Optical imaging and spectroscopy of G353.2+0.9, the brightest part of the giant H II region NGC 6357, shows that this H II region is optically thin, contains "300 M⊙ of ionized gas and is probably expanding into the surrounding medium. Its chemical composition is similar to that found in other H II regions at similar galactocentric distances if temperature fluctuations are significant. The inner regions are probably made of thin shells and filaments, whereas extended slabs of material, maybe shells seen edge-on, are found in the periphery. The radio continuum and Hα emission maps are very similar, indicating that most of the optical nebula is not embedded in the denser regions traced by molecular gas and the presence of IR sources. About 1050 UV photons per second are required to produce the Hβ flux from the 1l.3'x10' region surrounding the Pis 24 cluster that is south of G353.2+0.9. Most of the energy powering this region is produced by the 03-7 stars in Pis 24. Most of the 2MASS sources in the field with large infrared excesses are within G353.2+0.9, indicating that the most recent star forming process occured within it. The formation of Pis 24 preceded and caused the formation of this new generation of stars and may be responsible for the present-day morphology of the entire NGC 6357 region.

• JOAQUIN BOHIGAS(Instituto de Astronomfa, UNAM, Apdo.)
• MAURICIO TAPIA(Instituto de Astronomfa, UNAM, Apdo.)
• MIGUEL ROTH(Las Campanas Observatory, Carnegie Institute of Washington)
• MARIA TERESA RUlZ(Departamento de Astronomia, Universidad de Chile)