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Supernova - Some of the largest Explosions in the Universe
Main Articles - Space Disasters
June 04, 2007

What is a Supernova?

Supernova refers to several types of stellar explosions that produce extremely bright objects made of plasma that decline to invisibility over weeks or months. There are two possible routes to this end. A massive star may cease to generate fusion energy from fusing the nuclei of atoms in its core and collapses inward under the force of its own gravity, or a white dwarf star may accumulate material from a companion star until it reaches its Chandrasekhar limit and undergoes a thermonuclear explosion. In either case, the resulting supernova explosion expels much or all of the stellar material with great force.

The explosion drives a blast wave into the surrounding space, forming a supernova remnant. One famous example of this process is the remnant of SN 1604, shown below.


Supernovae are the main source of all the elements heavier than oxygen, and they are the only source of many important elements. In standard cosmology, the Big Bang produced hydrogen, helium, and traces of lithium, while all heavier elements are synthesized in stars and supernovae. For example, all the calcium in our bones and all the iron in our hemoglobin were synthesized in supernova explosions, billions of years ago. Supernovae inject these heavy elements into the interstellar medium, ultimately enriching the molecular clouds that are the sites of star formation. 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. Supernova production of heavy elements over cosmic time ultimately made possible the chemistry of life on Earth.

Supernovae generate tremendous temperatures, and under the right conditions, the fusion reactions that take place during the peak moments of a supernova can produce some of the heaviest elements like plutonium and californium.

"Nova" (pl. novae) is Latin for "new", referring to what appears to be a very bright new star shining in the celestial sphere; the prefix "super" distinguishes this from an ordinary nova, which also involves a star increasing in brightness, though to a lesser extent and through a different mechanism. However, it is misleading to consider a supernova as a new star, because it really represents the death of a star (or at least its radical transformation into something else).

Possible Supernova threats to Earth

Several prominent stars within a few hundred light years from the Sun are candidates for becoming supernovae in a relatively short time, perhaps as little as 1000 years into the future. Speculations as to the effects of a nearby supernova on Earth often focus on these large stars, such as Betelgeuse, a red supergiant at a distance of about 400 light years from Earth. Of interest is the conclusion that Type Ia supernovae are the most potentially dangerous, if they occur close enough to the Earth. Since these supernovae are the result of accretion onto relatively 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 predictable supernovae, such as Betelgeuse, while spectacular, will have little effect on Earth. There is an estimation that a Type Ia supernova would have to be closer than 1000 parsecs - roughly 3300 light years - to affect the Earth . There are likely to be many Type Ia candidates within this distance. However the typical rate for Type Ia supernovae in a galaxy is about 1 per 1000 years, and therefore the probability of one occurring within 1000 parsecs of Earth, given that the Milky Way is about 30,000 parsecs in diameter and 1000 parsecs thick, is probably less than 1 per 1 million years. The probability of a Type Ia within 100 parsecs is about 1 per billion years or less. Thus it is likely that a nearby Type Ia about 100-1000 parsecs away has occurred several times within the history of life on Earth, about 500 million years ago, but is unlikely to occur anytime within the lifespan of the human species.

Recent estimates predict that a Type II supernova would have to be closer than 8 parsecs, which is about 26 light years, to destroy half of the Earth's protective ozone layer . Such estimates are mostly concerned with atmospheric modelling and considered only the known radiation flux from SN 1987A, a Type II supernova in the Large Magellanic Cloud.

Near-Earth Supernova

The Crab Nebula is the remnant of a supernova that occurred about 6500 light-years away from the Earth.

A Near-Earth Supernova is an explosion resulting from the death of a star that occurs close enough to the Earth (roughly fewer than 100 light-years away) to have noticeable effects on its biosphere.

Supernovae are some of the largest and brightest explosions in the Universe. For a moment, a supernova will outshine its entire parent galaxy. Gamma rays are at the heart 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 ozone into nitrous oxide, depleting the ozone layer enough to expose the surface to harmful solar and cosmic radiation.


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