A superbubble is a cloud of superheated gas that can be formed when multiple stars that are relatively near each other die out at similar times. This situation can lead to an explosion that spans hundreds of light years in distance. Recall that a light year is the distance that light travels through space in a year, so we are talking about an absolutely huge explosion! The light generated from this explosion is often not in the visible range of the spectrum, so they can’t always be seen by the naked eye (not to mention the fact that they are also very, very far away).