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This paper is one of the invited reviews celebrating the centenary of the Astronomical Society of the Pacific. The paper by I. S. Bowen (004.068) of 60 years ago is reviewed and used as the starting point for a review of nebular astrophysics. Two important aspects in the growth of our understanding of nebulae are stressed: improved observational data covering a wide wavelength region, from the ultraviolet through the optical and infrared to the radio spectral region, and improved computational power based on ever-larger computers.
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Donald E. Osterbrock (1988) studied this question.
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