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.
Astronomers can study the motion and appearance of the material in planetary nebulae to deduce how the remnant stars have changed over time.
International Conference on Planetary Nebulae as Astronomical Tools (2005: Gdansk, Poland) Ed.
Planetary nebulae are glowing shells of gas around white dwarfs-Sun-like stars in the final stages of their lives.
Speakers at the meeting included the Director, who gave his review of the Section year and showed a selection of images received, Bob Winter on the death of stars, Robin Leadbeater on the spectra of dying stars, Owen Brazell on new planetary nebulae, Chris Longthorn on observing from New Mexico Skies, Andrew Robertson on building and using a large Dobsonian telescope and David Boyd on the behaviour of Gyulbudaghian's Nebula and PV Cephei.
Henize compiled a catalogue of over 2 000 Planetary Nebulae, now listed as He objects.
Poster topics include the magnesium abundance in 52 B stars, an infrared survey of neutron capture elements in planetary nebulae, new r-process enhanced stars found in the HK-II survey, and the origin of sulfur.
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.
This means that there are not many 'galactic' objects to be found here--bright or dark nebulae, open or globular clusters, planetary nebulae.
Studies published last year, including one by Garcia-Hernandez, showed that buckyballs were present in planetary nebulae rich in hydrogen.
There are several other planetary nebulae that have the right magnitude central star to the nebula's overall brightness to show off this effect.

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