Gamma-ray bursts (GRBs) are powerful flashes of energetic gamma-rays lasting from less than a second to several minutes. They release a tremendous amount of energy in this short time making them the most powerful events in the Universe. They are thought to be mostly associated with the explosion of stars that collapse into black holes.
The gamma-ray burst GRB 090423 was detected by the NASA/STFC/ASI Swift satellite during the morning (CEST) of Thursday 23 April 2009. The 10 second burst was located in the constellation of Leo (the Lion). It was soon being followed by a whole range of telescopes on the ground, including the 2.2-metre ESO/MPG telescope at La Silla and ESO’s Very Large Telescope (VLT) at Paranal, both in Chile.
VLT infrared observations, made 17 hours after the burst detection, allowed astronomers to establish the distance to the explosion. “We find that the light coming from the explosion has been stretched, or redshifted, considerably by the expansion of the Universe”, says Nial Tanvir, the leader of the team who made the VLT observations. “With a redshift of 8.2 this is the most remote gamma-ray burst ever detected, and also the most distant object ever discovered — by some way.”
Because light moves at a finite speed, looking farther into the Universe means looking back in time. The explosion occurred when the Universe was about 600 million years old, less than 5 percent of its current age. It is believed that the very first stars only formed when the Universe was between 200 and 400 million years old.
“This discovery proves the importance of gamma-ray bursts in probing the most distant parts of the Universe”, says Tanvir. “We can now be confident that even more remote bursts will be found in the future, which will open a window to studying the very first stars and the ultimate end of the Dark Age of the Universe.”