Formation of Blue Stragglers

‘Blue stragglers’ are stars that seem to violate the normal star life cycle of running out of fuel as they mature. At an age when they should be out of fuel, they are as hot and bright as younger stars.

After a decade of observation, Robert Mathieu of the University of Wisconsin-Madison has outlined three possible scenarios for creating blue stragglers.

1. If a star is part of a binary pair with a red giant, the red giant can lose its outer envelope to that companion star, turning it into a blue straggler.
2. Blue stragglers can form as a result of stellar collisions. Although single stars have little chance of colliding, stars in binary or larger systems can hit each other as the clusters cross paths.
3. A third star entering a binary system can cause the pair of stars to fuse into one blue straggler.

Mathieu studied the star cluster NGC 188 using the WIYN Telescope on Kitt Peak, Arizona. NGC 188 contains several thousand stars of uniform age and at least 21 blue stragglers. It even contains one binary pair of two blue stragglers. They were probably created independently as part of separate pairs, and then ejected their old partners to join up with each other.

Blue stragglers in NGC 6397, created by NASA and ESA