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.

M5 colour magnitude diagram
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.

Discovery

NGC 288 HST
The brightest stars in globular clusters such as NGC 288 are red giants

Red giants were identified early in the 20th century when the use of the Hertzsprung–Russell diagram made it clear that there were two distinct types of cool stars with very different sizes: dwarfs, now formally known as the main sequence; and giants.[1][2]

The term red-giant branch came into use during the 1940s and 1950s, although initially just as a general term to refer to the red-giant region of the Hertzsprung–Russell diagram. Although the basis of a thermonuclear main-sequence lifetime, followed by a thermodynamic contraction phase to a white dwarf was understood by 1940, the internal details of the various types of giant stars were not known.[3]

In 1968, the name asymptotic giant branch (AGB) was used for a branch of stars somewhat more luminous than the bulk of red giants and more unstable, often large-amplitude variable stars such as Mira.[4] Observations of a bifurcated giant branch had been made years earlier but it was unclear how the different sequences were related.[5] By 1970, the red-giant region was well understood as being made up from subgiants, the RGB itself, the horizontal branch, and the AGB, and the evolutionary state of the stars in these regions was broadly understood.[6] The red-giant branch was described as the first giant branch in 1967, to distinguish it from the second or asymptotic giant branch,[7] and this terminology is still frequently used today.[8]

Modern stellar physics has modelled the internal processes that produce the different phases of the post-main-sequence life of moderate-mass stars,[9] with ever-increasingly complexity and precision.[10] The results of RGB research are themselves being used as the basis for research in other areas.[11]

Evolution

Zams and tracks
Evolutionary tracks for stars of different masses:
• the 0.6 M track shows the RGB and stops at the helium flash.
• the 1 M track shows a short but long-lasting subgiant branch and the RGB to the helium flash.
• the 2 M track shows the subgiant branch and RGB, with a barely detectable blue loop onto the AGB.
• the 5 M track shows a long but very brief subgiant branch, a short RGB, and an extended blue loop.

When a star with a mass from about 0.4 M (solar mass) to 12 M (8 M for low-metallicity stars) exhausts its core hydrogen, it enters a phase of hydrogen shell burning during which it becomes a red giant, larger and cooler than on the main sequence. During hydrogen shell burning, the interior of the star goes through several distinct stages which are reflected in the outward appearance. The evolutionary stages vary depending primarily on the mass of the star, but also on its metallicity.

Subgiant phase

After a main-sequence star has exhausted its core hydrogen, it begins to fuse hydrogen in a thick shell around a core consisting largely of helium. The mass of the helium core is below the Schönberg–Chandrasekhar limit and is in thermal equilibrium, and the star is a subgiant. Any additional energy production from the shell fusion is consumed in inflating the envelope and the star cools but does not increase in luminosity.[12]

Shell hydrogen fusion continues in stars of roughly solar mass until the helium core increases in mass sufficiently that it becomes degenerate. The core then shrinks, heats up, and develops a strong temperature gradient. The hydrogen shell, fusing via the temperature-sensitive CNO cycle, greatly increases its rate of energy production and the stars is considered to be at the foot of the red-giant branch. For a star the same mass as the sun, this takes approximately 2 billion years from the time that hydrogen was exhausted in the core.[13]

Subgiants more than about 2 M reach the Schönberg–Chandrasekhar limit relatively quickly before the core becomes degenerate. The core still supports its own weight thermodynamically with the help of energy from the hydrogen shell, but is no longer in thermal equilibrium. It shrinks and heats causing the hydrogen shell to become thinner and the stellar envelope to inflate. This combination decreases luminosity as the star cools towards the foot of the RGB. Before the core becomes degenerate, the outer hydrogen envelope becomes opaque which causes the star to stop cooling, increases the rate of fusion in the shell, and the star has entered the RGB. In these stars, the subgiant phase occurs within a few million years, causing an apparent gap in the Hertzsprung–Russell diagram between B-type main-sequence stars and the RGB seen in young open clusters such as Praesepe. This is the Hertzsprung gap and is actually sparsely populated with subgiant stars rapidly evolving towards red giants, in contrast to the short densely populated low-mass subgiant branch seen in older clusters such as ω Centauri.[14][15]

Ascending the red-giant branch

Evolutionary track 1m
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 5m
Stars more massive than the sun do not have a degenerate core and leave the red giant branch before the tip when their core helium ignites without a flash.

Stars at the foot of the red-giant branch all have a similar temperature around 5,000 K, corresponding to an early to mid K spectral type. Their luminosities range from a few times the luminosity of the sun for the least massive red giants to several thousand times as luminous for stars around 8 M.[16]

As their hydrogen shells continue to produce more helium, the cores of RGB stars increase in mass and temperature. This causes the hydrogen shell to fuse more rapidly. Stars become more luminous, larger, and somewhat cooler. They are described as ascending the RGB.[17]

On the ascent of the RGB, there are a number of internal events that produce observable external features. The outer convective envelope becomes deeper and deeper as the star grows and shell energy production increases. Eventually it reaches deep enough to bring fusion products to the surface from the formerly convective core, known as the first dredge-up. This changes the surface abundance of helium, carbon, nitrogen, and oxygen.[18] A noticeable clustering of stars at one point on the RGB can be detected and is known as the RGB bump. It is caused by a discontinuity in hydrogen abundance left behind by the deep convection. Shell energy production temporarily decreases at this discontinuity, effective stalling the ascent of the RGB and causing an excess of stars at that point.[19]

Tip of the red-giant branch

For stars with a degenerate helium core, there is a limit to this growth in size and luminosity, known as the tip of the red-giant branch, where the core reaches sufficient temperature to begin fusion. All stars that reach this point have an identical helium core mass of almost 0.5 M, and very similar stellar luminosity and temperature. These luminous stars have been used as standard candle distance indicators. Visually, the tip of the red giant branch occurs at about absolute magnitude −3 and temperatures around 3,000 K at solar metallicity, closer to 4,000 K at very low metallicity.[16][20] Models predict a luminosity at the tip of 2.0–2.5 L thousand, depending on metallicity.[21] In modern research, infrared magnitudes are more commonly used.[22]

Leaving the red-giant branch

A degenerate core begins fusion explosively in an event known as the helium flash, but externally there is little immediate sign of it. The energy is consumed in lifting the degeneracy in the core. The star overall becomes less luminous and hotter and migrates to the horizontal branch. All degenerate helium cores have approximately the same mass, regardless of the total stellar mass, so the helium fusion luminosity on the horizontal branch is the same. Hydrogen shell fusion can cause the total stellar luminosity to vary, but for most stars at near solar metallicity, the temperature and luminosity are very similar at the cool end of the horizontal branch. These stars form the red clump at about 5,000 K and 50 L. Less massive hydrogen envelopes cause the stars to take up a hotter and less luminous position on the horizontal branch, and this effect occurs more readily at low metallicity so that old metal-poor clusters show the most pronounced horizontal branches.[13][23]

Stars initially more massive than 2 M have non-degenerate helium cores on the red-giant branch. These stars become hot enough to start triple-alpha fusion before they reach the tip of the red-giant branch and before the core becomes degenerate. They then leave the red-giant branch and perform a blue loop before returning to join the asymptotic giant branch. Stars only a little more massive than 2 M perform a barely noticeable blue loop at a few hundred L before continuing on the AGB hardly distinguishable from their red-giant branch position. More massive stars perform extended blue loops which can reach 10,000 K or more at luminosities of thousands of L. These stars will cross the instability strip more than once and pulsate as Type I (Classical) Cepheid variables.[24]

Properties

The table below shows the typical lifetimes on the main sequence (MS), subgiant branch (SB), and red-giant branch (RGB), for stars with different initial masses, all at solar metallicity (Z = 0.02). Also shown are the helium core mass, surface effective temperature, radius, and luminosity at the start and end of the RGB for each star. The end of the red-giant branch is defined to be when core helium ignition takes place.[8]

Mass
(M)
MS (GYrs) SB (MYrs) RGB
(MYrs)
RGBfoot
RGBend
Core mass (M) Teff (K) Radius (R) Luminosity (L) Core mass (M) Teff (K) Radius (R) Luminosity (L)
0.6 58.8 5,100 2,500 0.10 4,634 1.2 0.6 0.48 2,925 207 2,809
1.0 9.3 2,600 760 0.13 5,034 2.0 2.2 0.48 3,140 179 2,802
2.0 1.2 10 25 0.25 5,220 5.4 19.6 0.34 4,417 23.5 188
5.0 0.1 0.4 0.3 0.83 4,737 43.8 866.0 0.84 4,034 115 3,118

Intermediate-mass stars only lose a small fraction of their mass as main-sequence and subgiant stars, but lose a significant amount of mass as red giants.[25]

The mass lost by a star similar to the Sun affects the temperature and luminosity of the star when it reaches the horizontal branch, so the properties of red-clump stars can be used to determine the mass difference before and after the helium flash. Mass lost from red giants also determines the mass and properties of the white dwarfs that form subsequently. Estimates of total mass loss for stars that reach the tip of the red-giant branch are around 0.2–0.25 M. Most of this is lost within the final million years before the helium flash.[26][27]

Mass lost by more-massive stars that leave the red-giant branch before the helium flash is more difficult to measure directly. The current mass of Cepheid variables such as δ Cephei can be measured accurately because there are either binaries or pulsating stars. When compared with evolutionary models, such stars appear to have lost around 20% of their mass, much of it during the blue loop and especially during pulsations on the instability strip.[28][29]

Variability

Some red giants are large amplitude variables. Many of the earliest known variable stars are Mira variables with regular periods and amplitudes of several magnitudes, semiregular variables with less obvious periods or multiple periods and slightly lower amplitudes, and slow irregular variables with no obvious period. These have long been considered to be asymptotic giant branch (AGB) stars or supergiants and the red giant branch (RGB) stars themselves were not generally considered to be variable. A few apparent exceptions were considered to be low luminosity AGB stars.[30]

Studies in the late 20th century began to show that all giants of class M were variable with amplitudes of 10 milli-magnitudes of more, and that late K class giants were also likely to be variable with smaller amplitudes. Such variable stars were amongst the more luminous red giants, close to the tip of the RGB, but it was difficult to argue that they were all actually AGB stars. The stars showed a period amplitude relationship with larger amplitude variables pulsating more slowly.[31]

Microlensing surveys in the 21st century have provided extremely accurate photometry of thousands of stars over may years. This has allowed for the discovery of many new variable stars, often of very small amplitudes. Multiple period-luminosity relationships have been discovered, grouped into regions with ridges of closely spaced parallel relationships. Some of these correspond to the known Miras and semi-regulars, but an additional class of variable star has been defined: OGLE Small Amplitude Red Giants or OSARGs. OSARGs have amplitudes of a few thousandths of a magnitude and semi-regular periods of 10 – 100 days. The OGLE survey published up to three periods for each OSARG, indicating a complex combination of pulsations. Many thousands of OSARGs were quickly detected in the Magellanic Clouds, both AGB and RGB stars.[32] A catalog has since been published of 192,643 OSARGs in the direction of the Milky Way central bulge. Although around a quarter of Magellanic Cloud OSARgs show long secondary periods, very few of the galactic OSARGs do.[33]

The RGB OSARGs follow three closely spaced period-luminosity relations, corresponding to the first, second, and third overtones of radial pulsation models for stars of certain masses and luminosities, but that dipole and quadrupole non-radial pulsations are also present leading to the semi-regular nature of the variations.[34] The fundamental mode does not appear, and the underlying cause of the excitation is not known. Stochastic convection has been suggested as a cause, similar to solar-like oscillations.[32]

Two additional types of variation have been discovered in RGB stars: long secondary periods, which are associated with other variations but can show larger amplitudes with periods of hundreds or thousands of days; and ellipsoidal variations. The cause of the long secondary periods is unknown, but it has been proposed that they are due to interactions with low mass companions in close orbits.[35] The ellipsoidal variations are also thought to be created in binary systems, in this case contact binaries where distorted stars cause strictly periodic variations as they orbit.[36]

References

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Bibliography

External links

16 Aquarii

16 Aquarii, abbreviated 16 Aqr, is a star in the constellation of Aquarius. 16 Aquarii is the Flamsteed designation. It is a faint star, just visible to the naked eye, with an apparent visual magnitude of 5.869. Based upon an annual parallax shift of 9.5 mas, it is located about 342 light years away. It is moving closer to the Earth with a heliocentric radial velocity of −6 km/s, and is predicted to come within 220 light-years in 6.8 million years.At the estimated age of 740 million years, this is an aging giant star currently on the red giant branch with a stellar classification of G7 III. This indicates it has exhausted the supply of hydrogen at its core and is generating energy via hydrogen fusion along a shell surrounding a hot core of inert helium. The star has 2.3 times the mass of the Sun and has expanded to 8 times the Sun's radius. It is radiating 37 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 5,096 K.

47 Aquarii

47 Aquarii, abbreviated 47 Aqr, is a star in the zodiac constellation of Aquarius. 47 Aquarii is its Flamsteed designation. It is a faint star but visible to the naked eye in good seeing conditions, having an apparent visual magnitude of 5.135. Based upon an annual parallax shift of 18.0 mas, it is located 181 light years away. At that distance, the visual magnitude of the star is diminished by an extinction of 0.088 due to interstellar dust. It is moving further from the Earth with a heliocentric radial velocity of +48 km/s.This is an evolved giant star currently on the red giant branch with a stellar classification of K0 III. The star has 1.35 times the mass of the Sun and has expanded to 7.86 times the Sun's radius. It is radiating 30 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,750 K.

50 Aquarii

50 Aquarii, abbreviated 50 Aqr, is a single star in the zodiac constellation of Aquarius. 50 Aquarii is its Flamsteed designation. It is a faint star with an apparent visual magnitude of 5.76 that is barely visible to the naked eye under good seeing conditions. The star is located near the ecliptic and thus is subject to lunar occultations. Based upon an annual parallax shift of 12.2 mas as seen from Earth orbit, it is located 266 light years away. It is moving closer to the Earth with a heliocentric radial velocity of −21 km/s.This is an evolved giant star with a stellar classification of G7.5 III that is most likely (87% chance) on the red giant branch. As such, it is estimated to be 620 million years old with 2.5 times the mass of the Sun and has expanded to 14 times the Sun's radius. The star is radiating 104 times the Sun's luminosity from its expanded photosphere at an effective temperature of 4,897 K.

52 Cygni

52 Cygni is a giant star in the northern constellation of Cygnus with an apparent magnitude of 4.22. Based on its Hipparcos parallax, it is about 291 light-years (89 pc) away.

52 Cygni is a probable horizontal branch (red clump) star, fusing helium in its core, although there is a 25% chance that it is still on the red giant branch (RGB) and fusing hydrogen in a shell around an insert core. As a clump giant it would be 2.27 gyr old, but only 910 myr if it is an RGB star. It shines with a bolometric luminosity of about 90 L☉ at an effective temperature of 4,677 K. It has a radius of about 14 R☉.At an angular separation of 6.0″ from 52 Cygni is a faint magnitude 9.5 companion.

64 Aquilae

64 Aquilae, abbreviated 64 Aql, is a star in the equatorial constellation of Aquila. 64 Aquilae is its Flamsteed designation. It is a faint star that requires good viewing conditions to see, having an apparent visual magnitude of 5.97. The distance to 64 Aql, as determined from its annual parallax shift of 21.42 mas, is 152.2 light years. At that distance, the visual magnitude of the star is diminished by an extinction of 0.029 due to interstellar dust. It is moving closer to the Earth with a heliocentric radial velocity of −3.6 km/s.This is an evolved giant star currently on the red giant branch with a stellar classification of K1 III/IV. The luminosity class of 'III/IV' indicates the spectrum shows a blend of features matching a subgiant and giant star. It is around 6.2 billion years old with 1.17 times the mass of the Sun and has expanded to 4.5 times the Sun's radius. The star is radiating 11 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,786 K.

64 Arietis

64 Arietis is a possible binary star system in the northern constellation of Aries. 64 Arietis is the Flamsteed designation. It is faintly visible to the naked eye as a dim, orange-hued star with an apparent visual magnitude of +5.67. Based upon an annual parallax shift of 15.2 mas, this star is approximately 214 light-years (66 parsecs) distant from the Sun. It is receding from the Earth with a heliocentric radial velocity of +8.5 km/s.The visible component is an aging giant star with a stellar classification of K4 III, currently on the red giant branch. It is around 5.2 billion years old with 1.27 times the mass of the Sun. With the supply of hydrogen at its core exhausted, the star has expanded to 11 times the radius of the Sun and it shines with 42 times the Sun's luminosity. This energy is being radiated from the outer envelope at an effective temperature of 4,426 K, giving it the orange-hued glow of a K-type star.

Aquarius Dwarf

The Aquarius Dwarf is a dwarf irregular galaxy, first catalogued in 1959 by the DDO survey. It is located within the boundaries of the constellation of Aquarius. It is a member of the Local Group of galaxies, albeit an extremely isolated one; it is one of only a few known Local Group members for which a past close approach to the Milky Way or Andromeda Galaxy can be ruled out, based on its current location and velocity.

Local Group membership was firmly established only in 1999, with the derivation of a distance based on the tip of the red-giant branch method. Its distance from the Milky Way of 3.2 ±0.2 Mly (980 ±40 kpc) means that Aquarius Dwarf is quite isolated in space. It is one of the least luminous Local Group galaxies to contain significant amounts of neutral hydrogen and support to ongoing star formation, although it does so only at an extremely low level.

Because of its large distance, the Hubble Space Telescope is required in order to study its stellar populations in detail. RR Lyrae stars have been discovered in Aquarius Dwarf, indicating the existence of stars more than 10 billion years old, but the majority of its stars are much younger (median age 6.8 billion years). Among Local Group galaxies, only Leo A has a younger mean age, leading to the suggestion that delayed star formation could be correlated with galaxy isolation.

Blue loop

In the field of stellar evolution, a blue loop is a stage in the life of an evolved star where it changes from a cool star to a hotter one before cooling again. The name derives from the shape of the evolutionary track on a Hertzsprung–Russell diagram which forms a loop towards the blue (i.e. hotter) side of the diagram.

Blue loops can occur for red supergiants red giant branch stars, or asymptotic giant branch stars. Some stars may undergo more than one blue loop. Many pulsating variable stars such as Cepheids are blue loop stars. Stars on the horizontal branch are not generally referred to as on a blue loop even though they are temporarily hotter than on the red giant or asymptotic giant branches. Loops occur far too slowly to be observed for individual stars, but are inferred from theory and from the properties and distribution of stars in the H-R diagram.

DDO 190

DDO 190 (or UGC 9240) is a dwarf irregular galaxy in the vicinity of the Milky Way, as it is relatively small and lacks clear structure. It is 9.10 million light-years (2.79 Mpc) away from Earth and lies out of the Local Group, determined by the tip of the red giant branch method. The outskirts of the galaxy are harbouring older (reddish) stars, while the centre is crowded with younger (bluish) stars. Heated gas is observed at several places. DDO 190 still experiences some active star formation. The galaxy is categorised as a Magellanic dwarf galaxy of morphological type Im. Its metallicity is [Fe/H] = −1.55 ± 0.12.DDO 190 is small, but not tiny: about 15,000 light years across—about 1/6 the size of our galaxy. It is also well outside the Local Group, which contains nearby galaxies (the Andromeda galaxy is less than 3 million light years distant from Earth, for comparison), and is instead thought to be part of the M94 galaxy group. But if true it is fairly isolated even from the others on its team; the nearest neighbor appears to be another dwarf galaxy, DDO 187, at a distance of 3 million light-years (0.92 Mpc).

Dredge-up

A dredge-up is a period in the evolution of a star where a surface convection zone extends down to the layers where material has undergone nuclear fusion. As a result, the fusion products are mixed into the outer layers of the stellar atmosphere where they can appear in the spectrum of the star.

The first dredge-up occurs when a main-sequence star enters the red-giant branch. As a result of the convective mixing, the outer atmosphere will display the spectral signature of hydrogen fusion: the 12C/13C and C/N ratios are lowered, and the surface abundances of lithium and beryllium may be reduced.

The second dredge-up occurs in stars with 4–8 solar masses. When helium fusion comes to an end at the core, convection mixes the products of the CNO cycle. This second dredge-up results in an increase in the surface abundance of 4He and 14N, whereas the amount of 12C and 16O decreases.The third dredge-up occurs after a star enters the asymptotic giant branch and a flash occurs along a helium-burning shell. This dredge-up causes helium, carbon and the s-process products to be brought to the surface. The result is an increase in the abundance of carbon relative to oxygen, which can create a carbon star.The names of the dredge-ups are set by the evolutionary and structural state of the star in which each occurs, not by the sequence experienced by the star. As a result, lower-mass stars experience the first and third dredge-ups in their evolution but not the second.

Giant star

A giant star is a star with substantially larger radius and luminosity than a main-sequence (or dwarf) star of the same surface temperature. They lie above the main sequence (luminosity class V in the Yerkes spectral classification) on the Hertzsprung–Russell diagram and correspond to luminosity classes II and III. The terms giant and dwarf were coined for stars of quite different luminosity despite similar temperature or spectral type by Ejnar Hertzsprung about 1905.Giant stars have radii up to a few hundred times the Sun and luminosities between 10 and a few thousand times that of the Sun. Stars still more luminous than giants are referred to as supergiants and hypergiants.

A hot, luminous main-sequence star may also be referred to as a giant, but any main-sequence star is properly called a dwarf no matter how large and luminous it is.

HD 112410

HD 112410 is a star in the constellation Musca. Its apparent magnitude is 6.87. It is a yellow giant of spectral type G8III located around 439 light-years distant. With around 1.54 times the mass of our Sun, it is cooling and expanding along the Red Giant Branch, having left the main sequence after exhausting its core supply of hydrogen fuel.

It has a substellar companion calculated to have a mass 9.2 times that of Jupiter and an orbital period of 124.6 days at a distance of approximately 0.57 astronomical units (AU). This is the closest planet orbiting around any ascending Red Giant Branch star, and second-closest planet to a giant star after the companion of HIP 13044.

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.

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.

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.

Subgiant

A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as true giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution of a star.

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).

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