Tip of the red-giant branch (TRGB) is a primary distance indicator used in

astronomy Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...

. It uses the

luminosity Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a st ...

of the brightest red-giant-branch stars in a

galaxy A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

as a

standard candle The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A ''direct'' distance measurement of an astronomical object is possible o ...

to gauge the distance to that galaxy. It has been used in conjunction with observations from the

Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...

to determine the relative motions of the

Local Cluster The Local Group is the galaxy group that includes the Milky Way. It has a total diameter of roughly , and a total mass of the order of . It consists of two collections of galaxies in a "dumbbell" shape: the Milky Way and its satellites form ...

of galaxies within the

Local Supercluster The Virgo Supercluster (Virgo SC) or the Local Supercluster (LSC or LS) is a mass concentration of galaxies containing the Virgo Cluster and Local Group, which itself contains the Milky Way and Andromeda galaxies, as well as others. At leas ...

. Ground-based, 8-meter-class telescopes like the VLT are also able to measure the TRGB distance within reasonable observation times in the local universe.

Method

The

Hertzsprung–Russell diagram The 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 luminosity, luminosities versus their stellar classifications or eff ...

(HR diagram) is a plot of stellar luminosity versus surface temperature for a population of stars. During the core hydrogen burning phase of a Sun-like star's lifetime, it will appear on the HR diagram at a position along a diagonal band called the

main sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...

. When the hydrogen at the core is exhausted, energy will continue to be generated by hydrogen fusion in a shell around the core. The center of the star will accumulate the helium "ash" from this fusion and the star will migrate along an evolutionary branch of the HR diagram that leads toward the upper right. That is, the surface temperature will decrease and the total energy output (luminosity) of the star will increase as the surface area increases. At a certain point, the helium at the core of the star will reach a pressure and temperature where it can begin to undergo nuclear fusion through the

triple-alpha process The triple-alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei (alpha particles) are transformed into carbon. Triple-alpha process in stars Helium accumulates in the cores of stars as a result of the proton–pro ...

. For a star with less than 1.8 times the

mass of the Sun The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...

, this will occur in a process called the helium flash. The

evolutionary track In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar He ...

of the star will then carry it toward the left of the HR diagram as the surface temperature increases under the new equilibrium. The result is a sharp discontinuity in the evolutionary track of the star on the HR diagram. This discontinuity is called the tip of the red-giant branch. When distant stars at the TRGB are measured in the

I-band The NATO I band is the obsolete designation given to the radio frequencies from 8 000 to 10 000 MHz (equivalent to wavelengths between 3.75 and 3 cm) during the Cold War period. Since 1992 frequency allocations, allotment and assignments are ...

(in the infrared), their luminosity is somewhat insensitive to their composition of elements heavier than helium (

metallicity In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as a ...

) or their mass; they are a

with an I-band

absolute magnitude Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse Logarithmic scale, logarithmic Magnitude (astronomy), astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent mag ...

of –4.0±0.1. This makes the technique especially useful as a distance indicator. The TRGB indicator uses stars in the old stellar populations (

Population II During 1944, Walter Baade categorized groups of stars within the Milky Way into stellar populations. In the abstract of the article by Baade, he recognizes that Jan Oort originally conceived this type of classification in 1926: Baade noticed ...

References

External links

* * {{cite web , accessdate=February 14, 2005 , url=http://spaceref.com/news/viewsr.html?pid=10116 , title=NASA Hubble Space Telescope Daily Report # 3431 , work=SpaceRef, date=21 August 2003 Large-scale structure of the cosmos Physical cosmology Red giants Standard candles

Method

See also

References

External links