Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986amsci..74..244k&link_type=abstract
American Scientist (ISSN 0003-0996), vol. 74, May-June 1986, p. 244-252. NSF-supported research.
Astronomy and Astrophysics
Astronomy
2
Interstellar Gas, Planetary Nebulae, Stellar Evolution, Abundance, Astronomical Photography, Astronomical Spectroscopy, Emission Spectra, Forbidden Transitions
Scientific paper
The database on planetary nebulae (PN) is summarized in terms of the known facts of the processes of planetary formation. Approximately 1500 PN have been observed in the Galaxy, most taking either a bilobed or circularly symmetric form. The matter is 90 percent hydrogen heated by UV light from a central star to surface temperatures ranging from 25,000-100,000 K. The gas is also ionized, and the mechanisms by which the excitation energy is shed is traced. Consideration is given to the implications the observed optical spectra of such regions have for the physical processes occurring in PN. Spectral data furnish density and temperature information which in turn permits calculations of the chemical abundances, which have shown a surprisingly wide variation; heavier elements have been most associated with the youngest stars. The observational data of all the observed PNs indicates that the PN formation process begins with a star that has burned all its hydrogen, leading to collapse and He burning. Higher-mass stars may even burn Fe, but finally all are left with a C core and the PN, by a process which is not understood, forms. Observational data on various stellar objects on which the PN model has been based are discussed.
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