According to astronomical observations, galaxies like our own experience about one supernova (a violently-exploding star) every 25 years. The gas and dust remnants from such explosions (like the Crab Nebula) expand outward rapidly and should remain visible for over a million years. Yet the nearby parts of our galaxy in which we could observe such gas and dust shells contain only about 200 supernova remnants. That number is consistent with only about 7,000 years worth of supernovas.