Horizontal branch

On the horizontal branch of the Hertzsprung-Russell diagram there are low-metal stars of medium mass in the state of stable helium burning and hydrogen shell burning after the first helium flash . The horizontal branch appears pronounced in the HR diagrams of globular clusters and depicts the stellar development phase after the development along the giant red branch . The term horizontal branch refers to the approximately horizontal alignment in the Hertzsprung-Russell diagram.

development

Color-brightness diagram of the globular cluster Messier 3 . The stars on the horizontal branch have an apparent magnitude of approximately and are to the left of . The RR-Lyrae gap in the horizontal branch between and is due to the fact that variable stars with their non-constant brightnesses and spectra are not shown ${\displaystyle V=16}$ ${\displaystyle B-V=0{,}7}$ ${\displaystyle B-V=0{,}1}$ ${\displaystyle B-V=0{,}4}$

After a star on the main sequence has consumed the hydrogen in its core during central hydrogen burning, the hydrogen burning zone moves outwards in the shape of a shell. The star reacts to this by expanding its atmosphere and lowering the surface temperature. In the Hertzsprung-Russell diagram, the star moves up the giant red branch. The core of the star consists predominantly of helium and, in the absence of thermonuclear reactions, can only stabilize through further contraction. If the star has a mass between 0.5 and 2.2 solar masses , the temperature and pressure rise so much that a helium flash occurs, an explosive ignition of the helium burning in the core. The subsequent stable burning of helium generates more energy and the star then contracts. The new equilibrium is a luminosity 100 times that of the sun. The temperature of the star on the horizontal branch depends on the mass loss during the red giant phase. When the helium in the core is also exhausted, the star moves back to the right in the HR diagram in the direction of the asymptotic giant branch .

RR Lyrae Stars

The RR Lyrae stars are at the point where the instability strip crosses the horizontal branch . 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 drawn in, which apparently creates an RR-Lyrae gap . Usually there is not enough data available to determine the mean value of the brightness and the spectrum, which is why these variables are not recorded. From the change in the periods of these variables, an attempt was made to observe the development along the horizontal branch. With a contraction of the diameter, the star should move to the right into the area of ​​higher temperatures and the pulsation period should be shortened. But the vibrations of these stars are superimposed with a kind of white noise, so that there is still no evidence of the speed of development of the 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 of up to 30,000 K. They are located roughly in the middle between the white dwarfs and the early main sequence stars in the HR diagram . Between the extreme horizontal branch stars and the normal blue stars on the horizontal branch there is a gap with a temperature difference of 10,000 K, within which no stars are observed. The cause is believed to be that the extreme horizontal branch stars have a 15 percent higher helium content. In addition, some of the stars of the extreme horizontal branch belong to the blue sub- dwarfs with the spectral types sdB and sdO. They mainly arise through a mass transfer in a binary star system while a star is in the phase of a red giant. This exposes the core of the star and appears as a bright blue star with an unusually small radius, a blue subdwarf.

The second parameter problem

The position of the horizontal branch in the form of the expansion to the left in the Hertzsprung-Russell diagram as well as the luminosity of the stars and the slope of the horizontal branch is essentially determined by the metallicity of the stars. In addition to the metal content of the stars, there is at least one other parameter that helps determine the position of the horizontal branch, since the horizontal branches of globular clusters and dwarf galaxies with identical metallicity differ from one another. There are numerous hypotheses as to which second parameter is responsible for this:

• the age of the globular cluster and thus the mass of the stars on the horizontal branch
• the mean rotation speed and the resulting intermingling of the stars
• the total mass of the globular cluster
• the concentration of the globular cluster
• the proportion of helium in the stars' atmosphere
• the density in the core of the stars
• the mass loss in the previous red giant phase due to stellar wind

Red lump

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

Double horizontal branches

In the case of some metal-rich blue globular clusters such as NGC 6440 and NGC 6569, photometric observations show that the horizontal branch in these star associations consists of two separate horizontal branches with slightly different brightness. Since the main rows are already split in globular clusters, the double horizontal branches could also be a result of two different populations and metallicities . According to this, one population provided the original material from the formation of the star clusters, while the second generation emerged from the ashes of the massive stars of the first generation and remaining gas residues a few million years later. Alternatively, the two populations could also have a slightly different helium content of a few percent or an age difference of around a billion years.

Individual evidence

1. ↑ H. Scheffler, H. Elsässer: Physics of the sun and the stars . Bibliographisches Institut, Mannheim 1990, ISBN 3-411-14172-7 .
2. ↑ Rudolph Kippenhahn, A. Weigert: Stellar Structure and Evolution (Astronomy and Astrophysics Library) . Springer Verlag, Berlin 1994, ISBN 0-387-58013-1 .
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4. ↑ Young-Wook Lee et al .: Super-Helium-rich Populations and the Origin of Extreme Horizontal-Branch Stars in Globular Clusters . In: The Astrophysical Journal . tape 621 , no. 1 , March 2005, p. L57 – L60 , doi : 10.1086 / 428944 , arxiv : astro-ph / 0501500 .
5. ^ Péter Németh, Adéla Kawka, Stéphane Vennes: A selection of hot subluminous stars in the GALEX survey II. Subdwarf atmospheric parameters . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1211.0323 .
6. ↑ C. Moni Bidin, S. Villanova, G. Piotto, S. Moehler, S. Cassisi, Y. Momany: Spectroscopy of horizontal branch stars in Omega Centauri . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.1262 .
7. ^ RG Gratton, S. Lucatello, A. Sollima, E. Carretta, A. Bragaglia, Y. Momany, V. D'Orazi, S. Cassisi, A. Pietrinferni, M. Salaris: The Na-O anticorrelation in horizontal branch stars. III. 47 Tuc and M 5 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.4069 .
8. ^ Zhen-xin Lei, Xue-Fei Chen, Feng-Hui Zhang, and Z. Han: Effects of tidally enhanced stellar wind on the horizontal branch morphology of globular clusters . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1212.3063 .
9. ^ Andrew Gould: A Second Kelvin-Helmholtz Timescale of Post Helium-Flash Evolution . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1111.6970 .
10. ↑ S. Bilir et al .: Luminosity-Color Relations for Red Clump Stars . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1210.5352 .
11. ↑ Francesco Mauroa et al .: Double Horizontal Branches in NGC6440 and NGC6569 unveiled by the VVV Survey . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1211.3437 .
12. ↑ Andrea Kunder et al .: THE HORIZONTAL BRANCH OF NGC1851: CONSTRAINTS FROM ITS RR LYRAE VARIABLES . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1211.3447 .