A supernova is an extraordinarily powerful stellar explosion or explosive stellar event in which a star undergoes a catastrophic transformation and releases an enormous amount of energy, radiation and matter into surrounding space. The word supernova describes several physically distinct kinds of explosion rather than one single mechanism.
The two broad families are core collapse supernovae, which occur when sufficiently massive stars exhaust the nuclear fuel needed to support their cores, and thermonuclear supernovae, commonly associated with white dwarfs in binary systems. During a supernova, material that had been confined inside a star can be accelerated to thousands or tens of thousands of kilometres per second, while the explosion produces intense electromagnetic radiation, neutrinos and newly synthesised chemical elements. The resulting remnant can become a neutron star, a pulsar, a magnetar or a black hole in core collapse events, while thermonuclear explosions can leave no compact stellar remnant. Supernovae are therefore both endpoints of stellar evolution and major agents of cosmic chemical enrichment.
In a massive star, core collapse begins after the star has progressed through successive stages of nuclear fusion. Hydrogen fusion produces helium, helium fusion produces heavier nuclei, and increasingly advanced burning stages can create carbon, oxygen, neon, magnesium and silicon. The exact sequence depends on stellar mass and composition. Eventually the central region becomes dominated by iron group nuclei.
Iron is fundamentally different from lighter fusion fuels because fusing iron into heavier elements does not provide the net energy required to support the star against gravity. Once the core can no longer obtain sufficient pressure support from nuclear energy and related processes, gravity causes it to collapse extremely rapidly. Electrons are forced into protons through electron capture, producing neutrons and neutrinos, while the density rises enormously. The collapsing core can reach nuclear density, at which point the behaviour of extremely compressed matter changes and the inner core can halt its collapse and launch a shock. This core collapse is the central physical event behind most ordinary massive star supernovae.
The explosion of a core collapse supernova is not simply a giant conventional blast caused by the star running out of fuel. The collapse releases gravitational potential energy on an immense scale, with most of that energy initially carried away by neutrinos. The collapsing inner core becomes extremely compact while the outer core continues falling inward.
A shock wave forms when the inner core stiffens at approximately nuclear density, but the initial shock can lose energy by breaking apart heavy nuclei and can temporarily stall. Neutrino interactions are believed to play a crucial role in reviving the stalled shock under appropriate conditions, assisted by multidimensional fluid motions such as convection and the standing accretion shock instability. Magnetic fields and stellar rotation can also become important in particular explosions. The detailed mechanism remains an active research subject because the transition from core collapse to a successful explosion involves extremely dense matter, neutrino transport, nuclear physics and turbulent plasma flows that cannot be reproduced perfectly by simple one dimensional models.
Another major pathway is the thermonuclear supernova, most famously represented by a Type Ia supernova. A white dwarf is the compact remnant of a lower or intermediate mass star and is supported primarily by electron degeneracy pressure rather than ordinary thermal gas pressure. If a carbon oxygen white dwarf in a suitable binary system gains enough material, merges with another white dwarf, or otherwise reaches conditions favourable to runaway carbon burning, nuclear reactions can accelerate dramatically.
Because degenerate matter does not expand and cool in the normal way when heated, the burning can become unstable and propagate through the star. Carbon and oxygen are converted into heavier elements, particularly intermediate mass elements and iron group nuclei, releasing enormous nuclear energy. The energy unbinds the white dwarf rather than merely causing an outer layer to explode. Unlike a typical core collapse supernova, a normal Type Ia event is not expected to leave a neutron star or black hole at its centre.
Supernova classification historically relied heavily on the spectrum of the light produced by the explosion. Type II supernovae show hydrogen in their spectra and generally arise from massive stars that retained substantial hydrogen envelopes. Type Ib events lack strong hydrogen but show helium, while Type Ic events lack both strong hydrogen and helium signatures and are generally associated with strongly stripped massive stars. Type Ia explosions lack hydrogen features and display distinctive silicon signatures.
Astronomers also use classifications such as Type IIn and Type IIb to describe more specific spectral and environmental properties. Modern understanding recognises that the same broad spectral category can arise through different evolutionary histories, so classification does not always identify one unique progenitor mechanism. The light curve, spectra, surrounding circumstellar material, host environment, neutrino signals, gravitational wave observations and remnant properties can all provide additional evidence about what happened before and during the explosion.
Supernovae are responsible for important parts of the cosmic chemical inventory. Stellar nuclear fusion can produce elements up to approximately the iron group under normal stellar burning conditions, while explosive burning during supernovae can create additional intermediate mass and iron group nuclei. Some of the heaviest elements require neutron capture processes involving very neutron rich environments, and the relative contribution of different supernova mechanisms to those elements remains an area of active research.
Supernova ejecta mix newly produced elements into interstellar space, where they can later become incorporated into molecular clouds, stars, planets and living organisms. Elements such as oxygen, silicon, calcium and iron therefore have histories connected to earlier generations of stars. Supernova explosions also inject energy and momentum into galaxies, compressing some clouds, disrupting others and contributing to the turbulent structure of the interstellar medium. Their remnants can remain visible for thousands of years as expanding shells of hot gas and accelerated particles.
The observable brightness of a supernova comes from several physical processes. Immediately after explosion, shock heating produces extremely hot material and radiation. As the ejecta expand and cool, radioactive isotopes produced during the explosion can decay and provide a continuing source of energy. In Type Ia supernovae, radioactive nickel and its decay products are particularly important to the characteristic light curve.
In core collapse events, radioactive material, shock interaction and energy deposited by a central compact object can contribute to the observed emission. The explosion also produces neutrinos that can escape much more readily than ordinary photons and can provide a direct probe of the collapsing core. In rare cases, gravitational waves may be produced by asymmetric core collapse, although their detection from an individual supernova has not yet become routine. The expanding ejecta can interact with circumstellar gas lost by the progenitor before explosion, generating additional radiation across many wavelengths.
Supernovae have several important limitations as simple models of stellar death. Not every massive star necessarily produces a bright ordinary supernova, and some may undergo weak explosions, fallback events or direct collapse into black holes with comparatively faint electromagnetic signatures. Binary interactions can strip stars, transfer mass and radically alter their eventual evolution. Rotation, magnetic fields, metallicity, mass loss and internal mixing can all influence the outcome.
Astronomers therefore describe supernovae as a family of explosive stellar phenomena rather than a single standard event. The basic sequence for a massive star is nevertheless clear: nuclear evolution eventually leaves an unstable core, gravity overwhelms available support, the core collapses, and an enormous amount of energy is released while the outer layers are expelled or otherwise transformed. For thermonuclear supernovae, the key sequence instead involves a degenerate white dwarf undergoing runaway nuclear burning. Supernovae are consequently central to understanding stellar evolution, galactic chemistry, compact objects, cosmic radiation and the origin of many elements from which later planetary systems are built.
