The presence of oxygen in this nebula tells us something important about the role of supernova explosions in the evolution of the galaxy. Originally, the universe probably consisted of the two lightest elements: hydrogen and helium. Heavier elements, like those that make up the Earth, are forged in nuclear fires within stars. If they remained in stars, they would not be available to create the rocky worlds of planets.
Old stars can shed their outer layers of gas, although relatively small amounts of heavier elements in the starΓÇÖs core are liberated through this process. When a star explodes, however, it casts off a large fraction of its mass in an expanding shell of gas, propelling
