Since the discovery of the cosmic microwave background ( 1 84), it is gen­ erally assumed that the universe originated from a hot big bang (78). Detailed nucleosynthesis calculations based on the hot big bang model (e.g. 4, 1 82, 221 ) showed that no element heavier than 9Be could have been synthesized primordially with an abundance exceeding 101 4 by mass fraction. However, in the universe that we live in today, there are appreci­ able amounts of elements heavier than helium, called heavy elements or metals in general. These elements-accounting for about 2% of the visible mass-are believed to have been synthesized in stars or starlike objects (cf 26). Cosmic gas fragmented into huge gas clouds, forming galaxies­ further fragmentation of which possibly led to the formation of stars. Stars evolve on time scales ranging from several millions to tens of billions of years, synthesize elements in their central parts, and emit the processed elements into the interstellar medium (ISM) at various stages. The ISM is thus enriched with heavy elements, and the stars formed thereafter are born with a higher metallicity. This cycle should go on until all gas in the interstellar medium is exhausted (200a, 2 12a). Thus the study of chemical evolution of the galaxies involves understanding the spatial distribution and temporal evolution of various elements in the galaxies by taking into