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The table below lists the known reasons for core collapse in massive stars, the types of stars in which they occur, their associated supernova type, and the remnant produced. The metallicity is the proportion of elements other than hydrogen or helium, as compared to the Sun. The initial mass is the mass of the star prior to the supernova event, given in multiples of the Sun's mass, although the mass at the time of the supernova may be much lower. [100] Could a nearby supernova pose a threat to life on Earth? Yes, in theory—but the blast would have to be very close, and at the moment no such nearby stars are at risk of exploding. Which is a good thing, because the blast of radiation from a nearby supernova would be devastating. Over a period of weeks, the supernova would emit ultraviolet rays, X-rays and gamma rays, which wouldn’t necessarily reach the ground, but would still wreak havoc on the Earth’s protective ozone layer, explains Fields. “So it wouldn’t turn us into the Hulk—but it would strip the ozone layer off the stratosphere,” he says. Without the ozone layer, the Earth would be awash in deadly ultraviolet radiation from the sun; this could wipe out phytoplankton in the oceans, with the effects working their way up the food chain, possibly leading to a mass extinction, Fields says. Type Ib and Ic supernovas also undergo core collapse just as Type II supernovas do, but they have lost most of their outer hydrogen layer. In 2014, scientists detected the faint, hard-to-locate companion star to a Type Ib supernova. The search consumed two decades, as the companion star shone much fainter than the bright supernova. Main article: Type Ib and Ic supernovae Type Ib SN 2008D [120] at the far upper end of the galaxy, shown in X-ray (left) and visible light (right), [121] with the brighter SN 2007uy closer to the centre Either type of supernova can be so bright as to briefly outshine an entire galaxy. But Type II supernovas are particularly interesting because they release not only light but also enormous numbers of neutrinos. In fact, the emission of neutrinos can start a little bit ahead of the explosion itself, explains Kate Scholberg, an astronomer at Duke University.

Type Ib supernovae are the more common and result from Wolf–Rayet stars of type WC which still have helium in their atmospheres. For a narrow range of masses, stars evolve further before reaching core collapse to become WO stars with very little helium remaining, and these are the progenitors of type Ic supernovae. [124] On average, a supernova will occur once every 50 years in a galaxy the size of the Milky Way, according to research by the European Space Agency. This means a star explodes every 10 seconds or so somewhere in the universe, according to the U.S. Department of Energy. 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. [43] 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. [44] [45] Neutrinos are 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. [46] "A star set to explode", the SBW1 nebula surrounds a massive blue supergiant in the Carina Nebula. 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 (known as the Balmer series in the visual portion of the spectrum) it is classified Type II; otherwise it is Type I. In each of these two types there are subdivisions according to the presence of lines from other elements or the shape of the light curve (a graph of the supernova's apparent magnitude as a function of time). [60] [61] Supernova taxonomy [60] [61] Type I

A supernova occurs when there is a change in the core of a star, one much bigger than our sun. These changes can occur in two different ways, both of which result in a supernova. When 1987A blew up, neutrino science was in its infancy—even so, two dozen neutrinos were recorded by three detectors working at the time. If a supernova explodes within our galaxy now, the global network of detectors will record hundreds or even thousands of neutrinos. 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. [48] [49] Low redshift observations also anchor the low-distance end of the Hubble curve, which is a plot of distance versus redshift for visible galaxies. [50] [51]

A version of the periodic table indicating the origins – including stellar nucleosynthesis of the elements. (Photo Credit: Cmglee/Wikimedia Commons) Type I supernova: A star accumulates matter from a nearby neighbor until a runaway nuclear reaction ignites. A white dwarf star may accumulate sufficient material from a stellar companion to raise its core temperature enough to ignite carbon fusion, at which point it undergoes runaway nuclear fusion, completely disrupting it. There are three avenues by which this detonation is theorised to happen: stable accretion of material from a companion, the collision of two white dwarfs, or accretion that causes ignition in a shell that then ignites the core. The dominant mechanism by which type Ia supernovae are produced remains unclear. [74] Despite this uncertainty in how type Ia supernovae are produced, type Ia supernovae have very uniform properties and are useful standard candles over intergalactic distances. Some calibrations are required to compensate for the gradual change in properties or different frequencies of abnormal luminosity supernovae at high redshift, and for small variations in brightness identified by light curve shape or spectrum. [75] [76] Normal Type Ia [ edit ]One specific type of supernova originates from exploding white dwarfs, like type Ia, but contains hydrogen lines in their spectra, possibly because the white dwarf is surrounded by an envelope of hydrogen-rich circumstellar material. These supernovae have been dubbed type Ia/IIn, type Ian, type IIa and type IIan. [97]