Blue straggler

Sketch of HRD of a globular cluster, showing blue stragglers

This video shows the movement of blue straggler stars in globular clusters over time.

A Hubble Space Telescope image of NGC 6397, with a number of bright blue stragglers present.^[2]

Blue stragglers (BSS) are main-sequence stars in open or globular clusters that are more luminous and bluer than stars at the main-sequence turn-off point for the cluster. Blue stragglers were first discovered by Allan Sandage in 1953 while performing photometry of the stars in the globular cluster M3.^[5][6] Standard theories of stellar evolution hold that the position of a star on the Hertzsprung–Russell diagram should be determined almost entirely by the initial mass of the star and its age. In a cluster, stars all formed at approximately the same time, and thus in an H–R diagram for a cluster, all stars should lie along a clearly defined curve set by the age of the cluster, with the positions of individual stars on that curve determined solely by their initial mass. With masses two to three times that of the rest of the main-sequence cluster stars, blue stragglers seem to be exceptions to this rule.^[7] The resolution of this problem is likely related to interactions between two or more stars in the dense confines of the clusters in which blue stragglers are found.

Formation

Several explanations have been put forth to explain the existence of blue stragglers. The simplest is that blue stragglers formed later than the rest of the stars in the cluster, but evidence for this is limited.^[8] Another simple proposal is that blue stragglers are either field stars which are not actually members of the clusters to which they seem to belong, or are field stars which were captured by the cluster. This too seems unlikely, as blue stragglers often reside at the very center of the clusters to which they belong. Another theory is that blue stragglers are the result of stars that come too close to another star or similar mass object and collide.^[9]

Cluster interactions

NGC 6752 is a globular cluster that contains a high number of blue straggler stars.^[3]

The globular cluster NGC 6388, observed by Hubble.^[4]

The two most viable explanations put forth for the existence of blue stragglers both involve interactions between cluster members. One explanation 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 more massive star, potentially with a mass larger than that of stars at the main-sequence turn-off point. While a star born with a mass larger than that of stars at the turn-off point would have already evolved off of the main sequence, a more massive star which formed via merger would not have evolved as quickly. 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 and calculations of the expected number of collisions are consistent with the observed number of blue stragglers.^[9]

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. Some blue stragglers have been observed to rotate quickly, with one example in 47 Tucanae observed to rotate 75 times faster than the Sun, which is consistent with formation by collision.^[10]

The other explanation relies on mass transfer between two stars born in a binary star system. The more massive of the two stars in the system will evolve first and as it expands, will overflow its Roche lobe. Mass will quickly transfer from the initially more massive companion on to the less massive and like the collision hypothesis, would explain why there would be main-sequence stars more massive than other stars in the cluster which have already evolved off of the main sequence.^[11] Observations of blue stragglers have found that some are lacking in carbon and oxygen in their photospheres, which is evidence of having dredged up from the interior of a companion.^[12]

Overall, there is evidence in favor of both collisions and mass transfer between binary stars.^[13] In M3, 47 Tucanae and NGC 6752, both mechanisms seem to be operating, with collisional blue stragglers occupying the cluster cores and mass transfer blue stragglers at the outskirts.^[14] The discovery of low-mass white dwarf companions around two blue stragglers in the Kepler field suggests these two blue stragglers gained mass via stable mass transfer.^[15]

Other types

47 Tucanae contains at least 21 blue stragglers near its core.^[1]

"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.^[16]

See also

• Algol variable
• SX Phoenicis variable

References

v t e Star Evolution Formation Pre-main-sequence star Main sequence Horizontal branch Asymptotic giant branch Dredge-up Instability strip Red clump PG1159 star Mira variable Planetary nebula Protoplanetary nebula Luminous red nova Luminous blue variable Wolf–Rayet star Supernova impostor Supernova Hypernova Hertzsprung–Russell diagram Color–color diagram Protostars Molecular cloud H II region Bok globule Young stellar object Herbig–Haro object Hayashi track Hayashi limit Henyey track Orion T Tauri FU Orionis Herbig Ae/Be Luminosity class Subdwarf Dwarf Blue Red White Black Brown Subgiant Giant Blue Red Bright giant Supergiant Blue Red Yellow Hypergiant Yellow Blue straggler Spectral classification O B A F G K M Be OB Subdwarf B Late-type Peculiar Am Ap/Bp Oscillating Barium Carbon CH Extreme helium Lambda Boötis Lead Mercury-manganese S Shell Technetium Remnants White dwarf Black dwarf Helium planet Neutron star Pulsar Magnetar Stellar black hole Compact star Quark Exotic Stellar core: EF Eridani B Theoretical stars Dark-matter star Frozen star Quasi-star Thorne–Żytkow object Iron star Nucleosynthesis Alpha process Triple-alpha process Proton–proton chain Helium flash CNO cycle Lithium burning Carbon burning Neon burning Oxygen burning Silicon burning S-process R-process Fusor Nova Remnants Structure Core Convection zone Microturbulence Oscillations Radiation zone Atmosphere Photosphere Starspot Chromosphere Corona Stellar wind Bubble Asteroseismology Eddington luminosity Kelvin–Helmholtz mechanism Properties Designation Dynamics Effective temperature Kinematics Magnetic field Magnitude Absolute Mass Metallicity Rotation UBV color Variability Star systems Binary Contact Common envelope Multiple Accretion disc Planetary system Earth's Solar System Earth-centric observation of Pole star Circumpolar star Magnitude Apparent Extinction Photographic Color Radial velocity Proper motion Parallax Photometric-standard star Lists Star names Arabic Chinese Most massive Least massive Largest Brightest Historical Most luminous Nearest Nearest bright Stars with exoplanets Brown dwarfs Collapsars Planetary nebulae Novae Notable supernovae Supernova remnants Supernova candidates Timeline of stellar astronomy Related articles Substellar object Brown dwarf Sub-brown dwarf Planet Star cluster Association Open cluster Globular cluster Super star cluster Galactic year Galaxy Supercluster Helioseismology Guest star Constellation Asterism Gravity Intergalactic star Infrared dark cloud Star portal