Why are blue stragglers unusual in old globular clusters?

Blue stragglers are considered unusual in old globular clusters because their temperature and mass suggest they should have already evolved off the main sequence, similar to the vast majority of other stars in these ancient systems. Instead, these stars appear to "lag behind" in their evolution, remaining on the main sequence despite the cluster's advanced age.12

While the exact cause remains a subject of study, a leading theory suggests that a blue straggler may be the result of the coalescence of two lower-mass stars. This "born-again" scenario creates a single, more massive star that appears younger and hotter than its neighbors. These stars are also notable for being strongly concentrated toward the center of the cluster. Because no single theory currently accounts for all observed cases, they remain an enigmatic feature of stellar evolution.12