The fate of stars is not the same for all stellar objects in the Universe. While some will become cold corpses, others end their lives in gigantic explosions called supernovas. This is the case for white dwarfs that have passed the Chandrasekhar limit, as well as for sufficiently massive stars collapsing on themselves to form neutron stars or black holes. Supernovas are among the most violent phenomena in the cosmos, and some occur close enough to Earth to have noticeable effects there.
There are two types of supernovas. The first, called thermonuclear supernova (or type Ia supernova), results from the explosion of a white dwarf that has accreted enough matter to exceed the Chandrasekhar critical limit (1.4 solar masses); the thermonuclear reactions resume and run amok and, under the thermal pressure, the star's layers are gradually blown away until the core explodes, taking away the entire white dwarf.
The second, called core-collapse supernova (or type II, Ib and Ic supernova), concerns massive stars (at least 8 solar masses) that have reached the end of their life. The core of the star having exceeded the Chandrasekhar limit, collapses, causing a rebound of matter on its surface then the generation of a shock wave expelling the internal and peripheral layers; the star then explodes and, depending on its initial mass, leaves behind a neutron star or a black hole.
The majority of these events usually occur far from Earth (several tens of thousands and millions of light years). But some of them sometimes occur at a distance of between 30 and 1000 light years from our planet, close enough to imprint identifiable effects on the biosphere. Thus, over the past 11 million years, scientists estimate that around 20 supernovae have appeared within this distance range. Each event being correlated with a global warming of the planet of 3-4°C.
On Earth, supernova events are detectable through the study of metal isotopes in rock strata. For example, German researchers have reported an abundance of iron-60 in seafloor rocks in the Pacific Ocean; 33 atoms of this isotope have been found in 2 cm of Earth's crust, indicating that a supernova would have occurred in the last 5 million years, and close enough to the Solar System to have deposited as much iron 60 on Earth.
The study authors estimated that the event must have been about 50 light-years from our planet, which should have caused a mass extinction; however, no mass extinctions have been recorded in the last 5 million years. According to the researchers, this event must have occurred much further than the data suggests.
Gamma-ray bursts from near-Earth supernovae can have catastrophic consequences for the entire Earth biosphere. Scientists believe that a gamma-ray burst could thus be the cause of the Ordovician-Silurian mass extinction, 445 million years ago. Indeed, gamma ray fluxes are the main destructive factors for the atmosphere of a planet in the case of a supernova.
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The energy transported by gamma photons induces the radiolysis (molecular breakage) of atmospheric nitrogen and oxygen, leading to their conversion into nitrogen oxide and the destruction of the ozone layer. The surface of the planet is then swept by cosmic rays, in particular solar winds and UVs, leading to a generalized destruction of phytoplankton and coral populations, and therefore to a break in the marine food chain.
On average, scientists estimate that a supernova occurs at a distance of 30 light years every 240 million years. For type II supernovae, this estimate is between 0.5 and 10 supernovae every billion years. Currently, astrophysicists have detected six near-Earth supernova candidates, at a distance of less than 1000 light years.
