Astronomers have known about gamma-ray bursts for more than 20 years; however, tracking them has always been difficult because they happen without warning and typically last for only a few seconds and come from any direction in the sky.

Within only the past couple years, astronomers have found that these mysterious bursts of radiation are extremely far away and are caused by tremendous, still unexplained explosions. But they got their first break in early 1999. Using orbiting observatories and ground-based telescopes, astronomers tracked the visible glow of the most energetic gamma-ray burst ever recorded. For a brief moment, the light from the blast was equal to the radiance of one million galaxies.

When the Hubble Space Telescope observed the target a few days after the burst (left image), the object had already faded to one four-millionth of its original brightness. The telescope captured the fading fireball embedded in a galaxy located two-thirds of the way to the horizon of the observable universe. Further Hubble observations showed that the galaxy was neither a classic spiral nor an elliptical. It looked distorted as if its shape had been changed due to a collision with another galaxy, which would induce rapid star birth. The observations support the idea that these mysterious powerful explosions happen where vigorous star formation takes place.