapse supernovae can also provide information about massive star populations, star formation rates and the evolution of galaxies.
Neutrinos from a nearby core collapse event would provide a direct probe of the collapsing stellar core, as demonstrated by the detection of neutrinos from Supernova 1987A in the Large Magellanic Cloud. Gravitational waves from suitable asymmetric core collapse or compact object mergers can provide additional information about violent stellar events.
