Horizontal branch
Encyclopedia
The horizontal branch is a stage of stellar evolution
which immediately follows the red giant branch in stars whose masses are similar to the Sun's. The helium core flash that occurs to stars at the top of the red giant branch causes substantial changes in stellar structure, resulting in an overall reduction in luminosity, some contraction of the stellar envelope, and surfaces reaching higher temperatures. Horizontal branch stars are powered by helium fusion
in the core (via the triple-alpha reaction) and by hydrogen fusion
in a shell surrounding the core.
Horizontal branches were discovered with the first deep photographic photometric
studies of globular clusters
and were notable for being absent from all open clusters that had been studied up to that time.
The horizontal branch is so named because in low-metallicity
samples like globular clusters, HB stars lie along a roughly horizontal line in a color-magnitude diagram (CMD).
, so many stars in this region pulsate. These pulsating horizontal branch stars are known as RR Lyrae variable
stars and they are obviously variable in brightness with periods of up to 1.2 days
.
It requires an extended observing program to establish the star's true (that is, averaged over a full period) apparent magnitude and color. Such a program is usually beyond the scope of an investigation of a cluster's color-magnitude diagram. Because of this, while the variable stars are noted in tables of a cluster's stellar content from such an investigation, these variable stars are not included in the graphic presentation of the cluster CMD because data adequate to plot them correctly are unavailable. This omission often results in the RR Lyrae gap seen in many published globular cluster CMDs.
Different globular clusters often display different HB morphologies, by which is meant that the relative proportions of HB stars existing blue of the RR Lyr gap, within the gap, and to the red of the gap varies sharply from cluster to cluster. The underlying cause of different HB morphologies is a long-standing problem in stellar astrophysics. Chemical composition is one factor (usually in the sense that more metal-poor clusters have bluer HBs), but other stellar properties like age, rotation and helium content have also been suggested as affecting HB morphology. This has sometimes been called the "Second Parameter Problem" for globular clusters, because there exist pairs of globular clusters which seem to have the same metallicity yet have very different HB morphologies; one such pair is NGC 288
(which has a very blue HB) and NGC 362
(which has a rather red HB). The label "second parameter" acknowledges that some unknown physical effect is responsible for HB morphology differences in clusters that seem otherwise identical.
, which are the relatively younger (and hence more massive) and usually more metal-rich population I counterparts to HB stars (which belong to population II). Both HB stars and clump giants are fusing helium to carbon in their cores, but differences in the structure of their outer layers result in the different types of stars having different radii, effective temperatures, and color
. Since color index is the horizontal coordinate in a color-magnitude diagram, the different types of star appear in different parts of the CMD despite their common energy source. In effect, the red clump represents one extreme of horizontal branch morphology: all the stars are at the red end of the horizontal branch, and may be difficult to distinguish from stars ascending the red giant branch for the first time.
Stellar evolution
Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...
which immediately follows the red giant branch in stars whose masses are similar to the Sun's. The helium core flash that occurs to stars at the top of the red giant branch causes substantial changes in stellar structure, resulting in an overall reduction in luminosity, some contraction of the stellar envelope, and surfaces reaching higher temperatures. Horizontal branch stars are powered by helium fusion
Helium fusion
Helium fusion is a kind of nuclear fusion, with the nuclei involved being helium.The fusion of helium-4 nuclei is known as the triple-alpha process, because fusion of just two helium nuclei only produces beryllium-8, which is unstable and breaks back down to two helium nuclei with a half-life of...
in the core (via the triple-alpha reaction) and by hydrogen fusion
CNO cycle
The CNO cycle is one of two sets of fusion reactions by which stars convert hydrogen to helium, the other being the proton–proton chain. Unlike the proton–proton chain reaction, the CNO cycle is a catalytic cycle. Theoretical models show that the CNO cycle is the dominant source of energy in stars...
in a shell surrounding the core.
Horizontal branches were discovered with the first deep photographic photometric
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...
studies of globular clusters
and were notable for being absent from all open clusters that had been studied up to that time.
The horizontal branch is so named because in low-metallicity
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
samples like globular clusters, HB stars lie along a roughly horizontal line in a color-magnitude diagram (CMD).
The RR Lyrae "gap" and horizontal branch morphology
Globular cluster CMDs generally show horizontal branches that have a prominent gap in the HB. This gap in the CMD incorrectly suggests that the cluster has no stars in this region of its CMD. The gap occurs at the instability stripInstability strip
The Instability strip is a nearly vertical region in the Hertzsprung–Russell diagram which is occupied by pulsating variable stars .The instability strip intersects the main sequence in the region of A...
, so many stars in this region pulsate. These pulsating horizontal branch stars are known as RR Lyrae variable
RR Lyrae variable
RR Lyrae variables are periodic variable stars, commonly found in globular clusters, and often used as standard candles to measure galactic distances.This type of variable is named after the prototype, the variable star RR Lyrae in the constellation Lyra....
stars and they are obviously variable in brightness with periods of up to 1.2 days
.
It requires an extended observing program to establish the star's true (that is, averaged over a full period) apparent magnitude and color. Such a program is usually beyond the scope of an investigation of a cluster's color-magnitude diagram. Because of this, while the variable stars are noted in tables of a cluster's stellar content from such an investigation, these variable stars are not included in the graphic presentation of the cluster CMD because data adequate to plot them correctly are unavailable. This omission often results in the RR Lyrae gap seen in many published globular cluster CMDs.
Different globular clusters often display different HB morphologies, by which is meant that the relative proportions of HB stars existing blue of the RR Lyr gap, within the gap, and to the red of the gap varies sharply from cluster to cluster. The underlying cause of different HB morphologies is a long-standing problem in stellar astrophysics. Chemical composition is one factor (usually in the sense that more metal-poor clusters have bluer HBs), but other stellar properties like age, rotation and helium content have also been suggested as affecting HB morphology. This has sometimes been called the "Second Parameter Problem" for globular clusters, because there exist pairs of globular clusters which seem to have the same metallicity yet have very different HB morphologies; one such pair is NGC 288
NGC 288
NGC 288 is a globular cluster in the constellation Sculptor. Its visual appearance was described by John Dreyer in 1888. It is located about 1.8° southeast of the galaxy NGC 253, 37′ north-northeast of the South Galactic Pole, 15′ south-southeast of a 9th magnitude star, and encompassed by a...
(which has a very blue HB) and NGC 362
NGC 362
NGC 362 is a globular cluster located in the constellation Tucana in the Southern Hemisphere. It was discovered on August 1, 1826 by James Dunlop....
(which has a rather red HB). The label "second parameter" acknowledges that some unknown physical effect is responsible for HB morphology differences in clusters that seem otherwise identical.
Relationship to the red clump
A related class of stars is the clump giants, those belonging to the so-called red clumpRed clump
The red clump is a feature in the Hertzsprung-Russell diagram of stars. The red clump is considered the metal-rich counterpart to the horizontal branch. Stars in this part of the Hertzsprung-Russell diagram are sometimes called clump giants...
, which are the relatively younger (and hence more massive) and usually more metal-rich population I counterparts to HB stars (which belong to population II). Both HB stars and clump giants are fusing helium to carbon in their cores, but differences in the structure of their outer layers result in the different types of stars having different radii, effective temperatures, and color
Color index
In astronomy, the color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature...
. Since color index is the horizontal coordinate in a color-magnitude diagram, the different types of star appear in different parts of the CMD despite their common energy source. In effect, the red clump represents one extreme of horizontal branch morphology: all the stars are at the red end of the horizontal branch, and may be difficult to distinguish from stars ascending the red giant branch for the first time.