Helium flash

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 (between 0.8 solar masses (M) and 2.0 M[1]) 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 accreting white dwarf stars.

Low mass stars do not produce enough gravitational pressure to initiate normal helium fusion. As the hydrogen in the core is exhausted, some of the helium left behind is instead compacted into degenerate matter, supported against gravitational collapse by quantum mechanical pressure rather than thermal pressure. This increases the density and temperature of the core until it reaches approximately 100 million kelvin, which is hot enough to cause helium fusion (or "helium burning") in the core.

However, a fundamental quality of degenerate matter is that changes in temperature do not produce a change of volume of the matter until the thermal pressure becomes so very high that it exceeds degeneracy pressure. In main sequence stars, thermal expansion regulates the core temperature, but in degenerate cores this does not occur. Helium fusion increases the temperature, which increases the fusion rate, which further increases the temperature in a runaway reaction. This produces a flash of very intense helium fusion that lasts only a few minutes, but briefly emits energy at a rate comparable to the entire Milky Way galaxy.

In the case of normal low mass stars, the vast energy release causes much of the core to come out of degeneracy, allowing it to thermally expand, however, consuming as much energy as the total energy released by the helium flash, and any left-over energy is absorbed into the star's upper layers. Thus the helium flash is mostly undetectable to observation, and is described solely by astrophysical models. After the core's expansion and cooling, the star's surface rapidly cools and contracts in as little as 10,000 years until it is roughly 2% of its former radius and luminosity. It is estimated that the electron-degenerate helium core weighs about 40% of the star mass and that 6% of the core is converted into carbon.[2]

Helium flash
Fusion of helium in the core of low mass stars.

Red giants

White Dwarf Resurrection
Sakurai's Object is a white dwarf undergoing a helium flash.[3]

During the red giant phase of stellar evolution in stars with less than 2.0 M the nuclear fusion of hydrogen ceases in the core as it is depleted, leaving a helium-rich core. While fusion of hydrogen continues in the star's shell causing a continuation of the accumulation of helium ash in the core, making the core denser, the temperature still is unable to reach the level required for helium fusion, as happens in more massive stars. Thus the thermal pressure from fusion is no longer sufficient to counter the gravitational collapse and create the hydrostatic equilibrium found in most stars. This causes the star to start contracting and increasing in temperature until it eventually becomes compressed enough for the helium core to become degenerate matter. This degeneracy pressure is finally sufficient to stop further collapse of the most central material but the rest of the core continues to contract and the temperature continues to rise until it reaches a point (≈1×108 K) at which the helium can ignite and start to fuse.[4][5][6]

The explosive nature of the helium flash arises from its taking place in degenerate matter. Once the temperature reaches 100 million–200 million kelvin and helium fusion begins using the triple-alpha process, the temperature rapidly increases, further raising the helium fusion rate and, because degenerate matter is a good conductor of heat, widening the reaction region.

However, since degeneracy pressure (which is purely a function of density) is dominating thermal pressure (proportional to the product of density and temperature), the total pressure is only weakly dependent on temperature. Thus, the dramatic increase in temperature only causes a slight increase in pressure, so there is no stabilizing cooling expansion of the core.

This runaway reaction quickly climbs to about 100 billion times the star's normal energy production (for a few seconds) until the temperature increases to the point that thermal pressure again becomes dominant, eliminating the degeneracy. The core can then expand and cool down and a stable burning of helium will continue.[7]

A star with mass greater than about 2.25 M starts to burn helium without its core becoming degenerate, and so does not exhibit this type of helium flash. In a very low-mass star (less than about 0.5 M), the core is never hot enough to ignite helium. The degenerate helium core will keep on contracting, and finally becomes a helium white dwarf.

The helium flash is not directly observable on the surface by electromagnetic radiation. The flash occurs in the core deep inside the star, and the net effect will be that all released energy is absorbed by the entire core, leaving the degenerate state to become nondegenerate. Earlier computations indicated that a nondisruptive mass loss would be possible in some cases,[8] but later star modeling taking neutrino energy loss into account indicates no such mass loss.[9][10]

In a one solar mass star, the helium flash is estimated to release about 5×1041 J,[11] or about 0.3% of the energy release of a 1.5×1044 J Type Ia supernova,[12] which is triggered by an analogous ignition of carbon fusion in a carbon–oxygen white dwarf.

Binary white dwarfs

When hydrogen gas is accreted onto a white dwarf from a binary companion star, the hydrogen can fuse to form helium for a narrow range of accretion rates, but most systems develop a layer of hydrogen over the degenerate white dwarf interior. This hydrogen can build up to form a shell near the surface of the star. When the mass of hydrogen becomes sufficiently large, runaway fusion causes a nova. In a few binary systems where the hydrogen fuses on the surface, the mass of helium built up can burn in an unstable helium flash. In certain binary systems the companion star may have lost most of its hydrogen and donate helium-rich material to the compact star. Note that similar flashes occur on neutron stars.

Shell helium flash

Shell helium flashes are a somewhat analogous but much less violent, nonrunaway helium ignition event, taking place in the absence of degenerate matter. They occur periodically in asymptotic giant branch stars in a shell outside the core. This is late in the life of a star in its giant phase. The star has burnt most of the helium available in the core, which is now composed of carbon and oxygen. Helium fusion continues in a thin shell around this core, but then turns off as helium becomes depleted. This allows hydrogen fusion to start in a layer above the helium layer. After enough additional helium accumulates, helium fusion is reignited, leading to a thermal pulse which eventually causes the star to expand and brighten temporarily (the pulse in luminosity is delayed because it takes a number of years for the energy from restarted helium fusion to reach the surface[13]). Such pulses may last a few hundred years, and are thought to occur periodically every 10,000 to 100,000 years.[13] After the flash, helium fusion continues at an exponentially decaying rate for about 40% of the cycle as the helium shell is consumed.[13] Thermal pulses may cause a star to shed circumstellar shells of gas and dust.

In Fiction

In the science-fiction novella The Wandering Earth (Chinese: 流浪地球) written in 2000 by Liu Cixin, the prediction of a helium flash is what drives the plot to escape the solar system. This plot element was not in the 2019 movie based on the movie based on the novella.

See also

References

  1. ^ Chapter 9: Post-main sequence evolution through helium burning
  2. ^ Taylor, David. "The End Of The Sun". North Western.
  3. ^ "White Dwarf Resurrection". Retrieved 3 August 2015.
  4. ^ Hansen, Carl J.; Kawaler, Steven D.; Trimble, Virginia (2004). Stellar Interiors - Physical Principles, Structure, and Evolution (2 ed.). Springer. pp. 62–5. ISBN 978-0387200897.
  5. ^ Seeds, Michael A.; Backman, Dana E. (2012). Foundations of Astronomy (12 ed.). Cengage Learning. pp. 249–51. ISBN 978-1133103769.
  6. ^ Karttunen, Hannu; Kröger, Pekka; Oja, Heikki; Poutanen, Markku; Donner, Karl Johan, eds. (2007-06-27). Fundamental Astronomy (5 ed.). Springer. p. 249. ISBN 978-3540341437.
  7. ^ Deupree, R. G.; R. K. Wallace (1987). "The core helium flash and surface abundance anomalies". Astrophysical Journal. 317: 724–732. Bibcode:1987ApJ...317..724D. doi:10.1086/165319.
  8. ^ Deupree, R. G. (1984). "Two- and three-dimensional numerical simulations of the core helium flash". The Astrophysical Journal. 282: 274. Bibcode:1984ApJ...282..274D. doi:10.1086/162200.
  9. ^ Deupree, R. G. (1996-11-01). "A Reexamination of the Core Helium Flash". The Astrophysical Journal. 471 (1): 377–384. Bibcode:1996ApJ...471..377D. CiteSeerX 10.1.1.31.44. doi:10.1086/177976.
  10. ^ Mocák, M (2009). Multidimensional hydrodynamic simulations of the core helium flash in low-mass stars (PhD. Thesis). Technische Universität München. Bibcode:2009PhDT.........2M.
  11. ^ Edwards, A. C. (1969). "The Hydrodynamics of the Helium Flash". Monthly Notices of the Royal Astronomical Society. 146 (4): 445–472. Bibcode:1969MNRAS.146..445E. doi:10.1093/mnras/146.4.445.
  12. ^ Khokhlov, A.; Müller, E.; Höflich, P. (1993). "Light curves of Type IA supernova models with different explosion mechanisms". Astronomy and Astrophysics. 270 (1–2): 223–248. Bibcode:1993A&A...270..223K.
  13. ^ a b c Wood, P. R.; D. M. Zarro (1981). "Helium-shell flashing in low-mass stars and period changes in mira variables". Astrophysical Journal. 247 (Part 1): 247. Bibcode:1981ApJ...247..247W. doi:10.1086/159032.
14 Canis Minoris

14 Canis Minoris, also known as HD 65345, is a single star in the equatorial constellation of Canis Minor. It is faintly visible to the naked eye with an apparent visual magnitude of +5.30. The distance to this star, as determined from an annual parallax shift of 13.50±0.34 mas, is approximately 242 light years. 14 CMI has a relatively large proper motion, traversing the celestial sphere at the rate of 0.188 arcsecond/year. It is moving further from the Sun with heliocentric radial velocity of +42.6 km/s.This is an evolved G-type giant star with a stellar classification of G8 IIIb. At the age of around 550 million years old, it is a red clump giant, which means it has already undergone helium flash and is generating energy through helium fusion at its core. The star has an estimated 2.5 times the mass of the Sun and has expanded to 8.7 times the Sun's radius. It is radiating roughly 48 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 5,070 K.

56 Andromedae

56 Andromedae, abbreviated 56 And, is a probable binary star system in the northern constellation of Andromeda. 56 Andromedae is the Flamsteed designation. It has a combined apparent visual magnitude of 5.69, which is just bright enough to be dimly visible to the naked eye under good seeing conditions. The distance to this system can be ascertained from its annual parallax shift, measured at 9.9 mas with the Gaia space observatory, which yields a separation of 330 light years. It is moving further from the Earth with a heliocentric radial velocity of +62 km/s and is traversing the celestial sphere at a relatively high rate of 0.183″ per year. This pair is positioned near the line of sight to the open cluster NGC 752, located 1,490 light-years away.The brighter primary is an aging giant star with a stellar classification of K0 III, having exhausted the hydrogen at its core and evolved off the main sequence. It is a red clump giant, having undergone "helium flash" and is presently generating energy at its core through helium fusion. The star is about 3.1 billion years old with a negligible observable rotation rate, so the rotation axis of the star is likely pointing towards us. It has 1.3 times the mass of the Sun and has expanded to 11 times the Sun's radius The star is radiating 56 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,765 K.The faint secondary component is a magnitude 11.93 star located at an angular separation of 18.50″ along a position angle (PA) of 77°, as of 2001. This has changed little since 1903 when it was at a separation of 18.4″ along a PA of 80°.

Beta Fornacis

Beta Fornacis (Beta For, β Fornacis, β For) is the Bayer designation for a solitary star in the southern constellation of Fornax. It is visible to the naked eye with an apparent visual magnitude of 4.46. Based upon an annual parallax shift of 18.89 mas, it is located around 173 light years away from the Sun. At that distance, the visual magnitude is reduced by an interstellar extinction factor of 0.1.This is an evolved, G-type giant star with a stellar classification of G8 III. It is a red clump giant, which means it has undergone helium flash and is currently generating energy through the fusion of helium at its core. The star has an estimated 1.53 times the mass of the Sun and has expanded to 11 times the Sun's radius. It is radiating over 55 times the solar luminosity from its outer atmosphere at an effective temperature of 4,820 K.Beta Fornacis has a visual companion, CCDM J02491-3224B, which has an apparent visual magnitude of approximately 14.0. As of 1928, it lay at an angular separation of 4.80 arc seconds along a position angle of 67°. Located around three degrees to the southwest is the globular cluster NGC 1049.

Bright giant

The luminosity class II in the Yerkes spectral classification is given to bright giants. These are stars which straddle the boundary between ordinary giants and supergiants, based on the appearance of their spectra.

Carbon detonation

Carbon detonation or Carbon deflagration is the violent reignition of thermonuclear fusion in a white dwarf star that was previously slowly cooling. It involves a runaway thermonuclear process which spreads through the white dwarf in a matter of seconds, producing a Type Ia supernova which releases an immense amount of energy as the star is blown apart. The carbon detonation/deflagration process leads to a supernova by a different route from the better known Type II (core-collapse) supernova (the type II is caused by the cataclysmic explosion of the outer layers of a massive star as its core implodes).A white dwarf is the remnant of a small to medium size star (our sun is an example of these). At the end of its life, the star has burned its hydrogen and helium fuel, and thermonuclear fusion processes cease. The star does not have enough mass to either burn much heavier elements, or to implode into a neutron star or type II supernova as a larger star can, from the force of its own gravity, so it gradually shrinks and becomes very dense as it cools, glowing white and then red, for a period many times longer than the present age of the Universe.

Occasionally, a white dwarf gains mass from another source – for example, a binary star companion that is close enough for the dwarf star to siphon sufficient amounts of matter onto itself; or a collision with other stars, the siphoned matter having been expelled during the process of the companion's own late stage stellar evolution. If the white dwarf gains enough matter, its internal pressure and temperature will rise enough for carbon to begin fusing in its core. Carbon detonation generally occurs at the point when the accreted matter pushes the white dwarf's mass close to the Chandrasekhar limit of roughly 1.4 solar masses. This is the mass at which gravity can overcome the electron degeneracy pressure which had prevented the star from collapsing during its lifetime. The same also happens when two white dwarfs merge and the mass of the body formed is below the Chandrasekhar limit; if two white dwarves merge and the result is over the limit, a Type Ia supernova will occur.

A main sequence star supported by thermal pressure would expand and cool which automatically counterbalances an increase in thermal energy. However, degeneracy pressure is independent of temperature; the white dwarf is unable to regulate the fusion process in the manner of normal stars, so it is vulnerable to a runaway fusion reaction.

In the case of a white dwarf, the restarted fusion reactions releases heat, but the outward pressure that exists in the star and supports it against further collapse is initially due almost entirely to degeneracy pressure, not fusion processes or heat. Therefore, even when fusion recommences the outward pressure that is key to the star's thermal balance does not increase much. One result is that the star does not expand much to balance its fusion and heat processes with gravity and electron pressure, as it did when burning hydrogen (until too late). This increase of heat production without a means of cooling by expansion raises the internal temperature dramatically, and therefore the rate of fusion also increases extremely fast as well, a form of positive feedback known as thermal runaway.

A 2004 analysis of such a process states that:

A deflagration flame burning from the center of the white dwarf star outward leaves hot and light burnt material behind. The fuel in front of it is, however, cold and dense. This results in a density stratification inverse to the gravitational field of the star, which is therefore unstable. Thus, blobs of burning material form and ascend into the fuel. At their interfaces shear flows emerge. These effects lead to strong swirls. The resulting turbulent motions deform the flame and thus enlarge its surface. This increases the net burning rate of the flame and leads to the energetic explosion.

The flame accelerates dramatically, in part due to the Rayleigh–Taylor instability and interactions with turbulence. The resumption of fusion spreads outward in a series of uneven, expanding "bubbles" in accordance with Rayleigh–Taylor instability. Within the fusion area, the increase in heat with unchanged volume results in an exponentially rapid increase in the rate of fusion – a sort of supercritical event as thermal pressure increases boundlessly. As hydrostatic equilibrium is not possible in this situation, a "thermonuclear flame" is triggered and an explosive eruption through the dwarf star's surface that completely disrupts it, seen as a Ia supernova.

Regardless of the exact details of this nuclear fusion, it is generally accepted that a substantial fraction of the carbon and oxygen in the white dwarf is converted into heavier elements within a period of only a few seconds, raising the internal temperature to billions of degrees. This energy release from thermonuclear fusion (1–2×1044 J) is more than enough to unbind the star; that is, the individual particles making up the white dwarf gain enough kinetic energy to fly apart from each other. The star explodes violently and releases a shock wave in which matter is typically ejected at speeds on the order of 5,000–20000 km/s, roughly 6% of the speed of light. The energy released in the explosion also causes an extreme increase in luminosity. The typical visual absolute magnitude of Type Ia supernovae is Mv = −19.3 (about 5 billion times brighter than the Sun), with little variation.

This process, of a volume supported by electron degeneracy pressure instead of thermal pressure gradually reaching conditions capable of igniting runaway fusion, is also found in a less dramatic form in a helium flash in the core of a sufficiently massive red giant star.

Helium-weak star

Helium-weak stars are chemically peculiar stars which have a weak helium lines for their spectral type. Their helium lines place them in a later (ie. cooler) spectral type then their hydrogen lines.

Horizontal branch

The 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. The onset of core helium fusion at the tip 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 the surface reaching higher temperatures.

LS IV-14 116

LS IV-14 116 is a hot subdwarf located approximately 2,000 light years away on the border between the constellations Capricornus and Aquarius. It has a surface temperature of approximately 34,000 ± 500 kelvins. Along with stars HE 2359-2844 and HE 1256-2738, LS IV-14 116 forms a new group of star called heavy metal subdwarfs. These are thought to be stars contracting to the extended horizontal branch after a helium flash and ejection of their atmospheres at the tip of the red giant branch.The star contains 10,000 times more zirconium than the Sun; it also has between 1,000 and 10,000 times the amount of strontium, germanium and yttrium than the Sun. The heavy metals are believed to be in cloud layers in the atmosphere where the ions of each metal have a particular opacity that allows radiational levitation to balance gravitational settling.

Lead star

A lead star is a low-metallicity star with an overabundance of lead and bismuth as compared to other products of the S-process.

Q star

A Q-Star, also known as a grey hole, is a hypothetical type of a compact, heavy neutron star with an exotic state of matter. The Q stands for a conserved particle number. A Q-Star may be mistaken for a stellar black hole.

Red-giant branch

The 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 stars much larger and more luminous than main-sequence stars of the same temperature.

Red clump

The red clump is a clustering of red giants in the Hertzsprung–Russell diagram at around 5,000 K and absolute magnitude (MV) +0.5, slightly hotter than most red-giant-branch stars of the same luminosity. It is visible as a more dense region of the red giant branch or a bulge towards hotter temperatures. It is most distinct in many, but not all, galactic open clusters, but it is also noticeable in many intermediate-age globular clusters and in nearby field stars (e.g. the Hipparcos stars).

The red clump giants are cool horizontal branch stars, stars originally similar to the Sun which have undergone a helium flash and are now fusing helium in their cores.

Red giant

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. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around 5,000 K (4,700 °C; 8,500 °F) or lower. The appearance of the red giant is from yellow-orange to red, including the spectral types K and M, but also class S stars and most carbon stars.

The most common red giants are stars on the red-giant branch (RGB) that are still fusing hydrogen into helium in a shell surrounding an inert helium core. Other red giants are the red-clump stars in the cool half of the horizontal branch, fusing helium into carbon in their cores via the triple-alpha process; and the asymptotic-giant-branch (AGB) stars with a helium burning shell outside a degenerate carbon–oxygen core, and a hydrogen burning shell just beyond that.

Sakurai's Object

Sakurai's Object (V4334 Sagittarii) is a star in the constellation of Sagittarius. It is thought to have previously been a white dwarf that, as a result of a very late thermal pulse, swelled and became a red giant. It is located at the center of a planetary nebula and is believed to currently be in thermal instability and within its final shell helium flash phase.

At the time of its discovery, astronomers believed Sakurai's Object to be a slow nova. Later spectroscopic analysis suggested that the star was not a nova, but had instead undergone a very late thermal pulse similar to that of V605 Aquilae, causing it to vastly expand. V605 Aquilae, which was discovered in 1919, is the only other star known to have been observed during the high luminosity phase of a very late thermal pulse, and models predict that Sakurai's Object, over the next few decades, will follow a similar life cycle.

Sakurai's Object and other similar stars are expected to end up as helium-rich white dwarfs after retracing their evolution track from the "born-again" giant phase back to the white dwarf cooling track. There are few other suspected "born-again" objects, one example being FG Sagittae. Having erupted in 1995, it is expected that Sakurai's Object's final helium flash will be the first well-observed one.

Stellar evolution

Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the age of the universe. The table shows the lifetimes of stars as a function of their masses. All stars are born from collapsing clouds of gas and dust, often called nebulae or molecular clouds. Over the course of millions of years, these protostars settle down into a state of equilibrium, becoming what is known as a main-sequence star.

Nuclear fusion powers a star for most of its life. Initially the energy is generated by the fusion of hydrogen atoms at the core of the main-sequence star. Later, as the preponderance of atoms at the core becomes helium, stars like the Sun begin to fuse hydrogen along a spherical shell surrounding the core. This process causes the star to gradually grow in size, passing through the subgiant stage until it reaches the red giant phase. Stars with at least half the mass of the Sun can also begin to generate energy through the fusion of helium at their core, whereas more-massive stars can fuse heavier elements along a series of concentric shells. Once a star like the Sun has exhausted its nuclear fuel, its core collapses into a dense white dwarf and the outer layers are expelled as a planetary nebula. Stars with around ten or more times the mass of the Sun can explode in a supernova as their inert iron cores collapse into an extremely dense neutron star or black hole. Although the universe is not old enough for any of the smallest red dwarfs to have reached the end of their lives, stellar models suggest they will slowly become brighter and hotter before running out of hydrogen fuel and becoming low-mass white dwarfs.Stellar evolution is not studied by observing the life of a single star, as most stellar changes occur too slowly to be detected, even over many centuries. Instead, astrophysicists come to understand how stars evolve by observing numerous stars at various points in their lifetime, and by simulating stellar structure using computer models.

Stellar mass loss

Stellar mass loss is a phenomenon observed in some massive stars. It occurs when a triggering event causes the ejection of a large portion of the star's mass. Stellar mass loss can also occur when a star gradually loses material to a binary companion or into interstellar space.

Thermal runaway

Thermal runaway occurs in situations where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result. It is a kind of uncontrolled positive feedback.

In other words, "thermal runaway" describes a process which is accelerated by increased temperature, in turn releasing energy that further increases temperature. In chemistry (and chemical engineering), it is associated with strongly exothermic reactions that are accelerated by temperature rise. In electrical engineering, thermal runaway is typically associated with increased current flow and power dissipation, although exothermic chemical reactions can be of concern here too. Thermal runaway can occur in civil engineering, notably when the heat released by large amounts of curing concrete is not controlled. In astrophysics, runaway nuclear fusion reactions in stars can lead to nova and several types of supernova explosions, and also occur as a less dramatic event in the normal evolution of solar mass stars, the "helium flash".

There are also concerns regarding global warming that a global average increase of 3–4 degrees Celsius above the preindustrial baseline could lead to a further unchecked increase in surface temperatures. For example, releases of methane, a greenhouse gas more potent than CO2, from wetlands, melting permafrost and continental margin seabed clathrate deposits could be subject to positive feedback.

Tip of the red-giant branch

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.

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.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. 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. This makes the technique especially useful as a distance indicator. The TRGB indicator uses stars in the old stellar populations (Population II).

Yellow giant

A yellow giant is a luminous giant star of low or intermediate mass (roughly 0.5–11 solar masses (M)) in a late phase of its stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature as low as 5,200-7500 K. The appearance of the yellow giant is from white to yellow, including the spectral types F and G. About 10.6 percent of all giant stars are yellow giants.

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