Observations of star clusters combined with the theory of stellar evolution enable us to estimate the ages of stars while cosmological observations and theories give us a value for the age of the Universe. This is the most important interaction between cosmology and stellar evolution because it is clearly necessary that stars are younger than the Universe. Stellar evolution also plays an important rôle in relating the present chemical composition of the Universe to its original composition.

To the aim of reproducing the oxygen abundances and gradients observed in the disk of our Galaxy and four nearby spirals (M31, M33, M83, and M101), we have computed numerical models of galactic evolution assuming only two free parameters: the infall of gas from outside the disk and the law of star formation (e.g. proportional to some power of the gas density, decreasing with time, constant, etc.).

We have detected the 8.7 GHz hyperfine line of 3He+ in the HII regions W43, W51, and W3 and obtained significant upper limits for M17, W49, and Orion A. Even though the abundances derived from measured line parameters can have large uncertainty due to the detailed structure of the sources, it seems almost certain that the 3He+ abundance in W3, 12 kpc from the galactic center is significantly higher than in sources closer to the galactic center. If this is the result of general chemical evolution of the galaxy, it is exactly the opposite of what is expected.

We are engaged in a long term study of the stellar component in the nuclear bulge of the Galaxy. Regions of low obscuration at different galactic latitudes above the galactic nucleus are surveyed for M giants with a transmission grating and thin prism at the prime focus of the CTIO 4-m reflector. Photographic and infrared photometry are obtained for samples of the stars found. Spectroscopic observations are used to investigate band strengths and radial velocities.

Models of chemical evolution of the Galaxy have been computed by taking into account the different roles played by TypeI-1/2 (single stars suffering degenerate C-ignition) and Typell supernovae in the chemical enrichment. The overabundance of oxygen observed in the Halo stellar population has been well reproduced.

An evolutionary track for a metal-free star of intermediate mass with parameters (M/M⊙, X, Z)= (3,0.80,0.00) has been constructed. The evolutionary behaviour of the star in the theoretical H-R diagram is compared with that of a normal star with the same mass and its implication on the chemical evolution of the interstellar medium is discussed.

The absence of concensus on several of the problems which have been discussed during the meeting is perhaps the best demonstration of the usefulness of the Symposium.