For those who do not know: A gamma-ray burst (GRB) is a highly energetic explosion in space that produces intense bursts of gamma radiation. These bursts are the most luminous electromagnetic events known to occur in the universe, and they are thought to be associated with the deaths of massive stars or the mergers of compact objects like neutron stars.
A gamma-ray burst in hitting Earth would sterilize the planet or cause a mass extinction.
The Late Ordovician mass extinction has been
hypothesised by some researchers to have occurred as a result of such a gamma-ray burst.