Next Contents   The analysis of nebular spectra constitutes the
best, and in some cases the only one, method for the
determination of chemical abundances in spiral and irregular
galaxies, as well as in sites of recent star formation. The
abundances of several elements like He, O, N and S can in
principle be determined since strong emission lines of these
elements, some of them in their dominant ionization states, are
present in the optical region of the spectrum. This requires
knowledge of the electron temperature which can be obtained by
measuring appropriate line ratios like [OIII] [(lambda4363) /
(lambda4959 + lambda5007)], [NII] [(lambda5755) / (lambda6548 +
lambda6584)], [OII] [(lambda7327) / (lambda3727 + lambda3729)],
or [SIII] [(lambda6312) / (lambda9069 + lambda9532)].
Unfortunately, these line ratios usually involve one
intrisically weak line which can be detected and measured with
confidence only for the brighter and hotter objects and in many
cases - distant galaxies, low surface brightness objec