Complex sentence: All the bursts astronomers have recorded so far have come from distant galaxies far from the solar system and have been harmless to Earth, but if one occurred within the Milky Way galaxy and were aimed straight at Earth, the effects could be devastating for the planet.
Split into simpler sentences: All the bursts astronomers have recorded so far have come from distant galaxies far from the solar system and have been harmless to Earth. However, if a burst occurred within the Milky Way galaxy and were aimed straight at Earth, the effects could be devastating for the planet.