Globular clusters are aggregations of stars in a spherical shape. They are bound by their mutual gravity and have high star density towards their centers. They orbit the center of galaxies.

Globular clusters are considered to be old because they mostly contain low mass stars. It is important to understand that large stars have much shorter lives than low mass stars do. Smaller stars burn their fuel more slowly than larger stars. Also, large mass stars emit more blue light than do smaller stars so they appear to be blue.

Blue stragglers are young large mass stars in globular clusters. Their much younger age is incompatible with the age of the host globular cluster.

Astronomers believe that blue stragglers are the result of the merger of smaller stars in the cores of globular clusters. The relatively high concentration of stars in the cores of globular clusters facilitates these mergers.

NGC6752 and its blue straggles
NGC6752 and its blue straggles. Note the star in the top left of the cluster is located between Earth and the cluster (click photo for larger image).

Image source: Globular Star Cluster NGC 6752