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]
(astronomy)
An oval or round nebula of expanding concentric rings of gas associated with a hot central star.
References in periodicals archive ?
This intense radiation may destroy molecules that had previously been ejected by the star and that are bound up in the clumps or rings of material seen in the periphery of planetary nebulas.
In one study, led by Dr Isabel Aleman of the University of Leiden, the Netherlands, 11 planetary nebulas were analysed and the molecule was found in just three.
Although the radiation rapidly destroys molecules, astronomers studying a doughnut-shaped planetary nebula called K3-35 detected significant amounts of water vapor.
We infer that K3-35 is being observed at the very moment of its transformation from a giant star to a planetary nebula," the researchers say.
The radiation from the hot star and the interaction of its fast wind with the slower wind creates the complex and filamentary shell of a planetary nebula.
According to a popular model, planetary nebulas take their final form when the gaseous wind from a dying star plows into the shroudlike gas cloud that the star had previously ejected.
As reported in the June 20 ASTROPHYSICAL JOURNAL, Jacoby and his collaborators measured the light emitted by 486 candidate planetary nebulas in six galaxies belonging to the Virgo cluster.
Eventually, the red giant would shed its outer layers to produce a true planetary nebula.
Out of roughly 40 planetary nebulas studied so far, Bond and his colleagues have discovered about half a dozen in which the central stars prove to be close pairs of stars.
Until recently, astronomers thought red giant stars were the main source of planetary nebulas.
Spectroscopic observations lead to the supposition that the object is a very old giant star that is just on the point of becoming a planetary nebula.
The astronomers' "best guess' is that it is a star evolved past the giant or supergiant stage, undergoing a fast evolutionary phase that will make it a planetary nebula.

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