A supernova is an exceptionally energetic explosion associated with the death or disruption of a star. It produces a rapidly brightening astronomical source that subsequently fades, while expelled material expands into space. Some supernovae briefly rival the combined luminosity of an entire galaxy. The two principal mechanisms are gravitational collapse of a massive stellar core and runaway thermonuclear burning in a white dwarf. These mechanisms differ in their progenitors, energy sources, and surviving remnants. (science.nasa.gov)
Classification
Supernova classification is primarily observational rather than a direct description of the explosion mechanism. Through spectroscopy, astronomers identify absorption and emission features in the expanding ejecta. Type II supernovae show hydrogen lines, whereas Type I supernovae lack conspicuous hydrogen features. Type I is subdivided into Ia, Ib, and Ic: Type Ia displays a characteristic feature of singly ionized silicon near maximum brightness; Type Ib displays helium lines; and Type Ic lacks conspicuous hydrogen and helium lines. (indico.cern.ch)
This classification cuts across the physical distinction between explosion mechanisms. Types Ib, Ic, and II generally arise from core collapse. The absence of hydrogen in Ib and Ic indicates that the progenitor has lost its hydrogen-rich envelope; Ic progenitors are more extensively stripped. Mass loss and interactions with a companion can therefore give stars with broadly similar core-collapse mechanisms different observed spectra. Type Ia, by contrast, belongs to the thermonuclear family. (eso.org)
Core-collapse explosions
Core collapse occurs in stars born with roughly eight or more times the mass of the Sun, although the boundary depends on stellar evolution. During their lives, massive stars obtain support from the conditions maintained by nuclear fusion. Successive burning stages can build an iron-rich core. Further fusion of iron-group nuclei cannot provide the energy needed to sustain this core against gravity, and it eventually becomes unstable. (science.nasa.gov)
As the core contracts, electron capture and other processes produce enormous numbers of neutrinos. Collapse reaches nuclear densities, leaving a hot compact core and generating a shock. In neutrino-driven explosion models, energy deposited by neutrinos helps revive a shock that initially stalls. The details depend on multidimensional fluid motions and the progenitor’s structure, making the explosion mechanism a subject of computational and observational research. (arxiv.org)
The expelled envelope carries substantial kinetic energy, while most of the gravitational energy released during core collapse escapes as neutrinos. The compact remnant may become a neutron star or a black hole. Thus the visible display represents only part of the event’s energy budget, and an optical observation alone cannot reveal everything occurring at the center. (heasarc.gsfc.nasa.gov)
Thermonuclear explosions
Type Ia supernovae result from runaway nuclear burning involving a carbon–oxygen white dwarf in a binary star system. Possible pathways include material transferred from a companion and interactions or mergers between two white dwarfs. The relative importance of these channels remains under investigation; a single progenitor scenario should not be assumed for every Type Ia event. (nasa.gov)
White dwarfs are supported by electron degeneracy pressure. In the classical near-Chandrasekhar model, instability and ignition occur as the white dwarf approaches a limiting mass of about 1.4 solar masses. However, thermonuclear models also include explosions of substantially lighter white dwarfs. Rapid burning releases enough energy to disrupt a normal Type Ia progenitor, rather than forming the compact remnant characteristic of core collapse. (heasarc.gsfc.nasa.gov)
The explosion synthesizes radioactive nickel-56. Its decay through cobalt-56 to iron-56 supplies much of the subsequent heating, linking radioactivity to the evolving brightness. Radiation transport through the expanding ejecta also shapes the observed light curve, so brightness is not simply a direct measurement of the instantaneous nuclear burning rate. (arxiv.org)
Remnants and chemical enrichment
Ejected matter and swept-up surrounding gas form a supernova remnant. Initially, the ejecta expand almost freely; as they accumulate ambient material, their motion slows. Shocks heat gas to millions of degrees, producing strong X-ray emission. Remnant shapes reflect both the explosion and variations in the surrounding interstellar environment, including material expelled before the explosion. (heasarc.gsfc.nasa.gov)
Supernovae contribute to nucleosynthesis and disperse chemical elements made during stellar evolution and explosive burning. White-dwarf explosions are especially important sources of iron-group elements. Remnant shocks also accelerate charged particles, making these objects important laboratories for investigating the origin of high-energy cosmic rays. Chemical composition, shock structure, and particle acceleration can therefore be studied long after the original optical transient has faded. (supernova.eso.org)
Historical observations and cosmology
The supernova observed in 1054 produced the Crab Nebula. Other extensively studied historical explosions include those observed in 1572 and 1604. SN 1987A, in the Large Magellanic Cloud, provided a particularly important modern case: neutrinos detected alongside its optical appearance directly supported the core-collapse interpretation and enabled investigation of conditions inaccessible through light alone. (chandra.harvard.edu)
Type Ia supernovae are valuable distance indicators in cosmology. Their brightnesses are not identical, but light-curve measurements allow empirical standardization. Comparing inferred luminosity with apparent brightness provides distance estimates; combining these with redshift measurements probes cosmic expansion. Distant supernova observations established evidence for accelerating expansion, a discovery recognized by the 2011 Nobel Prize in Physics and central to investigations of dark energy. (lambda.gsfc.nasa.gov)