Earth has witnessed five mass extinction events in its history. The one that killed the dinosaurs 65 million years ago is one of the best-known ones. Then there were others, such as the Great Dying and the Late Devonian extinction. An asteroid's impact killed the dinosaurs and is the only mass extinction we know about that was caused by an outer space object.

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Now a new study has revealed that the death of two massive stars has caused at least two mass extinction events in the history of our planet. The two supernovae reportedly occurred 60 light-years from Earth and had a destructive impact on the atmosphere. The ozone layer bore the brunt that was completely stripped away. The vanishing of the protective layer exposed the environment to the Sun's deadly ultraviolet radiation, triggering a series of life-ending events.

Nick Wright, an astrophysics professor at Keele University in England, told Space.com, that the distance of the supernovae would have been "terrifying" since one at an even farther distance would have the potential to cause considerable loss of life.

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Study of star formation history led to the major revelation

The team of scientists led by Wright, conducted a virtual census of more than 24,000 extremely luminous stars in the universe. They took data on their locations from the Gaia satellite that is no longer in use. Focussing on stars located within 3,260 light-years of the sun, the study aimed at identifying new groups of young, massive stars and reconstructing nearby star formation history.

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However, they also figured out that the data could be used to learn about the supernova rate. What they discovered was that the rate of supernovae was very close to two mass extinctions. Two supernovae coincided with two mass extinction events on Earth - the Ordovician which was the first of the five major ones and occurred 445 million years ago, and the second was the late Devonian which occurred 372 million years ago.

Dying stars released rays that killed life on Earth

Plausible explanations for the events had been lacking till now. But the previous discovery of the radioactive isotope iron-60 in cosmic dust found in Antarctic snow and from the surface of the moon hinted that an interstellar event could be the only source.

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This led to the depletion of Earth's ozone layer, leading to the entry of cosmic rays released by the death of the stars.

As the ozone layer was removed, several marine species died. During the Devonian and Ordovician mass extinction events many mammals, seabirds, turtles, and sharks went extinct, with one such event occurring nearly 2.6 million years ago. 

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The study found that one to two supernovae occur every 100 years of so in galaxies like the Milky Way. The rate of supernovae was 2 to 2.5 per billion years when supernovae within 60 light-years from Earth were taken note of. This number almost matched with the Devonian and Ordovician extinctions, leading scientists to believe that the dying stars also likely killed life on Earth.