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Astronomy: Are Gamma-ray Flashes and Supernovae Threatening Earth?

We live in calm and peaceful times and we have the worst behind us. Italian researchers have come to this optimistic conclusion. Anyone who rubs their eyes in irritation at the current news situation should bear in mind that these researchers are astronomers. They think spatially and temporally in different dimensions than ordinary people on earth.

“The best place and the best time to live in the Milky Way” is the title of the studywho have favourited Ricardo Spinelli (University of Insubria) now at Europlanet Science Congress (EPSC) has presented. It deals with the threat to life on Earth-like planets from supernova explosions and gamma-ray bursts. In order to assess this risk more precisely, the celestial scientists have modeled the evolution of our home galaxy over billions of years.

Gamma-ray bursts are considered to be the most energetic events in the universe. They are attributed to the explosion of very massive and rapidly rotating stars and to the merging of two neutron stars or black holes. As much energy can be released within ten seconds as the entire Milky Way emits in a hundred years. Supernovae that mark the end of life of stars with at least eight times the solar mass or that arise when a white dwarf in a binary star system has attracted too much mass from its companion are less powerful. Its energy supply corresponds to that of the Milky Way over a few hours. But they are observed more often. Such an event in the cosmic neighborhood could have a negative impact on the living conditions on an Earth-like planet.

In fact, the first of the five great mass extinctions of the past 500 million years, in each of which at least 75 percent of the animal and plant species on earth died out within a short period of time, has been traced back to a cosmic cause by several studies. The radiation from a gamma-ray burst less than 3,300 light-years away could have impacted the atmosphere with more than 100 kilojoules per square meter and destroyed 90 percent of the ozone layer, according to Spinelli. As a result, living beings were exposed to the sun’s UV radiation without protection. The resulting nitrogen oxides would also have lowered the global temperature significantly.

The model calculations made by Spinelli and his colleagues support this theory. According to this, the entire Milky Way was shaped by frequent and high-energy star explosions until six billion years ago, which must have made it difficult for the development of life. After that, supernovae and short gamma-ray bursts lasting less than two seconds have shifted into the interior of the Milky Way, an area that is located within a radius of about 6500 light years from the center. Further away from the galactic center to a distance of 26,000 light years, where the sun is also located, extends the comfort zone, where life could develop largely unmolested. Further out, however, the threat of long gamma-ray bursts that last longer than two seconds increases. The lower frequency of heavy elements here favors the formation of massive and rapidly rotating stars, explains Spinelli.

The greatest evolutionary pressure would come from these gamma-ray flashes. “Although they occur much less frequently than supernovae, they can trigger mass extinctions from a greater distance,” said Spinelli. “As the most energetic events, they are the bazookas with the greatest range.” According to the model calculations, they occur in our cosmic neighborhood approximately 1.3 times in 500 million years. The Ordovician mass extinction 450 to 440 million years ago could therefore have been triggered by a long gamma-ray flash.

Even though the universe is now a little more calm, we shouldn’t feel too safe. Because species extinctions are not only caused by cosmic events. “The sixth mass extinction that is currently taking place,” emphasizes Spinelli, “is due to human activities.”


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