Zhong-Han Ai ; Wen-Ping Chen
An open cluster since its birth has destined to disintegration as a consequence of continuing stellar ejection and extraneous perturbation by Galactic tidal forces plus differential rotation. Conceivable massive star clusters exist only in infancy; those away from the "action zones", e.g., in the disk, or bulge would have a longer average life expectancy. NGC 6791 (Berkeley 46), appears to be such a senile oddity. With a heliocentric distance of 5 kpc, and an age about 8 Gyr, the cluster is one of the oldest, yet metal-rich, and populous Galactic star clusters. An analysis of Gaia eDR3 data of high-precision proper motion and parallax measurements led to identification of more than 2000 member candidates. Cluster parameters more reliable than reported in the literature are derived, including the distance, age, size, and morphological shape. We will discuss the possible explanation of the secret of the longevity of this elusive star cluster.