Supernovae
A supernova is a powerful and luminous explosion of a star. A supernova occurs during the final evolutionary stages of a massive star, or when a white dwarf is activated into runaway nuclear fusion. The original object, called the precursor, collapses into a neutron star or black hole, or is completely destroyed to form a diffuse nebula. The peak brightness of a supernova can be comparable to that of an entire galaxy before fading over the course of several weeks or months.
The last supernova observed directly in the Milky Way was Kepler's supernova in 1604, which appeared not long after Tycho's supernova in 1572, both of which were visible to the naked eye. The remnants of more recent supernovae have been found, and observations of supernovae in other galaxies suggest that they occur on average about three times per century in the Milky Way. A supernova in the Milky Way would almost certainly be visible through modern astronomical telescopes. The most recent naked-eye supernova was SN 1987A, the explosion of a blue supergiant in the Large Magellanic Cloud, a satellite galaxy of the Milky Way.
Theoretical studies indicate that most supernovae are caused by one of two basic mechanisms: the sudden reignition of nuclear fusion in a white dwarf, or the sudden gravitational collapse of a massive star's core.
Reignition of a white dwarf raises the object's temperature enough to trigger runaway nuclear fusion, completely disrupting the star. Possible causes include an accumulation of material from a binary companion due to accretion, or due to a stellar merger.
In the case of the sudden implosion of a massive star, the core of a massive star will suddenly collapse once it is unable to produce enough energy from fusion to counteract the star's own gravity, which should happen as soon as the star iron begins to fuse, but can happen during an earlier stage of metal fusion.
Supernovae can eject several solar masses of material at speeds up to a few percent of the speed of light. This drives an expanding shock wave into the surrounding interstellar medium, sweeping up an expanding shell of gas and dust that is observed as a supernova remnant. Supernovae are a major source of elements in the interstellar medium, from oxygen to rubidium. The expanding shock waves of supernovae can trigger the formation of new stars. Supernovae are a major source of cosmic rays. They can also produce gravitational waves, although gravitational waves have so far only been detected by the mergers of black holes and neutron stars.
Classification
Astronomers classify supernovae based on their light curves and the absorption lines of various chemical elements found in their spectra. If a supernova's spectrum contains hydrogen lines (known as the Balmer series in the visual part of the spectrum), it is classified as Type II; otherwise it is Type I. In each of these two types there are subdivisions according to the presence of lines of other elements or the shape of the light curve (a graph of the apparent magnitude of the supernova as a function of time).
Type I
Type I supernovae are subdivided based on their spectra, with type Ia showing a strong ionized silica absorption line. Type I supernovae without this strong line are classified as types Ib and Ic, with type Ib showing strong neutral helium lines and type Ic lacking them. Historically, the light curves of type I supernovae have been seen as all broadly similar, too much so to make useful distinctions. Although variations in light curves have been studied, classification is still made on spectral grounds rather than on the shape of the light curve.
A small number of Type Ia supernovae show unusual features, such as non-standard brightness or broadened light curves, and these are usually categorized by reference to the earliest example showing similar features. For example, the subluminous SN 2008ha is often referred to as SN 2002cx-like or class Ia-2002cx.
A small fraction of type Ic supernovae exhibit strongly broadened and mixed emission lines that are considered indicative of very high expansion rates for the ejecta. These are classified as type Ic-BL or Ic-bl.
Calcium-rich supernovae are a rare type of very fast supernova with unusually strong calcium lines in their spectra. Models suggest they form when material is accreted from a helium-rich companion rather than a hydrogen-rich star. Because of helium lines in their spectra, they may resemble type Ib supernovae, but they are thought to have very different precursors.
Type II
The type II supernovae can also be subdivided based on their spectra. While most Type II supernovae exhibit very broad emission lines indicating expansion rates of many thousands of kilometers per second, some, such as SN 2005gl, have relatively narrow features in their spectra. These are called type IIn, where the "n" stands for "narrow".
A few supernovae, such as SN 1987K and SN 1993J, appear to change type, showing hydrogen lines early on, but becoming dominated by helium lines over weeks to months. The term "type IIb" is used to describe the combination of characteristics normally associated with type II and Ib.
Type II supernovae with normal spectra dominated by broad hydrogen lines that remain throughout the decay lifetime are classified based on their light curves. The most common type shows a distinctive "plateau" in the light curve shortly after peak brightness, with visual brightness remaining relatively constant for several months before the decline resumes. These are called type II-P, referring to the plateau. Less common are type II-L supernovae which do not have a clear plateau. The "L" means "linear", although the light curve is not actually a straight line.
Supernovas that do not fit into normal classifications are referred to as peculiar or "pec".
Type III, IV and V
Zwicky defined additional supernova types based on a few examples that did not exactly fit the parameters for type I or type II supernovae. SN 1961i in NGC 4303 was the prototype and only member of the Type III supernova class, known for its broad light curve maximum and broad hydrogen Balmer lines that developed slowly in the spectrum. SN 1961f in NGC 3003 was the prototype and only member of the Type IV class, with a light curve similar to a Type II-P supernova, with hydrogen absorption lines but weak hydrogen emission lines. The Type V class was coined for SN 1961V in NGC 1058, an unusual faint supernova or supernova impostor with a slow rise in brightness, a maximum lasting for many months, and an unusual emission spectrum. The similarity of SN 1961V to the Eta Carinae Great Outburst was noted. [74] Supernovae in M101 (1909) and M83 (1923 and 1957) were also suggested as possible type IV or type V supernovae.
These types would now all be treated as peculiar type II supernovae (IIpec), of which many more examples have been discovered, although debate continues as to whether SN 1961V was a true supernova following an LBV burst or an impostor.
Role in stellar evolution
The Big Bang produced hydrogen, helium and trace amounts of lithium, while all heavier elements are synthesized in stars, supernovae and neutron star collisions (and thus indirectly due to supernovae). Supernovae tend to enrich the surrounding interstellar medium with elements other than hydrogen and helium, which astronomers commonly call "metals." These ejected elements ultimately enrich the molecular clouds that are the sites of star formation.
Each stellar generation therefore has a slightly different composition, ranging from an almost pure mixture of hydrogen and helium to a more metal-rich composition. Supernovae are the dominant mechanism for the distribution of these heavier elements, which are formed in a star during the period of nuclear fusion.
The different abundances of elements in the material that forms a star have important influences on the life of the star, and can influence the ability of planets to orbit it: more giant planets form around stars with higher metallicity.
The kinetic energy of an expanding supernova remnant can trigger star formation by compressing nearby, dense molecular clouds in space. The increase in turbulent pressure can also prevent star formation if the cloud cannot release the excess energy.
Evidence from daughter products of short-lived radioactive isotopes shows that a nearby supernova 4.5 billion years ago determined the composition of the Solar System and may even have caused the formation of this system.
Fast radio bursts (FRBs) are intense, transient pulses of radio waves that typically last no longer than milliseconds. Many explanations for these events have been proposed; Magnetars produced by core-collapse supernovae are prime candidates.
