A supernova (/ˌsuːpərnoʊvə/ plural: supernovae /ˌsuːpərnoʊviː/ or supernovas, abbreviations: SN and SNe) is a powerful and luminous stellar explosion. At its peak brightness, the optical luminosity of a supernova can be comparable to that of an entire galaxy, before fading over several weeks or months. A supernova is a transient astronomical event, occurring during the last evolutionary stages of a massive star or when a white dwarf is triggered into runaway nuclear fusion. The original star, called the progenitor, either collapses to a neutron star or black hole, or it is completely destroyed. Supernovae are more energetic than novae. In Latin, nova means 'new', referring astronomically 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 in 1931. Only three naked-eye supernova events have been observed in the Milky Way during the last thousand years. The most recent directly observed supernova in the Milky Way was Kepler's Supernova in 1604, but the remnants of recent supernovae have also been found. Observations of supernovae in other galaxies suggest they occur in the Milky Way on average about three times every century. These supernovae would almost certainly be observable with modern astronomical telescopes. The most recent naked-eye supernova was SN 1987A, the explosion of a blue supergiant star in the Large Magellanic Cloud, a satellite of the Milky Way. Theoretical studies indicate that most supernovae are triggered by one of two basic mechanisms: the sudden re-ignition of nuclear fusion in a degenerate star or the sudden gravitational collapse of a massive star's core. In the first class of events, the object's temperature is raised enough to trigger runaway nuclear fusion, completely disrupting it. Possible causes are accumulation of sufficient material from a binary companion through accretion, or a merger. In the massive star case, the core of a massive star may undergo sudden collapse, releasing gravitational potential energy as a supernova. While some observed supernovae are more complex than these two simplified theories, the astrophysical mechanics have been established and accepted by most astronomers for some time. Supernovae can expel several solar masses of material at speeds up to several percent of the speed of light. This drives an expanding and fast-moving 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 through to rubidium. The expanding shock waves of supernovae can trigger the formation of new stars. Supernova remnants might be a major source of cosmic rays. Supernovae might produce strong gravitational waves, though, thus far, the gravitational waves detected have been from the merger of black holes and neutron stars. The earliest possible recorded supernova, known as HB9, could have been viewed and recorded by unknown Indian observers in 4,500±1000 BC. Later, SN 185, was viewed by Chinese astronomers in 185 AD. The brightest recorded supernova was SN 1006, which occurred in 1006 AD and was described by observers across China, Japan, Iraq, Egypt, and Europe. The widely observed supernova SN 1054 produced the Crab Nebula. Supernovae SN 1572 and SN 1604, the latest to be observed with the naked eye in the Milky Way galaxy, had notable effects 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 October 17, 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 SN 1572 seen by Tycho Brahe in Cassiopeia). There is some evidence that the youngest galactic supernova, G1.9+0.3, occurred in the late 19th century, considerably more recently than Cassiopeia A from around 1680. Neither supernova was noted at the time. In the case of G1.9+0.3, high extinction along the plane of the galaxy could have dimmed the event sufficiently to go unnoticed. The situation for Cassiopeia A is less clear. Infrared light echos have been detected showing that it was a type IIb supernova and was not in a region of especially high extinction. Before the development of the telescope, only five supernovae were seen in the last millennium. Compared to a star's entire history, the visual appearance of a galactic supernova is very brief, perhaps spanning several months, so that the chances of observing one is 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, restricted to either those having large mass or extraordinarily rare kinds of binary stars containing white dwarfs.