planetary nebula

planetary nebula

An expanding and usually symmetrical cloud of gas that has been ejected from a dying star. Most are believed to be the ejected envelopes of red giant stars, shed as a result of instabilities late in their evolution. The gas cloud is ionized by the compact hot burnt-out stellar core that remains in the center of the cloud; the cloud is detected by virtue of the resulting light emission. Planetary nebulae are therefore a class of emission nebulae. They are usually ring-shaped or sometimes hourglass-shaped. They are generally less than 50 000 years old, eventually fading and dispersing into the interstellar medium. The name refers to their resemblance to planetary disks rather than pointlike stars under low magnification. They have a large size range: the smallest objects have a starlike appearance on photographs – and are thus called stellar planetaries – but can be identified by the characteristic spectral emission lines. Planetary nebulae occur in isolation and usually lie close to the galactic plane, concentrated toward the galactic center.

A planetary nebula is believed to form as part of the normal evolution of single stars with masses of up to 8 solar masses; the immediately preceding stage is probably a rapid mass loss OH/IR star. Instabilities eject a succession of planetary nebula shells, reducing the mass of the star until the core (the planetary nebula central star) is only about 0.6 solar masses. This degenerate core becomes a white dwarf. The recent discovery of planetary nebulae with close binary stars at the center suggests that some planetaries form as a result of interactions in a double star system. One star has expanded sufficiently to cocoon both in a common envelope, with the two star cores orbiting inside; frictional drag transfers energy from the orbiting stars to the surrounding gas and thus expels the envelope as a planetary nebula.

Although planetary nebulae are less massive and more symmetrical than H II (ionized hydrogen) regions, their optical spectra are similar. There are bright emission lines of oxygen, hydrogen, nitrogen, and other components, the characteristic green of the inner region being due to doubly ionized oxygen and the red of the outer periphery resulting from singly ionized nitrogen and from hydrogen alpha emission. About 1500 planetary nebulae are known in our Galaxy, the Ring nebula in Lyra being a typical example. See also nebula.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006

planetary nebula

[′plan·ə‚ter·ē ′neb·yə·lə]

(astronomy)

An oval or round nebula of expanding concentric rings of gas associated with a hot central star.

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.