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): http://oposite.stsci.edu/pubinfo/PR/97/35/content/9735a.jpg