Physics of Thermal Gaseous Nebulae: Physical Processes in by Lawrence H. Aller (auth.)

By Lawrence H. Aller (auth.)

Gaseous nebulae supply remarkable possibilities to atomic physicists, spectroscopists, plasma specialists, and to observers and theoreticians alike for the learn of attenuated ionized gases. those nebulae are usually dusty, heated by means of radiation fields and through shocks. they're short-lived phenomena at the scale of a stellar lifetime, yet their chemical compositions and inner kinematics can give very important clues to complex phases of stellar evolution. the fabric herein offered is predicated on lectures given on the college of Michigan, college of Queensland, collage of California, la, and in additional abbreviated shape on the Raman Institute, on the Scuola Internazionale di Trieste, and in different places. a lot of it really is derived origionally from the sequence "Physical strategies in Gaseous Nebulae" initiated on the Harvard collage Observatory within the overdue Thirties. i've got attempted to stress the fundamental physics of the mechanisms concerned and point out a few of the uncertainties that underlie calculations of many easy parameters. Emphasis is put on ionized plasmas with electron temperatures usually locally of 10,OOOoK. airborne dirt and dust and different materials of the chilly part of the interstellar medium are handled in brief from the viewpoint in their relation to sizzling plasmas of H II areas and planetaries. Chemical composition determinations for nebulae are mentioned in a few aspect whereas the final part bargains with interpretations of elemental abundances within the framework of stellar evolution and nucleogenesis. Gaseous nebulae provide a few fairly attractive possibilities for stories of stellar evolution.

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0 ;0 Z .... GlI'Tl ;00 -C I" ~ . ""'Gl I ,.. Gl OC",O -f-fN - - - ~ -- UlGl ' - • ; , ..... ] 5676 [ FeE: ) 5538[Clm) - - 5518[Clm] 5411 He II. 5335 [ FeE: ] 5270 [ Fe m ] 5199 [NI] _ - - - - __ - - ... ~ ~~: • j ~'':, ~ ". lrl' ~' ii , (>I "",0 cnz VI N .... ~ c: t""' t:d tTl 2: c: VI 0 tTl VI ;.. Cl "T1 0 ;.. 28 x 10- 14 • Thus, although the emission from the central star might resemble that of a black body with a temperature between 30,000oK and 150,000oK, this radiation would be attenuated by a factor of 10 14 • Atomic processes involving absorption of radiation, such as excitation to discrete levels or photoionization, will proceed at rates cut down roughly by a factor of Wg compared to corresponding rates in thermal equilibrium.

We can now grasp why the forbidden lines dominate the optical region of the spectra of gaseous nebulae. The emission per unit volume is governed by the number of atoms in the upper level. Under laboratory conditions this number would be given by Boltzmann's equation; in the gaseous nebula, the proportion in the metastable level would fall somewhat because atoms could now escape by radiation as well as by collision. On the other hand, a radiating column of 1015 meters vastly exceeds anything one could produce in the laboratory.

Forbidden transitions are of the magnetic dipole or electric quadrupole type. Magnetic quadrupole transitions also have been observed. 475) 21% 4S 0 422 (: min) t:. ,. 04eV ~ N o w SPECfRA OF GASEOUS NEBULAE a) 31 Spectroscopic parity cannot change; the jumps are even-to-even or odd-to-odd; b) The principal quantum number, n, cannot change, ~n 0 and likewise ~~ = 0; c) ~J = ± 1, 0, J = 0 + J = 0 is excluded. Electric quadrupole transitions obey rule a) above; however, ~n is arbitrary and ~~ can be ± 2 as well as O.

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