If stars which are significantly more massive than our Sun die, they explode in a supernova. Supernova remnants are the gas plume drifting through the universe after such an explosion and are excited by the Pulsar, which remains in the center of the explosion. Only a few supernova remnants can be seen fairly clear in our sky.
M 1, the Crab Nebula in Taurus, goes back to a supernova in the year 1054 and will disappear in about 50,000 years, when the gas masses have moved so far away from their origin that they will no longer be stimulated to emit light. So, it will become invisible in the near future – if you use astronomical dimensions. In binoculars you can barely see a little, disappointing and featureless speckle of fog. The Veil Nebula in Cygnus is also not an easy object, although it appears as a 1° long sickle.