A supernova is an energetic stellar explosion in which a star’s luminosity rises dramatically before fading over weeks to months, releasing about 10^51 ergs (10^44 joules) of kinetic energy and heavy elements into the interstellar medium. Such events can outshine their host galaxies and profoundly affect galactic ecology. NASA and
Britannica describe supernovae as catastrophic endpoints of stellar evolution or runaway thermonuclear detonations. ("What's a Nova?" NASA; Britannica, "supernova summary").
Nomenclature and discovery
The International Astronomical Union designates supernovae with the prefix “SN,” the year, and a letter or letter pair (for example, SN 1987A). Since 2016 the official reporting and naming of new transients, including confirmed supernovae, has been handled by the IAU’s Transient Name Server, succeeding the Central Bureau for Astronomical Telegrams system. (TNS;
IAU).
Physical mechanisms
Two principal channels dominate:
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Thermonuclear (Type Ia): a carbon–oxygen White Dwarf accretes matter from a companion or merges with another white dwarf until conditions trigger a runaway fusion front that unbinds the star. In the single‑degenerate scenario, instability is often linked to approach to the Chandrasekhar Limit, near 1.4 solar masses; double‑degenerate mergers are also strongly supported observationally. (
NASA Roman; Maoz, Mannucci & Nelemans, journal://Annual Review of Astronomy and Astrophysics|Observational Clues to the Progenitors of Type Ia Supernovae|2014;
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Core collapse (Types II, Ib, Ic): massive stars (initial mass ≳8 M☉) exhaust nuclear fuel, their iron cores collapse, and a shock revived by neutrino heating and hydrodynamic instabilities expels the envelope, leaving a Neutron Star or Black Hole. Reviews detail the neutrino‑driven mechanism and roles of convection and SASI (standing accretion shock instability). (Janka, journal://Annual Review of Nuclear and Particle Science|Explosion Mechanisms of Core‑Collapse Supernovae|2012; Woosley, Heger & Weaver, journal://Reviews of Modern Physics|The evolution and explosion of massive stars|2002;
A rare third channel, pair‑instability, affects very massive stars whose electron–positron pair creation softens the equation of state, leading to partial (pulsational) or total disruption. (Heger & Woosley, journal://The Astrophysical Journal|The Nucleosynthetic Signature of Population III|2002; journal://New Astronomy Reviews|Massive star evolution: nucleosynthesis and reaction‑rate uncertainties|2002).
Spectral classification and subtypes
Supernovae are divided by early‑time spectra: Type II show Balmer hydrogen, while Type I lack hydrogen; Type Ia exhibit strong Si II near 6355 Å, Type Ib show He I, and Type Ic show neither He nor Si II. Type II subdivide into II‑P (plateau), II‑L (linear), IIn (narrow lines from dense circumstellar interaction), and IIb (transitional). This spectroscopic framework remains standard. (Filippenko, journal://Annual Review of Astronomy and Astrophysics|OPTICAL SPECTRA OF SUPERNOVAE|1997).
Light curves, energetics, and radioactive power
The optical output of most events is powered by radioactive decay, especially 56Ni → 56Co → 56Fe. For Type Ia supernovae, the peak luminosity correlates with the post‑maximum decline rate (the Phillips relation), enabling “standardization” for distance measurements. (Arnett, journal://The Astrophysical Journal|Type I supernovae. I—Analytic solutions for the early part of the light curve|1982; Phillips, journal://The Astrophysical Journal Letters|The Absolute Magnitudes of Type Ia Supernovae|1993; Hamuy et al., arXiv:astro-ph/9609059). Typical explosion energies are ~10^51 ergs with ejecta of several solar masses for core‑collapse events; Type Ia ejecta are ~1 M☉ dominated by iron‑group elements. (Woosley, Heger & Weaver, journal://Reviews of Modern Physics|The evolution and explosion of massive stars|2002; Arnett, book://David Arnett|Supernovae and Nucleosynthesis|Princeton University Press|1996).
Progenitors and environments
Evidence favors carbon–oxygen white dwarfs as Type Ia progenitors, with both single‑ and double‑degenerate channels contributing; no single channel alone explains all observations. (Maoz, Mannucci & Nelemans, journal://Annual Review of Astronomy and Astrophysics|Observational Clues to the Progenitors of Type Ia Supernovae|2014). Core‑collapse supernovae trace recent star formation and are common in spiral arms and star‑forming galaxies. (Woosley, Heger & Weaver, journal://Reviews of Modern Physics|The evolution and explosion of massive stars|2002).
Historical and nearby supernovae
The “guest star” of 1054 CE produced the Crab Nebula, recorded in Chinese and other chronicles; modern observations identify its pulsar wind nebula and remnant at ~6,500 light‑years. (NASA Hubble/Crab). The closest modern supernova, SN 1987A in the Large Magellanic Cloud, provided decisive neutrino detections that confirmed the core‑collapse paradigm. Kamiokande II and IMB observed bursts hours before optical brightening, consistent with models in which ~99% of the collapse energy emerges as neutrinos. (Hirata et al., journal://Physical Review Letters|Observation of a neutrino burst from the supernova SN1987A|1987; Bionta et al., journal://Physical Review Letters|Observation of a neutrino burst in coincidence with supernova 1987A|1987;
NASA ADS).
Rates
In the Milky Way, gamma‑ray measurements of 26Al imply a core‑collapse rate of about 1.9 ± 1.1 per century, consistent with “about one every 50 years.” (Diehl et al., journal://Nature|Radioactive 26Al from massive stars in the Galaxy|2006; CERN Courier summary). Broader extragalactic studies indicate a total Galactic supernova rate of a few per century when Type Ia events are included. (
NASA Roman).
Supernovae in cosmology
After standardization, Type Ia supernovae map the luminosity‑distance–redshift relation. Two teams used high‑redshift SNe Ia to show cosmic expansion is accelerating, implying dark energy. (Riess et al., journal://The Astronomical Journal|Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant|1998; Perlmutter et al., journal://The Astrophysical Journal|Measurements of Ω and Λ from 42 High‑Redshift Supernovae|1999; IOPscience AJ).
Nucleosynthesis and feedback
Supernovae synthesize and disperse heavy elements: Type Ia events produce iron‑group nuclei, while core‑collapse explosions forge elements from oxygen through the iron group and contribute to heavier species via explosive burning and associated processes. (Arnett, book://David Arnett|Supernovae and Nucleosynthesis|Princeton University Press|1996; Rauscher et al., journal://The Astrophysical Journal|Nucleosynthesis in Massive Stars with Improved Nuclear and Stellar Physics|2002; Woosley, Heger & Weaver, journal://Reviews of Modern Physics|The evolution and explosion of massive stars|2002). Shock waves from supernova remnants are widely accepted as primary accelerators of Galactic Cosmic Rays up to the “knee” via diffusive shock acceleration. (Blandford & Ostriker, journal://The Astrophysical Journal Letters|Particle acceleration by astrophysical shocks|1978; Blandford & Eichler, journal://Physics Reports|Particle acceleration at astrophysical shocks|1987; Helder et al., journal://Science|Measuring the Cosmic‑Ray Acceleration Efficiency of a Supernova Remnant|2009; Science Perspective).
Multi‑messenger and time‑domain astronomy
Neutrino detections from SN 1987A inaugurated neutrino astronomy for stellar explosions and motivate global early‑warning networks for the next Galactic event. (Hirata et al., journal://Physical Review Letters|Observation of a neutrino burst from the supernova SN1987A|1987; Bionta et al., journal://Physical Review Letters|Observation of a neutrino burst in coincidence with supernova 1987A|1987). Modern surveys and facilities, coordinated via the IAU’s Transient Name Server, discover and classify thousands of supernovae, enabling statistical studies and rapid multi‑wavelength follow‑up. (TNS).
Relation to other transients
Supernovae differ from novae: novae are thermonuclear surface outbursts on white dwarfs that do not destroy the star, whereas supernovae are terminal explosions far brighter. (NASA). Some stripped‑envelope core‑collapse supernovae (Type Ic‑BL) accompany long‑duration Gamma-Ray Burst events, linking relativistic jets and massive‑star deaths. (Woosley & Bloom, journal://Annual Review of Astronomy and Astrophysics|The Supernova–Gamma‑Ray Burst Connection|2006).