Blue straggler

Blue stragglers ( more common English term: Blue straggler ) are stars that are bluer and more luminous compared to stars of the same age and metallicity.

Analysis of blue stragglers

Blue stragglers are first noticed in star clusters, where all the stars at the same time are the result of a homogeneous molecular cloud. According to the theory of stellar evolution all the stars in the Hertzsprung -Russell diagram according to the age and mass are arranged along a line, with Blue stragglers represent seemingly a contradiction to the theory of stellar evolution. Meanwhile, Blue stragglers were also in the halo of the Milky Way, are detected in the Galactic bulge and in dwarf galaxies. With many blue stragglers fast rotation speed has been detected in comparison to other stars apparently the same age.

An analysis of the properties of blue stragglers can be done by the pulsations, the rotation or Bedeckungsveränderlichkeit:

• Many Blue stragglers are in the Hertzsprung -Russell diagram in the instability strip of the Zwergcepheiden. These variable star form a pattern of vibrations, the analysis made ​​possible by means of an analysis of the Asteroseismology stellar structure, and thus the development.
• The rapid rotation of many blue stragglers can be detected by the rotational broadening of the spectral lines. In the case of an outer convective layer show the rapidly rotating blue stragglers all signs of stellar activity as Radioflares, X-ray emission and photometric variability due to star spots. The FK Comae Berenices - stars are probably the products of a merger of a close binary system.
• At Blue stragglers caused by a mass transfer in a close binary system, the mass- donating companion can be observed by radial velocity measurements, or the proper orientation in space as eclipsing binary star. In both cases, a derivation of absolute parameters, and thus the development of the binary system is possible.

Emergence of variants

Mass transfer in binary stars

Have stars in binary star systems via different masses, so runs their development on different time scales. The more massive star exhausts the supply of hydrogen in its core by hydrogen burning faster and develops into a red giant. Expands the star about his Roche limit, so flows in the following matter from the more massive star to his companion. The expanding star gets caught off balance and continues to expand, so that the matter transfer is increased. As a result of the lower-mass companion to the more massive star that is orbited by the core of the once massive star. Such binary systems consisting of a blue straggler and a low-mass white dwarf could be detected by the NASA satellite Kepler.

Star mergers

A second formation mechanism is necessary because blue stragglers also occur as single stars. As a precursor of his blue stragglers close contact systems are adopted. In these binary systems such as the W Ursae Majoris star pass the orbits of the two stars in a common sheath. You lose about gravitational radiation and magnetic interaction angular momentum and merge into an extremely rapidly rotating single star. With the merger of a binary system much energy is released, this event is called Luminous Red Nova or mergeburst. In the case of light Strengthening Red Nova V1309 Sco cover the light change of the contact system could be measured prior to the merger. In triple systems, the tracks may be unstable in the long term and lead to a merging of the central binary system also due to the Kozai effect or the Darwin instability.

Star collisions

Approximately 1% of all stars in open clusters and in globular clusters are blue stragglers. In particular, in the dense centers of globular clusters, some of the blue stragglers have been caused by collisions. If the relative velocity between the participating stars not very large, only a few percent of the atmosphere is lost in the collision, and it does not lead to mixing of the stars. The spectra of the Blue stragglers show chemical compositions that would be typical of a mix of two stars. Also at the Blue stragglers from collisions include close encounters of a star with a binary system, then change the orbital parameters of the binary system. If the path spacing decrease in the double star system, this can be due to mass transfer in the long term lead to a blue straggler.

Blue stragglers in globular clusters

The distribution of blue stragglers in globular clusters is characterized by a high concentration in the core, an adjoining zone of avoidance (English zone of avoidance ) and an elevated concentration in the outer region of the cluster. This is interpreted as a consequence of two different pathogenic mechanisms in the core and the outer region of the globular cluster or as a result of dynamic friction. Here is transmitted at a close encounter of a blue straggler with another star kinetic energy to the less massive star and, by definition, more massive blue stragglers loses kinetic energy. As a result, it sinks deep in the direction of the center of the cluster, and increases the concentration there. These dynamic effects make it difficult to determine the share of generation mechanisms. However, spectroscopic studies show up to 75 % of all blue stragglers in globular clusters a companion due to variations of the radial velocity. Therefore, mass transfer is likely to be in close binary systems in globular clusters, the dominant formation mechanism for blue stragglers.

If the blue stragglers of a globular cluster located in a color-magnitude diagram, then show two main sets: A red and a blue number. While in the blue series no variability is observed through light cover change is include about 50 percent of the members of the Red to the W Ursae Majoris stars. The blue sequence is interpreted as emerged from collisions blue stragglers, with two stars merge with nearly identical mass. The red number on the other hand is formed from the mass transfer in close binary stars, the high red component of the mass- donating companion is.

Red and Yellow stragglers

In addition to the blue stragglers and red or yellow stragglers are found in globular clusters and open clusters. These stars are in the Hertzspung -Russell diagram between the turn-off point at which the stars evolve from the main sequence towards the giant branch and the giant branch. Here, however, they are lighter than the normal subgiant of the star cluster. These stars could be former blue stragglers, giving a bright path of development to cover the giant branch due to their higher mass. Alternatively, it could also be involved unresolved binary star systems or overlays of constellations. The red stragglers are also called sub-sub- giant called because of their location in the HR diagram. These stars are often the brightest X-ray sources in the cluster with a luminosity of 1031 erg / s, which is interpreted as a sign of coronal activity. The sub-sub- giants are probably always be members of binary systems.