some even orbit in opposite directions. Because most heavy metals form in supernovae, not many were available when these clusters were born (few super-
novae had occurred so early in our galaxyΓÇÖs life). So the stars in globular clusters contain less metal than stars in the galaxyΓÇÖs plane, which form from material that has
been enriched by generations of supernovae. Because these stars appear to be the oldest in our
galaxy, determining their age will help us date the universe. Some estimates put the birth of the
oldest globular cluster at 10 to 13 billion years ago. Unfortunately, other methods suggest that the universe itself is only 7 billion
years old. When, and if, two substantially different methods
