Nasa Space Science: A Violent History Of Time

From mother Earth, the night sky can look peaceful and unchanging, but the universe as seen in gamma-rays is a place of sudden and chaotic violence. Using gamma-ray telescopes, astronomers witness short but tremendously intense explosions called gamma-ray bursts, and there is nothing more powerful.
No one is sure what causes gamma-ray bursts. Favored possibilities include the collision of two neutron stars or a sort of super-supernova that occurs when extremely massive stars explode. One thing is certain: gamma-ray bursts happen in galaxies far, far away — so far away that the distances are called “cosmological,” beyond ordinary comprehension.
Think about this: When you look up at the night sky, you are looking at the ultimate history book – one that goes back to the very beginning of what we call time. And each star is a chapter in the book. You are not really seeing the stars as they are now. You are looking at stars as they used to be when their light left them long ago. And the deeper we peer into space, the farther back in time we are looking. In fact, light from the galaxies farthest away is billions of years old.