SODIUM ABUNDANCE VARIATIONS IN MAIN-SEQUENCE STARS OF THE GLOBULAR-CLUSTER 47-TUCANAE

GALACTIC globular clusters were once thought to be chemically homogene ous, having formed quite rapidly from relatively small condensations o f primordial gas(29). In many clusters, significant star-to-star varia tions in light-element abundances have been observed(1-4) in evolved g iant stars. These variations have been attributed to the presence at t he stellar surfaces of nucleosynthesis products generated deep within the stars. But other observations(13) have suggested that some of this variability was established earlier in the stars' lifetimes, perhaps as a result of inhomogeneities in the gas cloud from which the cluster formed. Here we report the observation of variations in the sodium ab undances of unevolved (main-sequence) stars in the cluster 47 Tucanae. Although these variations are similar to those observed in evolved cl uster stars, they cannot be explained by mixing, in the framework of c urrent models of stellar evolution. This indicates either that the gas out of which 47 Tuc formed was chemically inhomogeneous, or that some mechanism for altering the surface element abundances of stars operat es while they are still on the main sequence.