A supernova (: supernovae) is a powerful and luminous explosion of a star. A supernova 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.
It is expected that supernovae in our galaxy occur on average once every 61 years, although the last to be observed was Kepler's Supernova in 1604. SN 1987A occurred in the Large Magellanic Cloud, a satellite galaxy of our galaxy, in 1987. Several thousand supernovae are typically seen in distant galaxies every year.
Theoretical studies indicate that most supernovae are triggered by one of two basic mechanisms: the sudden re-ignition of nuclear fusion in a white dwarf, or the sudden gravitational collapse of a massive star's core. In the re-ignition of a white dwarf, the object's temperature is raised enough to trigger runaway nuclear fusion, completely disrupting the star. Possible causes are an accumulation of material from a binary companion through accretion, or by a stellar merger. In the case of a massive star's sudden implosion, the core of a massive star will undergo sudden collapse once it is unable to produce sufficient energy from fusion to counteract the star's own gravity, which must happen once the star begins fusing iron, but may happen 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 an expanding shell of gas and dust 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 might also produce gravitational waves.
Occurrence The first supernovae to be studied by astronomical methods were Tycho's Supernova in 1572 and Kepler's Supernova in 1604, both of which were in the Milky Way and were visible to the naked eye. Analysis of the historical record suggests that, aside from telescope discoveries, fewer than 10 supernovae have been observed over the last 2,000 years.
Observations of recent supernova remnants within the Milky Way, coupled with studies of supernovae in other galaxies, suggest that these powerful stellar explosions occur in our galaxy approximately 1.6 to 4.6 times per century on average.
In 1987, the supernova SN 1987A appeared in the Large Magellanic Cloud, a satellite galaxy of the Milky Way in an easily studied part of the sky. Many astronomical observations were made on SN 1987A, including the only measurements of astronomical neutrinos other than the Sun's. The event was attributed to an explosion of a blue supergiant star.
Etymology The word supernova has the plural form supernovae () or supernovas and is often abbreviated as SN or SNe. It is derived from the Latin word , meaning , which refers to what appears to be a temporary new bright star. Adding the prefix "super-" distinguishes supernovae from ordinary novae, which are far less luminous. The word supernova was coined by Walter Baade and Fritz Zwicky, who began using it in astrophysics lectures in 1931. Its first use in a journal article came the following year in a publication by Knut Lundmark, who may have coined it independently.
Observation history
Compared to a star's entire history, the visual appearance of a supernova is very brief, sometimes spanning several months, so that the chances of observing one with the naked eye are roughly once in a lifetime. Only a tiny fraction of the 100 billion stars in a typical galaxy have the capacity to become a supernova, the ability being restricted to those having high mass and those in rare kinds of binary star systems with at least one white dwarf.
Early observations A rock carving in the Burzahama region of Kashmir, dated to showing what might be nova HB9 is the earliest of many claimed but unverifiable records of supernovae by prehistoric people. The first widely recorded supernova was SN 1006, observed in AD 1006 in the constellation of Lupus. This event was described by observers in China, Japan, Iraq, Egypt and Europe.
The supernova SN 1054, which produced the Crab Nebula, was recorded by Chinese astronomers in AD 1054. Supernovae SN 1572 and SN 1604, the latest Milky Way supernovae to be observed with the naked eye, had a notable influence on the development of astronomy in Europe because they were used to argue against the Aristotelian idea that the universe beyond the Moon and planets was static and unchanging. Johannes Kepler began observing SN 1604 at its peak on 17 October 1604, and continued to make estimates of its brightness until it faded from naked eye view a year later. It was the second supernova to be observed in a generation, after Tycho Brahe observed SN 1572 in Cassiopeia.
There is some evidence that the youngest known supernova in our galaxy, G1.9+0.3, occurred in the late 19th century, considerably more recently than Cassiopeia A from around 1680. Neither was noted at the time. In the case of G1.9+0.3, high extinction from dust along the plane of the galactic disk could have dimmed the event sufficiently for it to go unnoticed. The situation for Cassiopeia A is less clear; infrared light echoes have been detected showing that it was not in a region of especially high extinction.
+ Historical novaeyearobserved inmaximum apparent magnitudecertainty of theSN's identification185constellation of Centaurus−6possible SN, but may be a comet386constellation of Sagittarius +1.5uncertain whether SN or classical nova393constellation of Scorpius−3possible SN1006constellation of Lupus−7.5certain1054constellation of Taurus−6certain; remnant and pulsar known1181constellation of Cassiopeia−2likely Type Iax SN associated with the remnant Pa301572constellation of Cassiopeia−4certain; remnant known1604constellation of Ophiuchus−2certain; remnant known
Telescope findings With the development of the astronomical telescope, observation and discovery of fainter and more distant supernovae became possible. The first such observation was of SN 1885A in the Andromeda Galaxy. A decade later, two further supernovae, SN 1895A and SN 1895B, were discovered in NGC 4424 and NGC 5253 respectively. Early work on what was originally believed to be simply a new category of novae was performed during the 1920s. These were variously called "upper-class Novae", "Hauptnovae", or "giant novae". The name "supernovae" is thought to have been coined by Walter Baade and Fritz Zwicky in lectures at Caltech in 1931. It was used, as "super-Novae", in a journal paper published by Knut Lundmark in 1933, and in a 1934 paper by Baade and Zwicky. By 1938, the hyphen was no longer used and the modern name was in use.
Rudolph Minkowski and Fritz Zwicky developed the modern supernova classification scheme beginning in 1941. During the 1960s, astronomers found that the maximum intensities of supernovae could be used as standard candles, hence indicators of astronomical distances. Some of the most distant supernovae observed in 2003 appeared dimmer than expected. This supports the view that the expansion of the universe is accelerating. Techniques were developed for reconstructing supernovae events that have no written records of being observed. The date of the Cassiopeia A supernova event was determined from light echoes off nebulae, while the age of supernova remnant RX J0852.0-4622 was estimated from temperature measurements and the gamma ray emissions from the radioactive decay of .
The most luminous supernova ever recorded is ASASSN-15lh, at a distance of 3.82 gigalight-years. It was first detected in June 2015 and peaked at , which is twice the bolometric luminosity of any other known supernova. The nature of this supernova is debated and several alternative explanations, such as tidal disruption of a star by a black hole, have been suggested.
SN 2013fs was recorded three hours after the supernova event on 6 October 2013, by the Intermediate Palomar Transient Factory. This is among the earliest supernovae caught after detonation, and it is the earliest for which spectra have been obtained, beginning six hours after the actual explosion. The star is located in a spiral galaxy named NGC 7610, 160 million light-years away in the constellation of Pegasus.
The supernova SN 2016gkg was detected by amateur astronomer Victor Buso from Rosario, Argentina, on 20 September 2016. It was the first time that the initial "shock breakout" from an optical supernova had been observed. The progenitor star has been identified in Hubble Space Telescope images from before its collapse. Astronomer Alex Filippenko noted: "Observations of stars in the first moments they begin exploding provide information that cannot be directly obtained in any other way."
Discovery programs
Because supernovae are relatively rare events within a galaxy, occurring about three times a century in the Milky Way, obtaining a good sample of supernovae to study requires regular monitoring of many galaxies. Today, amateur and professional astronomers are finding about two thousand every year, some when near maximum brightness, others on old astronomical photographs or plates. Supernovae in other galaxies cannot be predicted with any meaningful accuracy. Normally, when they are discovered, they are already in progress. To use supernovae as standard candles for measuring distance, observation of their peak luminosity is required. It is therefore important to discover them well before they reach their maximum. Amateur astronomers, who greatly outnumber professional astronomers, have played an important role in finding supernovae, typically by looking at some of the closer galaxies through an optical telescope and comparing them to earlier photographs.
Toward the end of the 20th century, astronomers increasingly turned to computer-controlled telescopes and CCDs for hunting supernovae. While such systems are popular with amateurs, there are also professional installations such as the Katzman Automatic Imaging Telescope. The Supernova Early Warning System (SNEWS) project uses a network of neutrino detectors to give early warning of a supernova in the Milky Way galaxy. Neutrinos are subatomic particles that are produced in great quantities by a supernova, and they are not significantly absorbed by the interstellar gas and dust of the galactic disk.
Supernova searches fall into two classes: those focused on relatively nearby events and those looking farther away. Because of the expansion of the universe, the distance to a remote object with a known emission spectrum can be estimated by measuring its Doppler shift (or redshift); on average, more-distant objects recede with greater velocity than those nearby, and so have a higher redshift. Thus the search is split between high redshift and low redshift, with the boundary falling around a redshift range of z=0.1–0.3, where z is a dimensionless measure of the spectrum's frequency shift.
High-redshift searches for supernovae usually involve the observation of supernova light curves. These are useful for standard or calibrated candles to generate Hubble diagrams and make cosmological predictions. Supernova spectroscopy, used to study the physics and environments of supernovae, is more practical at low than at high redshift. Low redshift observations also anchor the low-distance end of the Hubble curve, which is a plot of distance versus redshift for visible galaxies.