Horizontal branchThe horizontal branch (HB) is a stage of stellar evolution that immediately follows the red-giant branch in stars whose masses are similar to the Sun's. Horizontal-branch stars are powered by helium fusion in the core (via the triple-alpha process) and by hydrogen fusion (via the CNO cycle) in a shell surrounding the core.Red-giant branchThe red-giant branch (RGB), sometimes called the first giant branch, is the portion of the giant branch before helium ignition occurs in the course of stellar evolution. It is a stage that follows the main sequence for low- to intermediate-mass stars. Red-giant-branch stars have an inert helium core surrounded by a shell of hydrogen fusing via the CNO cycle. They are K- and M-class but much larger and more luminous than main-sequence stars of the same temperature.
Effective temperatureThe effective temperature (akaET) of a body such as a star or planet is the temperature of a black body that would emit the same total energy as electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature when the body's emissivity curve (as a function of wavelength) is not known.
MetallicityIn astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal currently detectable (i.e. non-dark) matter in the universe is either hydrogen or helium, and astronomers use the word metals as convenient shorthand for all elements except hydrogen and helium. This word-use is distinct from the conventional chemical or physical definition of a metal as an electrically conducting element.
Hertzsprung–Russell diagramA Hertzsprung–Russell diagram (abbreviated as H–R diagram, HR diagram or HRD) is a scatter plot of stars showing the relationship between the stars' absolute magnitudes or luminosities and their stellar classifications or effective temperatures. It is also sometimes called a color magnitude diagram.
Absolute magnitudeMeasure of the luminosity of celestial objectsIn astronomy, absolute magnitude (M) is a measure of the luminosity of a celestial object on an inverse logarithmicastronomical magnitude scale; the more luminous (intrinsically bright) an object, the lower its magnitude number.
Helium flashA helium flash is a very brief thermal runawaynuclear fusion of large quantities of helium into carbon through the triple-alpha process in the core of low-mass stars (between 0.8 solar masses (M☉) and 2.0 M☉) during their red giant phase. The Sun is predicted to experience a flash 1.2 billion years after it leaves the main sequence. A much rarer runaway helium fusion process can also occur on the surface of accretingwhite dwarf stars.
Red giantHertzsprung–Russell diagramSpectral typeOBAFGKMLTBrown dwarfsWhite dwarfsRed dwarfsSubdwarfsMain sequence("dwarfs")SubgiantsGiantsRed giantsBlue giantsBright giantsSupergiantsRed supergiantHypergiantsabsolutemagni-tude(MV)A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses (M☉)) in a late phase of stellar evolution.
Open clusterAn open cluster is a type of star cluster made of tens to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, and many more are thought to exist. Each one is loosely bound by mutual gravitational attraction and becomes disrupted by close encounters with other clusters and clouds of gas as they orbit the Galactic Center.
Globular clusterA globular cluster is a spheroidal conglomeration of stars that is bound together by gravity, with a higher concentration of stars towards its center. It can contain anywhere from tens of thousands to many millions of member stars, all orbiting in a stable, compact formation. This contrasts with open clusters, where the stars are only loosely bound and more easily disrupted.
HipparcosHipparcos was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions and distances of celestial objects on the sky. This was the first practical attempt at all-sky absolute parallax measurement, something not possible with groundside observatories, and thus represented a fundamental breakthrough in astronomy.
Triple-alpha processThe triple-alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei (alpha particles) are transformed into carbon. In starsHelium accumulates in the cores of stars as a result of the proton–proton chain reaction and the carbon–nitrogen–oxygen cycle.*As an Amazon Associate I earn from qualifying purchases.