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 ?
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.
0) Red Rectangle Reflecting 06h 19m 8 -10[degrees]38 HD 44179 Nebula NGC 2237/8/9 Emission Nebula 06 32 3 +05 03 NGC 2244 Open Cluster 06 32 3 +04 51 NGC2251 Open Cluster 06 34 7 +08 23 NGC 2252 Open Cluster 06 35 0 +05 23 NGC 2261 Reflecting Neb 06 39 2 +08 44 NGC 2264 Emission Nebula/ 06 41 1 +09 53 Open Cluster NGC 2323 Open Cluster 07 03 2 -08 20 NGC 2346 Planetary Nebula 07 09 2 -00 48 COCD 1034 Multiple Star group 07 10 5 +06 04 NGC 2353 Open Cluster 07 14 6 -10 18 Moni 5 Asterism 07 18 4 +06 18 Streicher 20 Asterism 07 43 7 +04 50 NGC 2538 Galaxy 08 11 4 +03 37 Object Mag.
Menzel 3 is a planetary nebula whose central star is ejecting huge amounts of glowing gas out into space in opposite directions, giving the nebula a strange ant-like appearance.
NGC 6302 is a planetary nebula, which is in the final stage of the life of a star with a mass several times that of the Sun.
Between M46 and M93, and about 3 1/2 [degrees] south of M46 is the planetary nebula NGC 2440, which has a more lobed or elliptical appearance, about 32" long.
NGC 2392, nicknamed the "Eskimo Nebula," is what astronomers call a planetary nebula.
In the far western side of the constellation a special planetary nebula can be found.
Our Sun will eventually become a planetary nebula as it gives off layers of its outer atmoshere that will drift into space, while the central core of the Sun will become a white dwarf.
The cast-off material, known as a planetary nebula, will seed the cosmos with elements forged in the star's nuclear furnace, some of which have formed into complex organic molecules.
This phase is the last stage of nuclear burning before stars form a planetary nebula, where the gas and dust emitted through copius stellar winds are colourfully illuminated by radiation from the star's naked core.
I did mention that the area is rather devoid of local nebulae, and in fact there is only one object of note--the planetary nebula NGC 246.
What a nice surprise to find a bright and eye-catching planetary nebula just a degree north-west from NGC 1398.