Horizontal branch

On the horizontal branch of the Hertzsprung -Russell diagram is metal-poor Intermediate-mass stars in the state of stable helium burning and hydrogen shell burning are after the first helium flash. The horizontal branch appears pronounced in the HR diagrams of globular clusters and forms the stellar development phase of development along the Red giant branch from. The term horizontal branch refers to the approximately horizontal orientation in the Hertzsprung -Russell diagram.

Development

After a star on the main sequence at the central hydrogen burning has consumed the hydrogen in its core wanders the zone of the hydrogen burning shell-shaped outward. Then the star reacts with an expansion of its atmosphere and a lowering of the surface temperature. In the Hertzsprung -Russell diagram of the star moves up the red giant branch. The core of the star consists mostly of helium and can be stabilized only by the absence of thermonuclear reactions further contraction. If the star over a mass 0.5 to 2.2 solar masses rise as the temperature and pressure to the extent that it comes to a helium flash, an explosive ignition of helium burning in the core. The subsequent stable helium burning produces more energy and contracts as a result of the rating. The new equilibrium is at a luminosity of one hundred times the sun. The temperature of the star on the horizontal branch is dependent on the mass loss during the red giant phase. Although the helium is exhausted in the core of the star moves back to the right in the HR diagram in the direction of the asymptotic giant branch.

RR Lyrae stars

At the point at which the instability strip crosses the horizontal branch, are the RR Lyrae stars. These are radially pulsating variables with periods between 0.2 and 1.2 days. In the Hertzsprung -Russell diagram of a star system, these stars are often not shown, which appears to be a RR Lyrae gap. In most cases there are not enough data to determine the average value of the brightness and the spectrum, which is why these variables are not included. From the change of the periods of these variables has been attempted to develop along the horizontal branch observed. In a contraction of the diameter of the star should move to the right in the field of higher temperatures and thereby reduce the pulsation period. But the oscillations of these stars are superimposed with a kind of white noise, so there is still no evidence of the development rate of stars in this phase of development.

Extreme horizontal branch stars

The extreme horizontal branch stars are a small group of stars in some globular clusters with surface temperatures up to 30,000 K. They are located in the HR diagram roughly in the middle between the white dwarf and the early main-sequence stars. Between the extreme horizontal branch stars and the normal blue stars on the horizontal branch is a gap with a temperature difference of 10 000 K, within which no stars are observed. The cause is presumed that the extreme horizontal branch star having a higher content of 15 percent helium. In addition, include some of the stars of extreme horizontal branch to the blue sub- dwarfs with spectral types sdB and sdO. They arise principally through mass transfer in a binary system during a star in the phase of a Red Riesens is. Thus, the core of the star is exposed and appears as a luminous blue star with unusually small radius, a blue subdwarf.

The second - parameter problem

The location of the horizontal branch in the form of extension to the left in the Hertzsprung -Russell diagram and the luminosity of the star and the slope of the horizontal branch is mainly determined by the metallicity of the stars. In addition to the metal content of the star, there is at least one further parameter, affecting the position of the horizontal branch, since with identical metallicity vary the horizontal branches in globular clusters and dwarf galaxies. There are numerous hypotheses which second parameter that is responsible for:

• The age of the globular cluster, and thus the mass of the stars on the horizontal branch
• The mean speed of rotation and the consequent mixing of the star
• The total mass of the globular cluster
• The concentration of the globular cluster
• The proportion of helium in the atmosphere of the star
• The density in the core of the star
• The mass loss in the previous red giant phase by stellar wind

Red lumps

In metal-rich star clusters no horizontal branch can be observed. This is both a result of higher metal content and the low age of the population I stars on the red giant branch, which is why they currently have a higher mass than the Population II stars in the Milky Way. Therefore, the stars walk after the ignition of central helium burning up to a spectral type G8 III. In the Hertzsprung -Russell diagram, the resulting structure is a red lump (English red clump ) because the red giant branch in the HR diagram shows a small bulge. The absolute visual magnitude of the stars of the red clump is 1 for the spectral type K2 III and increases only slightly to 0.7 for the spectral type G8 III with a small dependence on the metallicity. Because of this relationship, the color brightness of the red star lumps are used as a distance indicator.

Double horizontal branches

In some metal-rich blue globular clusters like NGC 6440 and NGC 6569 show photometric observations indicate that the horizontal branch, with these stellar associations of two separate horizontal branches, which have a slightly different brightness. As already known split main sequence in globular clusters and the double horizontal branches may be a consequence of two different populations and metallicities. Thus, provided the a population because the original material from the formation of star clusters, while the second generation from the ashes of the massive stars of the first generation and still existing residual gas is evolved several million years later. Alternatively, the two populations may also have a slightly different helium content of a few percent or have an age difference of about a billion years.