M57 (The Ring Nebula)

A remarkable example of a planetary nebula. M57 is called the Ring Nebula because of its donut shaped appearance. It is easy to find but a challenge to see in binoculars. M57 is tiny (1' in diameter) but bright. Because its brightness is spread over a small area, it is best under high magnification. The 14th magnitude central star (a white dwarf) is difficult to identify in smaller telescopes. Astronomers estimate that the shell of this planetary nebula was blown off about 20 000 years ago. The different colours of the shell visible in photographs represent different elements. Helium gas emits blue light, oxygen emits green light, and carbon emits red light. The apparently void region between the shell and the central star is also filled with gas. This area only appears empty because it is so hot that the gas emits most of its energy as ultraviolet rays instead of visible light.