A near-Earth supernova is an explosion resulting from the death of a star that occurs close enough to the Earth (roughly less than 10 to 300 parsecs (30 to 1000 light-years) away) to have noticeable effects on its biosphere.
Effects on Earth
On average, a supernova explosion occurs within 10 parsecs (33 light-years) of the Earth every 240 million years.[a] Gamma rays are responsible for most of the adverse effects a supernova can have on a living terrestrial planet. In Earth's case, gamma rays induce a chemical reaction in the upper atmosphere, converting molecular nitrogen into nitrogen oxides, depleting the ozone layer enough to expose the surface to harmful solar and cosmic radiation (mainly ultra-violet). Phytoplankton and reef communities would be particularly affected, which could severely deplete the base of the marine food chain.
Risk by supernova type
Speculation as to the effects of a nearby supernova on Earth often focuses on large stars as Type II supernova candidates. Several prominent stars within a few hundred light years of the Sun are candidates for becoming supernovae in as little as a millennium. Although they would be spectacular to look at, were these "predictable" supernovae to occur, they are thought to have little potential to affect Earth.
It is estimated that a Type II supernova closer than eight parsecs (26 light-years) would destroy more than half of the Earth's ozone layer. Such estimates are based on atmospheric modeling and the measured radiation flux from SN 1987A, a Type II supernova in the Large Magellanic Cloud. Estimates of the rate of supernova occurrence within 10 parsecs of the Earth vary from 0.05–0.5 per Ga to 10 per Ga. Several studies assume that supernovae are concentrated in the spiral arms of the galaxy, and that supernova explosions near the Sun usually occur during the ~10 million years that the Sun takes to pass through one of these regions. Examples of relatively near supernovae are the Vela Supernova Remnant (~800 ly, ~12,000 years ago) and Geminga (~550 ly, ~300,000 years ago).
Type Ia supernovae are thought to be potentially the most dangerous if they occur close enough to the Earth. Because Type Ia supernovae arise from dim, common white dwarf stars, it is likely that a supernova that could affect the Earth will occur unpredictably and take place in a star system that is not well studied. The closest known candidate is IK Pegasi. It is currently estimated, however, that by the time it could become a threat, its velocity in relation to the Solar System would have carried IK Pegasi to a safe distance.
Evidence from daughter products of short-lived radioactive isotopes shows that a nearby supernova helped determine the composition of the Solar System 4.5 billion years ago, and may even have triggered the formation of this system. Supernova production of heavy elements over astronomic periods of time ultimately made the chemistry of life on Earth possible.
Past supernovae might be detectable on Earth in the form of metal isotope signatures in rock strata. Subsequently, iron-60 enrichment has been reported in deep-sea rock of the Pacific Ocean by researchers from the Technical University of Munich. 23 increased concentrations of this iron isotope were found in the top 2 cm of crust, and these date from about 13 million years ago. It is estimated that the supernova must have occurred in the last 5 million years or else it would have had to happen very close to the solar system to account for so much iron-60 still being here. A supernova occurring so close would have probably caused a mass extinction, which didn't happen in that time frame. The quantity of iron seems to indicate that the supernova was less than 30 parsecs away. On the other hand, the authors estimate the frequency of supernovae at a distance less than D (for reasonably small D) as around (D/10 pc)3 per Ga, which gives a probability of only around 5% for a supernova within 30 pc in the last 5 million years. They point out that the probability may be higher because our solar system is entering the Orion Arm of the Milky Way.
Gamma ray bursts from "dangerously close" supernova explosions occur two or more times per billion years, and this has been proposed as the cause of the end Ordovician extinction, which resulted in the death of nearly 60% of the oceanic life on Earth.
In 1998 a supernova remnant, RX J0852.0-4622, was found in front (apparently) of the larger Vela Supernova Remnant. Gamma rays from the decay of titanium-44 (half-life about 60 years) were independently discovered emanating from it, showing that it must have exploded fairly recently (perhaps around 1200 CE), but there is no historical record of it. The flux of gamma rays and x-rays indicates that the supernova was relatively close to us (perhaps 200 parsecs or 660 ly). If so, this is a surprising event because supernovae less than 200 parsecs away are estimated to occur less than once per 100,000 years.
- Since a radius of 100 light years contains approximately 27.8 times as much volume as one of 33 light years, a supernova should occur within a radius of 100 light years from Earth approximately once every 8.6 million years. A supernova would occur within a radius of 200 light years approximately once every million years, within 500 light years every 69,000 years, and within 1,000 light years roughly every 8,625 years.[original research?]
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