Supernova
A supernova is a powerful and luminous explosion of a star. This event occurs during the last evolutionary stages of a massive star or when a white dwarf is triggered into runaway nuclear fusion. The original object, called the progenitor, either collapses to a neutron star or black hole, or is completely destroyed to form a diffuse nebula. The peak optical luminosity of a supernova can be comparable to that of an entire galaxy before fading over several weeks or months. Astronomers distinguish this phenomenon from ordinary novae by adding the prefix super- to the word nova. Ordinary novae are far less luminous and do not destroy the underlying star.
Chinese astronomers documented SN 185 in AD 185 within the constellation of Centaurus. The brightest recorded supernova was SN 1006, which appeared in AD 1006 in the constellation of Lupus with a maximum apparent magnitude of negative 7.5. Observers in China, Japan, Iraq, Egypt, and Europe described this event. The widely observed supernova SN 1054 produced the Crab Nebula in the constellation of Taurus. Johannes Kepler began observing SN 1604 at its peak on the 17th of October 1604. He continued to make estimates of its brightness until it faded from naked eye view a year later. Tycho Brahe observed SN 1572 in Cassiopeia, which was the second supernova to be observed in a generation after Kepler's Star. These events were used to argue against the Aristotelian idea that the universe beyond the Moon and planets was static and unchanging.
Theoretical studies indicate that most supernovae are triggered by one of two basic mechanisms. The first involves the sudden re-ignition of nuclear fusion in a white dwarf. Possible causes include an accumulation of material from a binary companion through accretion or a stellar merger. In the case of a massive star's sudden implosion, the core will undergo collapse once unable to produce sufficient energy from fusion to counteract gravity. This must happen once the star begins fusing iron but may occur during an earlier stage of metal fusion. Supernovae can expel several solar masses of material at speeds up to several percent of the speed of light. This drives an expanding shock wave into the surrounding interstellar medium, sweeping up gas and dust observed as a supernova remnant.
Astronomers classify supernovae according to their light curves and the absorption lines of different chemical elements that appear in their spectra. If a supernova's spectrum contains lines of hydrogen it is classified Type II; otherwise it is Type I. Type Ia supernovae show a strong ionized silicon absorption line near peak light. Type Ib shows a non-ionized helium line while Type Ic lacks these features. A small number of Type Ia supernovae exhibit unusual features such as non-standard luminosity or broadened light curves. These are typically categorized by referring to the earliest example showing similar features like SN 2008ha which is often referred to as SN 2002cx-like. Calcium-rich supernovae are a rare type with unusually strong calcium lines in their spectra. The term Type IIb describes the combination of features normally associated with Type II and Type Ib supernovae.
Type Ia supernovae are produced from white dwarf stars in binary star systems and occur in all galaxy types. Core collapse supernovae result from short-lived massive stars found only in galaxies undergoing current or very recent star formation. Red supergiants are the progenitors for the vast majority of core collapse supernovae but have been observed only at relatively low masses below about 15 solar masses. Blue supergiants form an unexpectedly high proportion of confirmed supernova progenitors partly due to their high luminosity and easy detection. Wolf-Rayet stars are known as the progenitors of Type Ib and Type Ic supernovae after losing most of their outer hydrogen envelopes. Models have had difficulty showing how blue supergiants lose enough mass to reach supernova without progressing to a different evolutionary stage.
Supernovae are a major source of elements in the interstellar medium from oxygen through to rubidium. The expanding shock waves can trigger the formation of new stars by compressing nearby dense molecular clouds. Galactic cosmic rays are generated by supernova explosions. In the modern universe old asymptotic giant branch stars are the dominant source of dust from oxides carbon and s-process elements. However in the early universe before AGB stars formed supernovae may have been the main source of dust. 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 depending on local conditions.
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Common questions
What is a supernova and how does it differ from an ordinary nova?
A supernova is a powerful and luminous explosion of a star that occurs during the last evolutionary stages of a massive star or when a white dwarf triggers runaway nuclear fusion. Astronomers distinguish this phenomenon from ordinary novae by adding the prefix super- to the word nova because ordinary novae are far less luminous and do not destroy the underlying star.
Which historical supernovae were documented by Chinese astronomers and what dates did they occur on?
Chinese astronomers documented SN 185 in AD 185 within the constellation of Centaurus and recorded the brightest supernova SN 1006 which appeared in AD 1006 in the constellation of Lupus with a maximum apparent magnitude of negative 7.5. Observers in China, Japan, Iraq, Egypt, and Europe described these events while Johannes Kepler began observing SN 1604 at its peak on the 17th of October 1604.
How do theoretical studies explain the two basic mechanisms that trigger most supernovae?
Theoretical studies indicate that most supernovae are triggered by either the sudden re-ignition of nuclear fusion in a white dwarf through material accumulation or stellar merger or the core collapse of a massive star once it begins fusing iron. This process expels several solar masses of material at speeds up to several percent of the speed of light into the surrounding interstellar medium.
What criteria do astronomers use to classify different types of supernovae based on their spectra?
Astronomers classify supernovae according to their light curves and the absorption lines of different chemical elements that appear in their spectra where Type II contains hydrogen lines and Type I lacks them. Specific subtypes include Type Ia which shows strong ionized silicon absorption near peak light and Type Ib which displays non-ionized helium lines.
Which stars serve as progenitors for specific categories of supernovae such as Type Ia or Type Ib?
Type Ia supernovae are produced from white dwarf stars in binary star systems while Wolf-Rayet stars act as the progenitors of Type Ib and Type Ic supernovae after losing most of their outer hydrogen envelopes. Red supergiants form the progenitors for the vast majority of core collapse supernovae but have been observed only at relatively low masses below about 15 solar masses.