Blue straggler

Blue stragglers (BSS) are stars in open or globular clusters that are hotter and bluer than other cluster stars having the same luminosity. Thus, they are separate from other stars on the cluster's Hertzsprung-Russell diagram. Blue stragglers appear to violate standard theories of stellar evolution, which holds that stars formed at the same time in a cluster should lie along a clearly defined curve in the Hertzsprung-Russell diagram, with their positions on that curve determined solely by their initial mass. Since blue stragglers often lie well off this curve, they may undergo atypical stellar evolution.

A ground and Hubble Space Telescope comparison, illuminating a few blue stragglers.

Formation

A number of theories have been proposed for the origin of blue stragglers.^[1] The leading hypothesis is that they are current or former binary stars that are in the process of merging or have already done so. The merger of two stars would create a single star with larger mass, making it hotter and more luminous than stars of similar age. If this is correct, then blue stragglers would not violate stellar evolution theory, given the resulting star would have more hydrogen in its core making it behave like a much younger star. There is evidence in favor of this view, notably that blue stragglers appear to be much more common in dense regions of clusters, especially in the cores of globular clusters. Since there are more stars per unit volume, collisions and close encounters are far more likely in clusters than among field stars.

One way to test this hypothesis is to study the pulsations of variable blue stragglers. The asteroseismological properties of merged stars may be measurably different from those of typical pulsating variables of similar mass and luminosity. However, the measurement of pulsations is very difficult, given the scarcity of variable blue stragglers, the small photometric amplitudes of their pulsations, and the crowded fields in which these stars are often found.

Recent research has shown that stars near blue stragglers have significantly less carbon and oxygen than their neighbors.^[2] This suggests that a star may become hotter and bluer by incorporating matter from an orbiting star. The donor star thus has its deeper regions exposed, showing areas where carbon has fused into heavier elements.

Blue stragglers rotate at a high rate, some 75 times that of the Sun. They appear to be two to three times the mass of the other cluster stars present.

Other types

"Yellow stragglers" or "red stragglers" are stars with colors between that of the turnoff and the red giant branch but brighter than the subgiant branch. Such stars have been identified in open and globular star clusters. These stars may be former blue straggler stars that are now evolving toward the giant branch.^[3]

References

External links

• Origin of Strange 'Blue Straggler' Stars Pinned Down - Astronomers discover evidence that the stars are stealing fuel from their neighbors; Space.com, 5 October 2006.

• "NASA's Hubble Space Telescope Finds "Blue Straggler" Stars in the Core of a Globular Cluster". Hubble News Desk. 1991-07-24. http://hubblesite.org/newscenter/newsdesk/archive/releases/1991/12/text/. Retrieved 2006-05-24. 

• The radial distribution of blue straggler stars and the nature of their progenitors, M. Mapelli, S. Sigurdsson, F. R. Ferraro, M. Colpi, A. Possenti, B. Lanzoni, 2006.

• Becker, Kate. Rejuvenating Ancient Stars. Sky & Telescope. Jan. 2007: 24.

• Hubble Catches up with a Blue Straggler Star, Weaver, Donna, Mike Shara, and Rex Saffer, 1997.

• Astronomy Picture of the Day – Blue Straggler Stars, Nemiroff, Robert, and Jerry Bonnell, 2000