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Planetary Nebulae

Primary Mechanism, Collisional Excitation Mechanism, Bowen's Fluorescent Mechanism, Continuous Spectra Mechanism

High-density interstellar dust or clouds are referred to as nebulae. These nebulae, both dark and luminous, are equally important since the chemical analyses of these objects contribute significantly to the study of cosmic abundances. Bright or incandescent nebulae, just as dark nebulae, are not self-luminous.

It is the star or stars imbedded in these nebulae which produce the luminous objects and are responsible for the atomic processes that may take place. Nebulae may be divided into four groups: dark, reflection, diffuse, and planetary, with the latter three representing the luminous objects.

The study of bright-line spectra of gaseous nebulae, namely diffuse and planetary, is important because it contributes in no small way to the determination of cosmic abundances. It has been suggested that these objects can be studied with greater ease since all portions of a nebula are observable, and even though departures from thermodynamic equilibrium are significant, the processes seem to be well understood and can be treated theoretically.

A disadvantage in using gaseous nebulae is that many of them possess a filamentary structure that is due to non-uniform density and temperature, from point-to-point. In instances where stratification occurs, the temperature The Cat's Eye Nebula (NGC 6543) as seen from the Hubble Space Telescope. The shells of gas were expelled from a dying star (center) during its last stages of life. It has been suggested, in order to explain the intricate features seen in the shells, that another star is orbiting around the dying star. The knots and thin filaments seen along the periphery of the gas (bottom right and top left) might have been formed by a pair of high-speed jets ejected by the companion star interacting with the gas in the shells. U.S. National Aeronautics and Space Administration (NASA). and excitation level will be different for the inner and outer parts of the nebula. Also, an element may be observed in one or two stages of ionization and yet may exist in several unobserved stages of ionization.

In the study of nebulae there are four fundamental quantities that are needed at the outset: distance, mass, electron temperature, and density. Of these, the distance parameter is probably the most important one because without it the real dimensions of the nebula cannot be determined from the apparent ones. To determine the mass it is necessary to know the density, and this can be determined, in some cases, from forbidden line data.

For diffuse nebulae, the distances are found from the stars with which they are associated, and the most commonly used methods are statistical parallaxes and moving clusters. However, for planetary nebulae none of these methods apply because they are too far away for a direct trigonometric measurement; they are not members of moving clusters, and statistical parallaxes are inapplicable since they do not appear to move randomly. Instead, the approach is to obtain parallaxes of the individual objects, or by special methods in which the mass of the nebular shell is assumed constant, or the absolute magnitude of nebula is assumed constant.

From the bright-line spectra of gaseous nebulae the abundances of the elements and ions can be determined, the contribution to the elements and ions can be determined, Figure 1. Illustration by Hans & Cassidy. Courtesy of Gale Group.

and the contribution to the cosmic abundances can be assessed. The mechanism of excitation (ionization), and recombination that operate is well understood, so that from these spectra reliable results can be expected. Physically, the electron from the ionized atom, for example hydrogen, moves about freely for approximately 10 years, and during that period it will collide with other electrons, thereby altering its energy. Also, periodically it will excite ions to the metastable levels. Since the electron undergoes so many energy exchanges with other electrons, the velocity distribution turns out to be Maxwellian so that the gas kinetic temperature, and specifically the electron temperature, is of physical significance. It must be noted, also, that an atom in the nebula is subjected to dilute or attenuated temperature radiation from a star that subtends a very small angle. The energy distribution or quality of this radiation corresponds to temperatures ranging from 36,000–180,000°F (20,000–100,000°C). However, the density of this radiation is attenuated by a factor of 1014.

The mechanisms that are operating in gaseous nebulae are as follows:

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