Discovered in 1953 by Allan Sandage, Blue Stragglers, or BSSs, are known for their striking color and luminosity in otherwise homogeneous galactic and globular clusters. What is so peculiar about BSSs is that for their temperature, and therefore mass and luminosity, they should have long since evolved off of the main sequence, into Red Giant and later stages. Stars that are members of large clusters are all formed around the same time. As they age along the main sequence there line above a certain luminosity above which no stars will exist since they will have long since fused their hydrogen and helium cores and died.

Clusters are relatively barren of any gas or dust that might allow for new star formation and observed BSSs. While BSSs could be pollutants, field stars from outside the cluster, they tend to be concentrated in the core of clusters and no observed BSSs have radial velocities consistent with this theory.

The leading theory, and observation supports this, states that BSSs are a result of collisions and mass transfer. As unstable binary star systems gravitate towards one another, mass from the smaller star is slowly coalesced with the larger star. This provides it with more fuel and disturbs other gases in the star causing new hydrogen to be introduced into the core. This process gives the larger star a new lease on life and more mass causes it to burn hotter and brighter.

One bright BSS and several fainter ones can be seen in this star field(image of Globular Cluster 47 Tucanae):

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