PARIS ? It took 13 billion years to reach Earth, but astronomers have seen the light of an exploding mega-star that is the most distant object ever detected, two studies published Thursday reported.

The stunning gamma-ray burst (GRB) was observed by two teams of researchers in April, and opens a window onto a poorly known period when the Universe was in its infancy.

GRBs are the most violent explosions known to exist, and can be 10 million times more luminous than the brightest of galaxies.

They accompany the catastrophic death of a massive star, and are probably triggered by the collapse of the star's centre into a black hole.

Dubbed GRB 090423, the new discovery was first spotted by the NASA satellite Swift.

Astronomers alerted to the find trained several of Earth's largest telescopes skyward just in time to see the gamma-ray burst's fading afterglow.

The discovery is especially exciting for scientists because the explosion occurred during the so-called "cosmic dark ages", which started a mere 400,000 years after the Big Bang set the Universe in motion some 13.7 billion years ago.

During this period, free electrons and protons combined to form neutral atoms with the same number of positive and negative charges, resulting in an opaque ? or "dark" ? universe.

Not until 800 to 900 million years after the Big Bang were atoms and molecules "re-ionized", or electrically charged, resulting in the relatively transluscent inter-galactic medium we see today.

GRB 090423 flashed and crashed toward the end of these dark ages, making it the oldest object ever seen.

"This observation allows us to begin exploring the last blank space on our map of the Universe," said Nial Tanvir, a professor at the University of Leicester and lead author of one of the studies.

"It is tremendously exciting to be looking back in time to an era when the first stars were just switching on," said Andrew Levan, a professor at the University of Warwick in Britain and co-author of the same study.

The previous record holder for oldest object is at least 150 million years younger than the newly discovered gamma-ray burst.

Both studies were published in the British science journal Nature.