The most powerful explosion ever witnessed in the universe was bright enough to be seen with the naked eye this week. It blew up half way across the universe, making it also the most distant object visible without a telescope.

The blast – first detected by Nasa’s Swift satellite on Wednesday – happened 7.5 billion light-years away. That means it actually occurred 7.5 billion years ago, before the Earth even formed, and its light has been racing to meet us ever since.

By contrast, the light of the Moon takes just one and a quarter seconds to travel 240,000 miles to our eyes.

Astronomers term such an explosion a gamma ray blast, caused when a massive star collapses to form a black hole. The discovery shatters the record for the furthest object visible with the naked eye alone.

Swift is an orbiting observatory operated by Nasa in conjunction with, in the UK, the University of Leicester and Mullard Space Sciences Laboratory in Surrey.

Its principal investigator, Neil Gehrels, of Nasa’s Goddard Space Flight Centre in Maryland, said: “This burst was a whopper. It blows away every gamma ray burst we’ve seen so far.” Amazingly the blast was one of four gamma ray bursts witnessed in one day by Swift, whose work we have reported before. The first such blast viewed live happened in February 2006.

Telescopes around the world raced to catch the afterglow from the record explosion, labelled GRB 080319B, in the constellation of Bootes, the Herdsman. Several reported that it briefly became as bright as faint stars visible to the naked eye in clear dark skies away from light pollution.

Experts say the blast was 2.5 million times more luminous than the brightest supernova ever observed. The most distant object usually visible to the keen-sighted is a galaxy called M33 in Triangulum, just under 3 million light-years from Earth.

Swift science team member Stephen Holland said: “No other known object or type of explosion could be seen by the naked eye at such an immense distance. If someone just happened to be looking at the right place at the right time, they saw the most distant object ever seen by human eyes without optical aid.”

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Picture: The extremely luminous afterglow of GRB 080319B imaged by Swift’s X-ray Telescope (left) and Optical/Ultraviolet Telescope (right). Credit: NASA/Swift/Stefan Immler, et al.