planetary nebula

An expanding shell of gas surrounding a star in a late stage of stellar evolution. The name derives from the description given by William Herschel, who thought their circular shapes to be reminiscent of the discs of the planets as seen through a small telescope. There is no connection between planets and planetary nebulae. Planetary nebulae are formed in the process of mass loss during which red giant stars ultimately become white dwarfs. Typically, the gaseous shell contains a few tenths of a solar mass of material and is moving outwards with a velocity of 20 km/s. The lifetime is perhaps 35,000 years before the shell becomes too tenuous to be visible. The spectra show emission lines from the glowing gas combined with the spectrum of the central star, which may contain absorption or emission lines, or both. These central stars are essentially burnt-out cores in the process of becoming white dwarfs, with temperatures of up to 125,000 K, and their diverse characteristics reflect those observed in white dwarfs. The nebulae take a variety of forms - ring-shaped, circular, dumbbell-like or irregular. Notable examples are the Ring Nebula, the Helix Nebula and the Dumbbell Nebula.