Astronomers say they have been stunned by the amount of energy released in a star explosion on the far side of our galaxy, 50,000 light-years away. The flash of radiation on 27 December was so powerful that it bounced off the Moon and lit up the Earth's atmosphere. ...If the explosion had been within just 10 light-years, Earth could have suffered a mass extinction. ...SGR 1806-20 is sited in the southern constellation Sagittarius. Its distance puts it beyond the centre of the Milky Way and a safe distance from Earth. "Had this happened within 10 light-years of us, it would have severely damaged our atmosphere and would possibly have triggered a mass extinction," said Dr Bryan Gaensler, of the Harvard-Smithsonian Center for Astrophysics, who is the lead author on one of the forthcoming Nature papers. "Fortunately there are no magnetars anywhere near us."

The previous biggest explosion obeserved by humans would've been the supernova of 1604.

Source: BBC