Planetary Nebulae and Supernovae Remnants

When nuclear fusion in the core of stars like the Sun ends, the stars will swell and become red giant or red supergiant stars. The outer atmospheric gases in those stars reach escape velocity as the star increases in diameter. These gases will eventually form a nebula around the star. These nebulae are called planetary nebulae since many appear as unresolved blue disks in small telescopes similar to the planets Uranus and Neptune. Stars ten times more massive than the Sun and larger have a more violent fate when fusion ends in their cores. The weight of one of these massive stars is so great that when fusion ends in the core, the star collapses on itself. The compression causes the star to rebound in a massive explosion called a supernova. The supernova creates a shell of expanding gases that appears as a nebula. These nebulae are called supernovae remnants. Planetary nebulae and supernovae remnants are transient phenomena. Eventually, the gases spread out to the point where they are no longer visible.