Supernova

Full definition

A massive star does not die quietly. It explodes. For a few days, its brightness can rival that of an entire galaxy of a hundred billion stars — that is a supernova. The energy released, about 10⁴⁴ joules, exceeds everything the Sun will produce over its 10-billion-year lifetime. Most of this energy escapes as neutrinos (99 %); the rest goes into light and the kinetic ejection of outer layers at 10,000 km/s.

Two distinct physical engines drive a supernova, even though the optical outcome looks similar.

The first, core collapse, strikes massive stars (> 8 M☉). Their iron core can no longer fuse elements for energy, pressure collapses, and the core contracts in seconds under gravity. It rebounds off nuclear density and launches a shock wave that tears the star apart. The remnant is a neutron star or, for very high masses, a black hole.

The second, thermonuclear, hits a white dwarf that exceeds the Chandrasekhar limit (1.44 M☉) by accreting gas from a companion — or by merging with another white dwarf. The core carbon ignites at once and blows the entire star to pieces. No compact remnant survives.

These explosions are what spread carbon, oxygen, iron and all 'metallic' elements across the cosmos — every calcium atom in your bones, every iron atom in your blood comes from a supernova.

Numbers and orders of magnitude

Total energy released: ~10⁴⁴ J (10⁵¹ erg). Equivalent: the energy produced by the Sun in... 10 billion years. Of this, 99 % in neutrinos, ~1 % in kinetic energy of ejecta, 0.01 % in visible light.

Peak luminosity: absolute magnitude -16 to -20 depending on type (~10⁹ to 10¹⁰ L☉). A Type Ia peaks at -19.3 ± 0.3: that tight dispersion is what makes them cosmological standard candles.

Ejecta velocity: 3,000 to 30,000 km/s (0.01 to 0.1 c).

Ejected mass: 1 to 15 M☉ depending on the progenitor.

Frequency: about 1-2 per century in a galaxy like the Milky Way; ~1 per second across the observable Universe. Big extragalactic surveys (ZTF, LSST, Pan-STARRS) detect several thousand per year.

Last SN visible to the naked eye: SN 1987A, on February 24, 1987 in the Large Magellanic Cloud (166,000 ly), magnitude 3 at peak. Last well-documented galactic SN: SN 1604 (Kepler), observed in broad daylight for more than a year.

The different types

Historical classification (Minkowski 1941, then Zwicky) rests on optical spectra, not directly on mechanism.

Type Ia. No hydrogen, silicon lines (Si II 6355 Å). Mechanism: accreting white dwarf or WD-WD binary reaching 1.44 M☉. No compact remnant. Very standardized light curve → cosmological standard candle. The discovery by Perlmutter, Riess and Schmidt that distant Type Ia SNe are fainter than expected revealed the accelerating cosmic expansion (2011 Nobel Prize).

Type Ib and Ic. No hydrogen (Ib: helium present; Ic: neither H nor He). Core collapse of a massive star that lost its outer envelope (Wolf-Rayet star or binary). Some broad-line Ic SNe are linked to long gamma-ray bursts.

Type II. Hydrogen present. Core collapse of a red supergiant that kept its envelope. Subtypes IIP (plateau light curve), IIL (linear decline), IIn (narrow lines = interaction with dense circumstellar environment).

Historic naked-eye supernovae. SN 1006 (-7.5, the brightest ever recorded), SN 1054 (-6, remnant = Crab Nebula), SN 1572 (Tycho Brahe, -4), SN 1604 (Kepler, -3), SN 1987A (+3, Large Magellanic Cloud). The next galactic event has been awaited since 1604 — statistically overdue.

How do we detect them?

Supernovae are transient events: they shine for weeks to months, then fade. Detecting them requires continuous sky monitoring.

Large automated surveys. The Zwicky Transient Facility (ZTF, Palomar, 2018) and now the Vera C. Rubin Observatory (LSST, first survey 2025) scan the visible sky several times a week. They detect about 10,000 SNe per year, vs a few dozen per year in the 1990s.

Space telescopes. Hubble imaged SN 1987A remnants in high resolution for nearly 40 years. James Webb (JWST, 2021) observes distant supernovae in the infrared, probing the first galaxies. Gaia (ESA) also detects supernovae through photometry in its all-sky survey.

Neutrinos. 99 % of a core-collapse SN's energy leaves as neutrinos. Super-Kamiokande (Japan), IceCube (South Pole) and soon Hyper-K can detect the next galactic SN before its light. The SNEWS system coordinates a worldwide alert. SN 1987A was the first historical detection (24 neutrinos in Kamiokande II).

Gravitational waves. Asymmetric core-collapse SNe might be detected by LIGO/Virgo at galactic distances — no detection yet.

What about amateur astronomy? Remnants are perfect targets: Crab Nebula (M1, magnitude 8.4, Taurus), Veil Nebula in Cygnus, Vela SNR (southern sky). Extragalactic supernova hunting is a real sport: Tim Puckett, Tom Boles and Koichi Itagaki have each discovered over 100. Our sky map tool helps locate candidate galaxies.

Not to be confused with

Several explosive phenomena are related but distinct.

Nova. A surface explosion on a white dwarf accreting gas from a companion: the gas ignites without destroying the star. Absolute magnitude ~-8 (10⁴ to 10⁵ times less energetic than a supernova). Can recur. Archetype: T Coronae Borealis, expected between 2024 and 2027.

Hypernova / long gamma-ray burst. An extreme case of a very energetic Ic-BL SN (~10⁴⁵ J), often linked to the formation of a black hole and a relativistic jet. When that jet points at us, we see a long gamma-ray burst.

Kilonova. Merger of two neutron stars. Intermediate energy (10⁴¹-10⁴² J), duration of 1-2 weeks, spectrum dominated by heavy elements (lanthanides). First observed during GW170817 (August 2017).

Planetary nebula. Gentle expulsion of a low-mass star's outer layers — not an explosion. The remnant is a white dwarf, not a neutron star. The Ring Nebula (M57) is the archetype.

Solar flares and stellar flares. Magnetic events on the surface of an active star, negligible in brightness compared with a SN. No direct physical link.