Once stars like our Sun have exhausted all of their available fuel, they expand to huge diameters (as big as the orbit of the Earth). Then, their outer layers are gently ejected into space. The remnant hot core energizes the surrounding gas, producing the planetary nebula. This is a very brief phase - lasting just 100,000 years, after which the nebula disperses and the core cools. In these images, the greenish tinge is given by the light of glowing oxygen gas, while red signifies the light from hydrogen.