Tip of the red-giant branch

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Sun-like stars have a degenerate core on the red-giant branch and ascend to the tip before starting core helium fusion with a flash. Evolutionary track 1m.svg
Sun-like stars have a degenerate core on the red-giant branch and ascend to the tip before starting core helium fusion with a flash.

Tip of the red-giant branch (TRGB) is a primary distance indicator used in astronomy. It uses the luminosity of the brightest red-giant-branch stars in a galaxy as a standard candle to gauge the distance to that galaxy. It has been used in conjunction with observations from the Hubble Space Telescope to determine the relative motions of the Local Cluster of galaxies within the Local Supercluster. 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. [1]



Hertzsprung-Russell diagram for globular cluster M5. The red-giant branch runs from the thin horizontal subgiant branch to the top right, with a number of the more luminous RGB stars marked in red. M5 colour magnitude diagram.png
Hertzsprung–Russell diagram for globular cluster M5. The red-giant branch runs from the thin horizontal subgiant branch to the top right, with a number of the more luminous RGB stars marked in red.

The Hertzsprung–Russell diagram (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. 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. [2]

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. For a star with less than 1.8 times the mass of the Sun, this will occur in a process called the helium flash. The evolutionary track 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. [2] This discontinuity is called the tip of the red-giant branch.

When distant stars at the TRGB are measured in the I-band (in the infrared), their luminosity is somewhat insensitive to their composition of elements heavier than helium (metallicity) or their mass; they are a standard candle with an I-band absolute magnitude of –4.0±0.1. [3] This makes the technique especially useful as a distance indicator. The TRGB indicator uses stars in the old stellar populations (Population II). [4]

See also

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<span class="mw-page-title-main">Star</span> Astronomical object

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<span class="mw-page-title-main">Stellar evolution</span> Changes to a star over its lifespan

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<span class="mw-page-title-main">Supergiant</span> Type of star that is massive and luminous

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<span class="mw-page-title-main">Red supergiant</span> Stars with a supergiant luminosity class

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<span class="mw-page-title-main">Blue giant</span> Giant star of early spectral type

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<span class="mw-page-title-main">Helium flash</span> Brief thermal runaway nuclear fusion in the core of low mass stars

A helium flash is a very brief thermal runaway nuclear fusion of large quantities of helium into carbon through the triple-alpha process in the core of low mass stars during their red giant phase. A much rarer runaway helium fusion process can also occur on the surface of accreting white dwarf stars.

<span class="mw-page-title-main">Giant star</span> Type of star, larger and brighter than the Sun

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<span class="mw-page-title-main">Horizontal branch</span>

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<span class="mw-page-title-main">IK Pegasi</span> Variable star in the constellation Pegasus

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<span class="mw-page-title-main">Yellow supergiant</span>

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<span class="mw-page-title-main">Red giant</span> Type of large cool star that has exhausted its core hydrogen

A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around 5,000 K or lower. The appearance of the red giant is from yellow-white to reddish-orange, including the spectral types K and M, sometimes G, but also class S stars and most carbon stars.

<span class="mw-page-title-main">Hertzsprung–Russell diagram</span> Scatter plot of stars showing the relationship of luminosity to stellar classification

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 luminosities versus their stellar classifications or effective temperatures. The diagram was created independently in 1911 by Ejnar Hertzsprung and by Henry Norris Russell in 1913, and represented a major step towards an understanding of stellar evolution.

A stellar core is the extremely hot, dense region at the center of a star. For an ordinary main sequence star, the core region is the volume where the temperature and pressure conditions allow for energy production through thermonuclear fusion of hydrogen into helium. This energy in turn counterbalances the mass of the star pressing inward; a process that self-maintains the conditions in thermal and hydrostatic equilibrium. The minimum temperature required for stellar hydrogen fusion exceeds 107 K (10 MK), while the density at the core of the Sun is over 100 g/cm3. The core is surrounded by the stellar envelope, which transports energy from the core to the stellar atmosphere where it is radiated away into space.

<span class="mw-page-title-main">Blue loop</span> Stage of stellar evolution

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A super-AGB star is a star with a mass intermediate between those that end their lives as a white dwarf and those that end with a core collapse supernova, and properties intermediate between asymptotic giant branch (AGB) stars and red supergiants. They have initial masses of 7.5–9.25 M in stellar-evolutionary models, but have exhausted their core hydrogen and helium, left the main sequence, and expanded to become large, cool, and luminous.


  1. Müller, Oliver; Rejkuba, Marina; Jerjen, Helmut (2018). "Tip of the red giant branch distances to the dwarf galaxies dw1335-29 and dw1340-30 in the Centaurus group". Astronomy & Astrophysics. 615: A96. arXiv: 1803.02406 . Bibcode:2018A&A...615A..96M. doi:10.1051/0004-6361/201732455. S2CID   67754889.
  2. 1 2 Harpaz, Amos (1994). Stellar evolution. Peters Series. A K Peters, Ltd. pp. 103–110. ISBN   978-1-56881-012-6.
  3. Sakai, S (1999). Katsuhiko Sato (ed.). The Tip of the Red Giant Branch as a Population II Distance Indicator. Proceedings of the 183rd symposium of the International Astronomical Union. Dordrecht, Boston: Kluwer Academic. Bibcode:1999IAUS..183...48S.
  4. Ferrarese, Laura; Ford, Holland C.; Huchra, John; Kennicutt, Robert C., Jr.; Mould, Jeremy R.; Sakai, Shoko; et al. (2000). "A Database of Cepheid Distance Moduli and Tip of the Red Giant Branch, Globular Cluster Luminosity Function, Planetary Nebula Luminosity Function, and Surface Brightness Fluctuation Data Useful for Distance Determinations". The Astrophysical Journal Supplement Series (abstract). 128 (2): 431–459. arXiv: astro-ph/9910501 . Bibcode:2000ApJS..128..431F. doi:10.1086/313391. S2CID   121612286.{{cite journal}}: CS1 maint: multiple names: authors list (link)