Supernovae are violent explosions in stars that release more energy than the Sun would in 10 billion years. They occur in two types: Type I and Type II. Type I occurs when a white dwarf accumulates enough mass to exceed the Chandrasekhar limit, while Type II occurs when a supergiant star fuses elements in its core down to iron. Type I supernovae are sometimes used as standard candles in astronomy to measure distance.
A supernova is a violent explosion that occurs as a stage of development in some stars. A supernova lasts from a few weeks to months, and during that time it can release more energy than the Sun would emit in 10 billion years. Supernovae are capable of eclipsing their host galaxies. In a galaxy the size of the Milky Way, supernovae occur about once every fifty years.
If a supernova occurred 26 light-years away from Earth, it would wipe out half of our ozone layer. Some paleontologists blame a nearby supernova for the Ordovician-Silurian extinction event, which occurred about 444 million years ago, during which 60% of oceanic life died. The brightest supernova in human history was observed in 1006 by people across Eurasia, with the most detailed records coming from China. With a brightness between one-quarter and one-half that of the full Moon, this supernova was so bright that it cast shadows.
Supernovae occur in two ways and are correspondingly divided into types: type I supernovae and type II supernovae.
A Type I supernova occurs when a carbon-oxygen white dwarf, an Earth-sized stellar remnant left over from millions of years of burning hydrogen and helium, accumulates enough mass to exceed the Chandrasekhar limit, which is 1.44 solar masses per a non-rotating star. Above this limit, the electron shells in the atoms that make up the dwarf can no longer repel each other and the star collapses. A stellar object containing about the mass of the Sun in a space equal to the Earth becomes even smaller, until it reaches the temperature and density necessary for carbon ignition. Within seconds, a major percentage of the carbon in the star fuses into oxygen, magnesium and neon, releasing energy equivalent to 1029 megatons of TNT. This is enough to make the star explode at about 3% of the speed of light.
A type II supernova is also called a core collapse supernova. It happens when a supergiant star of more than nine solar masses fuses elements in its core down to iron, which no longer provides a net energy gain through fusion. Without net energy production, no nuclear chain reaction can occur and an iron core builds up to the previously mentioned Chandrasekhar limit. At this point, it collapses to form a neutron star, an object containing the mass of a sun in an area about 30 km (18.6 mi) in diameter, the size of a large city. Most of the star outside the core also begins to collapse, but it rebounds against the neutron star’s super-dense matter, rapidly fusing all remaining light nuclei and creating an explosion of a similar scale to a Type I supernova.
Because Type I supernovae have a relatively predictable energy release, they are sometimes used as standard candles in astronomy, to measure distance. Since their absolute magnitude is known, the relationship between absolute and apparent magnitude can be used to determine the distance to the supernova.
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