Aaron Warren (Rutgers University), James Lombardi (Vassar College)

Globular clusters are spherically distributed balls of hundreds of thousands of stars. The stars all formed in the cluster at about the same epoch. The clusters orbit large galaxies, such as our own Milky Way, and are generally the densest collections of stars in the cosmos. Each globular cluster has an upper limit to how massive its stars can be, called the turn-off mass. But sometimes abnormally hot stars called blue stragglers are found in them, which are higher than the turn-off mass. How do they get there? Could a collision between two older, smaller stars create these larger, younger blue stragglers?

Star formation in globular clusters

This movie shows the result of what happens when two stars in a globular cluster collide. The stars originally have masses of 0.6 and 0.8 times the mass of our sun. The images that compose the simulations are generated by taking a two-dimensional slice in the equatorial plane of the three-dimensional simulation. The colors represents different densities: dark red corresponds to low density regions while the bright yellow corresponds to a high density region. According to this calculation, it is indeed possible that these small stars can join together without being destroyed, breathing new life into an old cluster. Apparently, if you smash old, dim stars, you just might end up with a young, bright star—a blue straggler!

Gordon Myers