Planetary nebulae are clouds of dust and gas ejected from older stars as they move on to the next stage in their evolution. When a star uses up all its hydrogen fuel, and begins to burn helium, the core heats dramatically, causing the outer layers of the star to inflate dramatically. The outer layers of the star escape its gravity, and the solar wind of high energy particles and radiation accelerates the still superheated plasma out into space where it forms a glowing, expanding shell. Depending on the properties of the original star, these shells can take on one of three forms: elliptical with an aligned internal structure, elliptical without an aligned internal structure, and bipolar. Viewed through a small telescope, they often appear as small blue-green disks. This is similar to how many planets look, and is where the name comes from. Larger telescopes reveal a wide range of shapes (mostly the hourglass seen from different angles) and colours, and they are some of the prettiest objects in the universe.