planetary nebula

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planetary nebula:

see nebulanebula
[Lat.,=mist], in astronomy, observed manifestation of a collection of highly rarefied gas and dust in interstellar space. Prior to the 1960s this term was also applied to bodies later discovered to be galaxies, e.g.
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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.

planetary nebula

[′plan·ə‚ter·ē ′neb·yə·lə]
An oval or round nebula of expanding concentric rings of gas associated with a hot central star.
References in periodicals archive ?
Dr Frew, lead author on the paper, said: "For many decades, measuring distances to Galactic planetary nebulae has been a serious, almost intractable problem because of the extremely diverse nature of the nebulae themselves and their central stars.
Kostyakova EB: Study of Photometric and Spectral Variability of Planetary Nebulae during 1968-1996 (abstract) Proc 180th Symp of the IAU, Aug 26-30, 1996 (Habing HJ and Lamers HJGLM - eds) Boston: Kluwer Academic Publishers, p 25, 1997.
The model we propose can help explain the presence of bipolar planetary nebulae, the presence of knotty jet-like structures in many of these objects, and even multipolar planetary nebulae.
The Dumbbell Nebula (M27) in Vulpecula is one of the brightest planetary nebulae in the sky and can be recognized even in binoculars and finderscopes.
Astronomers can study the motion and appearance of the material in planetary nebulae to deduce how the remnant stars have changed over time.
Andrea Tasselli has become an expert at imaging planetary nebulae and in teasing out the fine structure within their haloes, his images appearing many times in these pages.
In his early years at Mount Wilson, he conducted a survey to find new planetary nebulae (PNe) using an objective prism mounted on the 10-inch Cooke wide-angle camera.
Planetary nebulae are glowing shells of gas around white dwarfs-Sun-like stars in the final stages of their lives.
Planetary nebulae are some of the most beautiful objects in the sky and many being small and bright are ideal targets for lighter summer skies.
Instead, they represent huge knots of gas that formed when spherical shells of material, known as planetary nebulae, were ejected from the surface of a dying, sunlike star.
If you haven't picked up on it already, LPR filters are only good for observing nebulae--and then only emission and planetary nebulae.
1 in the 1967 Perek & Kohoutek catalogue of galactic planetary nebulae, is an ancient large faint planetary that images relatively easily with narrow-band filters and CCDs.

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