Supernova

A supernova (pl.: 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 nova, meaning 'new', 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 4500±1000 BC 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.

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