Supernova: The Explosion That Shapes the Universe

The Formation Process of a Supernova

Core-collapse supernovae occur following the collapse of the core of a massive star, generally at least eight times more massive than the Sun. When the star's nuclear fuel is exhausted, the pressure caused by nuclear fusion reactions is no longer sufficient to counteract the star's gravity, causing its core to collapse in a fraction of a second.

The resulting compact core can become a neutron star or, for the most massive stars, a black hole, while the outer layers of the star are ejected into space at speeds reaching several tens of thousands of kilometers per second, thus creating a supernova.

The Different Types of Supernovae

Observational classification of supernovae is based on the presence or absence of certain lines in their light spectrum, particularly those of hydrogen. This classification partially, but not entirely, overlaps with the two physical explosion mechanisms.

Type Ia: The Thermonuclear Explosion of a White Dwarf

Type Ia supernovae do not show hydrogen lines in their spectrum. They result from the complete thermonuclear explosion of a carbon-oxygen white dwarf, a member of a binary system, which accretes matter from a companion star until it approaches the Chandrasekhar limit (about 1.4 solar masses). The entire star is then destroyed: unlike other types, no compact remnant (neutron star or black hole) remains. Their very homogeneous peak luminosity makes them standard candles used to measure cosmic distances, which led to the discovery of the accelerating expansion of the Universe.

Type Ib and Ic: Massive Stars Stripped of Their Envelope

Type Ib and type Ic supernovae also lack hydrogen lines; they could long be confused with type Ia on this sole criterion. However, they result from a totally different mechanism: the core collapse of a massive star that has lost its outer hydrogen envelope (and, for type Ic, also its helium envelope), either through an intense stellar wind or mass transfer to a companion star. Type Ib retains helium lines in its spectrum, which type Ic has also lost.

Type II: Core Collapse of a Massive Star That Retained Its Hydrogen

Type II supernovae, like types Ib and Ic, result from the core collapse of a massive star, but this one has retained a significant part of its hydrogen envelope, whose lines clearly appear in the spectrum. They are more frequent in regions of recent star formation in spiral galaxies.

N.B.:
A common misconception systematically associates "type I" with "white dwarf explosion" and "type II" with "core collapse". This equivalence is only true for type Ia (white dwarf) on one side, and II on the other: types Ib and Ic, although classified among type I supernovae due to the absence of hydrogen in their spectrum, do result from a core collapse of a massive star, just like type II.

Super Explosion: A Phenomenon Beyond the Supernova

Super explosions refer to events even more extreme than supernovae, often linked to phenomena like hypernovae or gamma-ray bursts (GRBs). A hypernova is a particularly powerful core-collapse supernova, producing energy quantities far greater than a classic supernova, generally associated with the direct collapse of a very massive star into a black hole.

Gamma-ray bursts, on the other hand, are incredibly intense and brief discharges of energy that can outshine the entire galaxy for a few seconds to a few minutes. So-called "long" gamma-ray bursts are associated with the core collapse of a rapidly rotating massive star giving birth to a black hole, while "short" gamma-ray bursts most often originate from the merger of two neutron stars.

The Energy Released by a Supernova

Generally, a supernova releases an astronomical amount of energy, on the order of 10⁴⁴ to 10⁴⁶ joules. This energy is emitted mostly in the form of neutrinos (over 99% in the case of a core collapse), with the rest distributed between the kinetic energy of the ejected matter and visible electromagnetic radiation. This corresponds to about 10,000 to 100,000 times the energy that our Sun emits over its entire lifetime (about 10 billion years).

Duration of the Supernova

A supernova is not an instantaneous event, although it unfolds rapidly in astronomical terms. The core collapse itself lasts a fraction of a second, but the visible light peak is reached in a few days to a few weeks, then declines over several months. The remnants of the explosion, called supernova remnants, continue to emit light and energy for thousands of years, gradually expanding into the interstellar medium.

Historical Supernovae Observed from Earth

A few supernovae that occurred in our Galaxy or nearby have marked the history of astronomy through their direct observation.

SN 1572, Tycho Brahe's Supernova

First observed on November 11, 1572, in the constellation Cassiopeia, this supernova is named after Danish astronomer Tycho Brahe, whose precise measurements of its position demonstrated it lay beyond the lunar sphere, contradicting the Aristotelian dogma of the immutability of the heavens. Spectroscopic analysis of light reflected by surrounding dust (a "light echo" observed more than four centuries after the explosion) confirmed in 2008 that it was a standard type Ia thermonuclear supernova. Its remnant is located, depending on measurement methods, between about 8,000 and 13,000 light-years from Earth.

SN 1987A, the Supernova That Opened the Era of Neutrino Astronomy

On February 23, 1987, the collapse of a blue supergiant star in the Large Magellanic Cloud, a satellite galaxy of the Milky Way located about 160,000 light-years away, produced the type II supernova SN 1987A. About three hours before the detection of its visible light, about twenty neutrinos from the stellar core collapse were detected by the Kamiokande-II (Japan), IMB (USA), and Baksan (Russia) instruments, a historic first that experimentally confirmed theoretical models of core collapse and gave birth to neutrino astronomy.

Comparative Table of Some Notable Supernovae

The following table compares some of the best-documented supernovae, whether observed historically or identified more recently thanks to their remnants (remains of the explosion that continue to emit light and energy for months).

N.B.:
SN 1181 was long the only historical supernova without a definitively identified remnant; its association with the Pa 30 nebula was only established in 2021, based on the position, age, and brightness inferred from Chinese and Japanese observations of the time.

The Impacts of Supernovae on the Universe

Nucleosynthesis of Heavy Elements

Supernovae play a crucial role in the formation of heavy elements, but also in the formation of many isotopes, ions, and other chemical species. Due to the extremely high temperatures and extreme pressure conditions prevalent in these explosions, a wide variety of elements, ionized forms, and isotopes are produced. Some of the heaviest elements, like gold, lead, or uranium, are thought to be produced rather by a rapid neutron capture process (r-process) occurring during neutron star mergers, a phenomenon complementary to supernovae identified since the observation of the GW170817 event in 2017. All these elements, once formed, are dispersed in space, thus enriching the interstellar medium. This contributes to the creation of new stars, planets, and solar systems, and allows the formation of the matter from which planets and life are made.

Formation of Molecules in Ejected Gases

Although the formation of complex molecules is more common in colder conditions (like in molecular clouds), supernovae can also initiate the formation of certain molecules in ejected gases, notably simple molecules such as carbon monoxide (CO) or carbon dioxide (CO2), as well as cosmic dust grains.