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.

HR-diag-no-text-2

Characteristics

Mira 1997
Mira, a variable asymptotic giant branch red giant

Red giants are stars that have exhausted the supply of hydrogen in their cores and have begun thermonuclear fusion of hydrogen in a shell surrounding the core. They have radii tens to hundreds of times larger than that of the Sun. However, their outer envelope is lower in temperature, giving them a reddish-orange hue. Despite the lower energy density of their envelope, red giants are many times more luminous than the Sun because of their great size. Red-giant-branch stars have luminosities up to nearly three thousand times that of the Sun (L), spectral types of K or M, have surface temperatures of 3,000–4,000 K, and radii up to about 200 times the Sun (R). Stars on the horizontal branch are hotter, with only a small range of luminosities around 75 L. Asymptotic-giant-branch stars range from similar luminosities as the brighter stars of the red giant branch, up to several times more luminous at the end of the thermal pulsing phase.

Among the asymptotic-giant-branch stars belong the carbon stars of type C-N and late C-R, produced when carbon and other elements are convected to the surface in what is called a dredge-up.[1] The first dredge-up occurs during hydrogen shell burning on the red-giant branch, but does not produce a large carbon abundance at the surface. The second, and sometimes third, dredge up occurs during helium shell burning on the asymptotic-giant branch and convects carbon to the surface in sufficiently massive stars.

The stellar limb of a red giant is not sharply defined, contrary to their depiction in many illustrations. Rather, due to the very low mass density of the envelope, such stars lack a well-defined photosphere, and the body of the star gradually transitions into a 'corona'.[2] The coolest red giants have complex spectra, with molecular lines, emission features, and sometimes masers, particularly from thermally pulsing AGB stars.[3]

Another noteworthy feature of red giants is that, unlike Sun-like stars whose photospheres have a large number of small convection cells (solar granules), red-giant photospheres, as well as those of red supergiants, have just a few large cells, the features of which cause the variations of brightness so common on both types of stars.[4]

Evolution

The life cycle of a Sun-like star (annotated)
This image tracks the life of a Sun-like star, from its birth on the left side of the frame to its evolution into a red giant on the right after billions of years.

Red giants are evolved from main-sequence stars with masses in the range from about 0.3 M to around 8 M.[5] When a star initially forms from a collapsing molecular cloud in the interstellar medium, it contains primarily hydrogen and helium, with trace amounts of "metals" (in stellar structure, this simply refers to any element that is not hydrogen or helium i.e. atomic number greater than 2). These elements are all uniformly mixed throughout the star. The star reaches the main sequence when the core reaches a temperature high enough to begin fusing hydrogen (a few million kelvin) and establishes hydrostatic equilibrium. Over its main sequence life, the star slowly converts the hydrogen in the core into helium; its main-sequence life ends when nearly all the hydrogen in the core has been fused. For the Sun, the main-sequence lifetime is approximately 10 billion years. More-massive stars burn disproportionately faster and so have a shorter lifetime than less massive stars.[6]

When the star exhausts the hydrogen fuel in its core, nuclear reactions can no longer continue and so the core begins to contract due to its own gravity. This brings additional hydrogen into a zone where the temperature and pressure are adequate to cause fusion to resume in a shell around the core. The outer layers of the star then expand greatly, thus beginning the red-giant phase of the star's life. As the star expands, the energy produced in the burning shell of the star is spread over a much larger surface area, resulting in a lower surface temperature and a shift in the star's visible light output towards the red—hence it becomes a red giant. At this time, the star is said to be ascending the red-giant branch of the Hertzsprung–Russell (H–R) diagram.[6]

Seeing into the Heart of Mira A and its Partner
Mira A is an old star, already shedding its outer layers into space.

The evolutionary path the star takes as it moves along the red-giant branch, which ends with the complete collapse of the core, depends on the mass of the star. For the Sun and stars of less than about 2 M[7] the core will become dense enough that electron degeneracy pressure will prevent it from collapsing further. Once the core is degenerate, it will continue to heat until it reaches a temperature of roughly 108 K, hot enough to begin fusing helium to carbon via the triple-alpha process. Once the degenerate core reaches this temperature, the entire core will begin helium fusion nearly simultaneously in a so-called helium flash. In more-massive stars, the collapsing core will reach 108 K before it is dense enough to be degenerate, so helium fusion will begin much more smoothly, and produce no helium flash.[6] The core helium fusing phase of a star's life is called the horizontal branch in metal-poor stars, so named because these stars lie on a nearly horizontal line in the H–R diagram of many star clusters. Metal-rich helium-fusing stars instead lie on the so-called red clump in the H–R diagram.[8]

An analogous process occurs when the central helium is exhausted and the star collapses once again, causing helium in a shell to begin fusing. At the same time hydrogen may begin fusion in a shell just outside the burning helium shell. This puts the star onto the asymptotic giant branch, a second red-giant phase.[9] The helium fusion results in the build up of a carbon–oxygen core. A star below about 8 M will never start fusion in its degenerate carbon–oxygen core.[7] Instead, at the end of the asymptotic-giant-branch phase the star will eject its outer layers, forming a planetary nebula with the core of the star exposed, ultimately becoming a white dwarf. The ejection of the outer mass and the creation of a planetary nebula finally ends the red-giant phase of the star's evolution.[6] The red-giant phase typically lasts only around a billion years in total for a solar mass star, almost all of which is spent on the red-giant branch. The horizontal-branch and asymptotic-giant-branch phases proceed tens of times faster.

If the star has about 0.2 to 0.5 M,[7] it is massive enough to become a red giant but does not have enough mass to initiate the fusion of helium.[5] These "intermediate" stars cool somewhat and increase their luminosity but never achieve the tip of the red-giant branch and helium core flash. When the ascent of the red-giant branch ends they puff off their outer layers much like a post-asymptotic-giant-branch star and then become a white dwarf.

Stars that do not become red giants

Very low mass stars are fully convective[10][11] and may continue to fuse hydrogen into helium for up to a trillion years[12] until only a small fraction of the entire star is hydrogen. Luminosity and temperature steadily increase during this time, just as for more-massive main-sequence stars, but the length of time involved means that the temperature eventually increases by about 50% and the luminosity by around 10 times. Eventually the level of helium increases to the point where the star ceases to be fully convective and the remaining hydrogen locked in the core is consumed in only a few billion more years. Depending on mass, the temperature and luminosity continue to increase for a time during hydrogen shell burning, the star can become hotter than the Sun and tens of times more luminous than when it formed although still not as luminous as the Sun. After some billions more years, they start to become less luminous and cooler even though hydrogen shell burning continues. These become cool helium white dwarfs.[5]

Very-high-mass stars develop into supergiants that follow an evolutionary track that takes them back and forth horizontally over the HR diagram, at the right end constituting red supergiants. These usually end their life as a type II supernova. The most massive stars can become Wolf–Rayet stars without becoming giants or supergiants at all.[13][14]

Planets

Red giants with known planets: the M-type HD 208527, HD 220074 and, as of February 2014, a few tens[15] of known K-giants including Pollux, Gamma Cephei and Iota Draconis.

Prospects for habitability

Although traditionally it has been suggested the evolution of a star into a red giant will render its planetary system, if present, uninhabitable, some research suggests that, during the evolution of a 1 M star along the red-giant branch, it could harbor a habitable zone for several billion years at 2 AU out to around 100 million years at 9 AU out, giving perhaps enough time for life to develop on a suitable world. After the red-giant stage, there would for such a star be a habitable zone between 7 and 22 AU for an additional 109 years.[16] Later studies have refined this scenario, showing how for a 1 M star the habitable zone lasts from 108 years for a planet with an orbit similar to that of Mars to 2.1×108 yr for one that orbits at Saturn's distance to the Sun, the maximum time (3.7×108 yr) corresponding for planets orbiting at the distance of Jupiter. However, for planets orbiting a 0.5 M star in equivalent orbits to those of Jupiter and Saturn they would be in the habitable zone for 5.8×109 yr and 2.1×109 yr respectively; for stars more massive than the Sun, the times are considerably shorter.[17]

Enlargement of planets

As of June 2014, 50 giant planets have been discovered around giant stars. However, these giant planets are more massive than the giant planets found around solar-type stars. This could be because giant stars are more massive than the Sun (less massive stars will still be on the main sequence and will not have become giants yet) and more massive stars are expected to have more massive planets. However, the masses of the planets that have been found around giant stars do not correlate with the masses of the stars; therefore, the planets could be growing in mass during the stars' red giant phase. The growth in planet mass could be partly due to accretion from stellar wind, although a much larger effect would be Roche lobe overflow causing mass-transfer from the star to the planet when the giant expands out to the orbital distance of the planet.[18]

Well known examples

Many of the well known bright stars are red giants, because they are luminous and moderately common. The red giant branch variable star Gamma Crucis is the nearest M class giant star at 88 light years.[19] The K0 red giant branch star Arcturus is 36 light years away.[20]

Red-giant branch

Red-clump giants

Asymptotic giant branch

The Sun as a red giant

Sun red giant
The current size of the Sun (now in the main sequence) compared to its estimated maximum size during its red-giant phase in the future

In about 5 to 6 billion years, the Sun will have depleted the hydrogen fuel in its core. It will shrink, with the hydrogen outside the core able to compress enough for hydrogen there to fuse, and will begin to expand into a subgiant. Eventually, the pressure builds up so much that the core will begin to fuse helium, and will expand even more into a red giant. At its largest, its surface (photosphere) will approximately reach the current orbit of Earth. It will then lose its atmosphere completely; its outer layers forming a planetary nebula and the core a white dwarf.

The evolution of the Sun into and through the red-giant phase has been extensively modelled, but it remains unclear whether Earth will be engulfed by the Sun or will continue in orbit. The uncertainty arises in part because as the Sun burns hydrogen, it loses mass causing Earth (and all planets) to orbit farther away. There are also significant uncertainties in calculating the orbits of the planets over the next 5–6.5 billion years, so the fate of Earth is not well-understood. At its brightest, the red-giant Sun will be several thousand times more luminous than today but its surface will be at about half the temperature.

References

  1. ^ Boothroyd, A. I.; Sackmann, I. ‐J. (1999). "The CNO Isotopes: Deep Circulation in Red Giants and First and Second Dredge‐up". The Astrophysical Journal. 510: 232–250. arXiv:astro-ph/9512121. Bibcode:1999ApJ...510..232B. doi:10.1086/306546.
  2. ^ Suzuki, Takeru K. (2007). "Structured Red Giant Winds with Magnetized Hot Bubbles and the Corona/Cool Wind Dividing Line". The Astrophysical Journal. 659 (2): 1592. arXiv:astro-ph/0608195. Bibcode:2007ApJ...659.1592S. doi:10.1086/512600.
  3. ^ Habing, Harm J.; Olofsson, Hans (2003). "Asymptotic giant branch stars". Asymptotic giant branch stars. Bibcode:2003agbs.conf.....H.
  4. ^ Schwarzschild, Martin (1975). "On the scale of photospheric convection in red giants and supergiants". Astrophysical Journal. 195: 137–144. Bibcode:1975ApJ...195..137S. doi:10.1086/153313.
  5. ^ a b c Laughlin, G.; Bodenheimer, P.; Adams, F. C. (1997). "The End of the Main Sequence". The Astrophysical Journal. 482: 420. Bibcode:1997ApJ...482..420L. doi:10.1086/304125.
  6. ^ a b c d Zeilik, Michael A.; Gregory, Stephan A. (1998). Introductory Astronomy & Astrophysics (4th ed.). Saunders College Publishing. pp. 321–322. ISBN 0-03-006228-4.
  7. ^ a b c Fagotto, F.; Bressan, A.; Bertelli, G.; Chiosi, C. (1994). "Evolutionary sequences of stellar models with new radiative opacities. IV. Z=0.004 and Z=0.008". Astronomy and Astrophysics Supplement Series. 105. Bibcode:1994A&AS..105...29F.
  8. ^ Alves, David R.; Sarajedini, Ata (1999). "The Age-dependent Luminosities of the Red Giant Branch Bump, Asymptotic Giant Branch Bump, and Horizontal Branch Red Clump". The Astrophysical Journal. 511: 225. arXiv:astro-ph/9808253. Bibcode:1999ApJ...511..225A. doi:10.1086/306655.
  9. ^ Sackmann, I. -J.; Boothroyd, A. I.; Kraemer, K. E. (1993). "Our Sun. III. Present and Future". The Astrophysical Journal. 418: 457. Bibcode:1993ApJ...418..457S. doi:10.1086/173407.
  10. ^ Reiners, A.; Basri, G. (2009). "On the magnetic topology of partially and fully convective stars". Astronomy and Astrophysics. 496 (3): 787. arXiv:0901.1659. Bibcode:2009A&A...496..787R. doi:10.1051/0004-6361:200811450.
  11. ^ Brainerd, Jerome James (16 February 2005). "Main-Sequence Stars". Stars. The Astrophysics Spectator. Retrieved 29 December 2006.
  12. ^ Richmond, Michael. "Late stages of evolution for low-mass stars". Retrieved 29 December 2006.
  13. ^ Crowther, P. A. (2007). "Physical Properties of Wolf-Rayet Stars". Annual Review of Astronomy and Astrophysics. 45 (1): 177–219. arXiv:astro-ph/0610356. Bibcode:2007ARA&A..45..177C. doi:10.1146/annurev.astro.45.051806.110615.
  14. ^ Georges Meynet; Cyril Georgy; Raphael Hirschi; Andre Maeder; et al. (12–16 July 2010). G. Rauw; M. De Becker; Y. Naz\'e; J.-M. Vreux; et al. (eds.). "Red Supergiants, Luminous Blue Variables and Wolf-Rayet stars: The single massive star perspective". Societe Royale des Sciences de Liege, Bulletin (Proceedings of the th Liege Astrophysical Colloquium). v1. Li'ege. 80 (39): 266–278. arXiv:1101.5873. Bibcode:2011BSRSL..80..266M.
  15. ^ http://exoplanetarchive.ipac.caltech.edu/cgi-bin/ExoTables/nph-exotbls?dataset=planets
  16. ^ Lopez, Bruno; Schneider, Jean; Danchi, William C. (2005). "Can Life Develop in the Expanded Habitable Zones around Red Giant Stars?". The Astrophysical Journal. 627 (2): 974–985. arXiv:astro-ph/0503520. Bibcode:2005ApJ...627..974L. doi:10.1086/430416.
  17. ^ Ramirez, Ramses M.; Kaltenegger, Lisa (2016). "Habitable Zones of Post-Main Sequence Stars". The Astrophysical Journal. 823 (1): 6. arXiv:1605.04924. Bibcode:2016ApJ...823....6R. doi:10.3847/0004-637X/823/1/6.
  18. ^ Jones, M. I.; Jenkins, J. S.; Bluhm, P.; Rojo, P.; Melo, C. H. F. (2014). "The properties of planets around giant stars". Astronomy & Astrophysics. 566: A113. arXiv:1406.0884. Bibcode:2014A&A...566A.113J. doi:10.1051/0004-6361/201323345.
  19. ^ Ireland, M. J.; et al. (May 2004). "Multiwavelength diameters of nearby Miras and semiregular variables". Monthly Notices of the Royal Astronomical Society. 350 (1): 365–374. arXiv:astro-ph/0402326. Bibcode:2004MNRAS.350..365I. doi:10.1111/j.1365-2966.2004.07651.x.
  20. ^ Abia, C.; Palmerini, S.; Busso, M.; Cristallo, S. (2012). "Carbon and oxygen isotopic ratios in Arcturus and Aldebaran. Constraining the parameters for non-convective mixing on the red giant branch". Astronomy & Astrophysics. 548: A55. arXiv:1210.1160. Bibcode:2012A&A...548A..55A. doi:10.1051/0004-6361/201220148.
  21. ^ Alves, David R. (2000). "K-Band Calibration of the Red Clump Luminosity". The Astrophysical Journal. 539 (2): 732. arXiv:astro-ph/0003329. Bibcode:2000ApJ...539..732A. doi:10.1086/309278.

External links

Media related to Red giants at Wikimedia Commons

Asymptotic giant branch

The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (0.6–10 solar masses) late in their lives.

Observationally, an asymptotic-giant-branch star will appear as a bright red giant with a luminosity ranging up to thousands of times greater than the Sun. Its interior structure is characterized by a central and largely inert core of carbon and oxygen, a shell where helium is undergoing fusion to form carbon (known as helium burning), another shell where hydrogen is undergoing fusion forming helium (known as hydrogen burning), and a very large envelope of material of composition similar to main-sequence stars.

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.

Future of Earth

The biological and geological future of Earth can be extrapolated based upon the estimated effects of several long-term influences. These include the chemistry at Earth's surface, the rate of cooling of the planet's interior, the gravitational interactions with other objects in the Solar System, and a steady increase in the Sun's luminosity. An uncertain factor in this extrapolation is the ongoing influence of technology introduced by humans, such as climate engineering, which could cause significant changes to the planet. The current Holocene extinction is being caused by technology and the effects may last for up to five million years. In turn, technology may result in the extinction of humanity, leaving the planet to gradually return to a slower evolutionary pace resulting solely from long-term natural processes.Over time intervals of hundreds of millions of years, random celestial events pose a global risk to the biosphere, which can result in mass extinctions. These include impacts by comets or asteroids, and the possibility of a massive stellar explosion, called a supernova, within a 100-light-year radius of the Sun. Other large-scale geological events are more predictable. Milankovitch theory predicts that the planet will continue to undergo glacial periods at least until the Quaternary glaciation comes to an end. These periods are caused by variations in eccentricity, axial tilt, and precession of the Earth's orbit. As part of the ongoing supercontinent cycle, plate tectonics will probably result in a supercontinent in 250–350 million years. Some time in the next 1.5–4.5 billion years, the axial tilt of the Earth may begin to undergo chaotic variations, with changes in the axial tilt of up to 90°.The luminosity of the Sun will steadily increase, resulting in a rise in the solar radiation reaching the Earth. This will result in a higher rate of weathering of silicate minerals, which will cause a decrease in the level of carbon dioxide in the atmosphere. In about 600 million years from now, the level of carbon dioxide will fall below the level needed to sustain C3 carbon fixation photosynthesis used by trees. Some plants use the C4 carbon fixation method, allowing them to persist at carbon dioxide concentrations as low as 10 parts per million. However, the long-term trend is for plant life to die off altogether. The extinction of plants will be the demise of almost all animal life, since plants are the base of the food chain on Earth.In about one billion years, the solar luminosity will be 10% higher than at present. This will cause the atmosphere to become a "moist greenhouse", resulting in a runaway evaporation of the oceans. As a likely consequence, plate tectonics will come to an end, and with them the entire carbon cycle. Following this event, in about 2–3 billion years, the planet's magnetic dynamo may cease, causing the magnetosphere to decay and leading to an accelerated loss of volatiles from the outer atmosphere. Four billion years from now, the increase in the Earth's surface temperature will cause a runaway greenhouse effect, heating the surface enough to melt it. By that point, all life on the Earth will be extinct. The most probable fate of the planet is absorption by the Sun in about 7.5 billion years, after the star has entered the red giant phase and expanded beyond the planet's current orbit.

G-type main-sequence star

A G-type main-sequence star (Spectral type: G-V), often (and imprecisely) called a yellow dwarf, or G dwarf star, is a main-sequence star (luminosity class V) of spectral type G. Such a star has about 0.84 to 1.15 solar masses and surface temperature of between 5,300 and 6,000 K., Tables VII, VIII. Like other main-sequence stars, a G-type main-sequence star is converting the element hydrogen to helium in its core by means of nuclear fusion. The Sun, the star to which the Earth is gravitationally bound in the Solar System and the object with the largest apparent magnitude, is an example of a G-type main-sequence star (G2V type). Each second, the Sun fuses approximately 600 million tons of hydrogen to helium, converting about 4 million tons of matter to energy. Besides the Sun, other well-known examples of G-type main-sequence stars include Alpha Centauri A, Tau Ceti, and 51 Pegasi.The term yellow dwarf is a misnomer, because G-type stars actually range in color from white, for more luminous types like the Sun, to only very slightly yellow for the less massive and luminous G-type main-sequence stars. The Sun is in fact white, and its spectrum peaks in blue and green light, but it can often appear yellow, orange or red through Earth's atmosphere due to atmospheric Rayleigh scattering, especially at sunrise and sunset. In addition, although the term "dwarf" is used to contrast yellow main-sequence stars from giant stars, yellow dwarfs like the Sun outshine 90% of the stars in the Milky Way (which are largely much dimmer orange dwarfs, red dwarfs, and white dwarfs, the last being a stellar remnant).

A G-type main-sequence star will fuse hydrogen for approximately 10 billion years, until it is exhausted at the center of the star. When this happens, the star expands to many times its previous size and becomes a red giant, such as Aldebaran (or Alpha Tauri). Eventually the red giant sheds its outer layers of gas, which become a planetary nebula, while the core rapidly cools and contracts into a compact, dense white dwarf.

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 65750

HD 65750, also known as V341 Carinae is a bright red giant star in the constellation Carina. It is surrounded by a prominent reflection nebula, known as IC 2220, nicknamed the Toby Jug Nebula.

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.

Long-period variable star

The descriptive term long-period variable star refers to various groups of cool luminous pulsating variable stars. It is frequently abbreviated to LPV.

RS Ophiuchi

RS Ophiuchi (RS Oph) is a recurrent nova system approximately 5,000 light-years away in the constellation Ophiuchus. In its quiet phase it has an apparent magnitude of about 12.5. It has been observed to erupt in 1898, 1933, 1958, 1967, 1985, and 2006 and reached about magnitude 5 on average. A further two eruptions, in 1907 and 1945, have been inferred from archival data. The recurrent nova is produced by a white dwarf star and a red giant in a binary system. About every 20 years, enough material from the red giant builds up on the surface of the white dwarf to produce a thermonuclear explosion. The white dwarf orbits close to the red giant, with an accretion disc concentrating the overflowing atmosphere of the red giant onto the white dwarf.

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 flying squirrel

The red giant flying squirrel or common giant flying squirrel (Petaurista petaurista) is a species of flying squirrel, found in northern South Asia, southern China and Southeast Asia. It is a dark red colour with black extremities and can grow to a head-and-body length of 42 cm (17 in). The tail is long and provides stability when it glides between trees. It is nocturnal, feeding mainly on leaves, fruits and nuts, and occasionally insects. This squirrel faces no particular threats apart from ongoing destruction of suitable habitat. It has a wide range and is relatively common, and the International Union for Conservation of Nature lists it as a "least-concern species".

Red supergiant star

Red supergiants are stars with a supergiant luminosity class (Yerkes class I) of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Antares are the brightest and best known red supergiants (RSGs), indeed the only first magnitude red supergiant stars.

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.

Subdwarf B star

A B-type subdwarf (sdB) is a kind of subdwarf star with spectral type B. They differ from the typical subdwarf by being much hotter and brighter. They are situated at the "extreme horizontal branch" of the Hertzsprung–Russell diagram. Masses of these stars are around 0.5 solar masses, and they contain only about 1% hydrogen, with the rest being helium. Their radius is from 0.15 to 0.25 solar radii, and their temperature is from 20,000 to 40,000K.

These stars represent a late stage in the evolution of some stars, caused when a red giant star loses its outer hydrogen layers before the core begins to fuse helium. The reasons why this premature mass loss occurs are unclear, but the interaction of stars in a binary star system is thought to be one of the main mechanisms. Single subdwarfs may be the result of a merger of two white dwarfs. The sdB stars are expected to become white dwarfs without going through any more giant stages.

Subdwarf B stars, being more luminous than white dwarfs, are a significant component in the hot star population of old stellar systems, such as globular clusters, spiral galaxy bulges and elliptical galaxies. They are prominent on ultraviolet images. The hot subdwarfs are proposed to be the cause of the UV upturn in the light output of elliptical galaxies.

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.

Symbiotic nova

Symbiotic novae are slow irregular eruptive variable stars with very slow nova-like outbursts with an amplitude of between 9 and 11 magnitudes. The symbiotic nova remains at maximum for one or a few decades, and then declines towards its original luminosity. Variables of this type are double star systems with one red giant, which probably is a Mira variable, and one white dwarf, with markedly contrasting spectra and whose proximity and mass characteristics indicate it as a symbiotic star. The red giant fills its Roche lobe so that matter is transferred to the white dwarf and accumulates until a nova-like outburst occurs, caused by ignition of thermonuclear fusion. The temperature at maximum is estimated to rise up to 200,000 K, similar to the energy source of novae, but dissimilar to the dwarf novae. The slow luminosity increase would then be simply due to time needed for growth of the ionization front in the outburst.It is believed that the white dwarf component of a symbiotic nova remains below the Chandrasekhar limit, so that it remains a white dwarf after its outburst.One example of a symbiotic nova is V1016 Cygni, whose outburst in 1971–2007 clearly indicated a thermonuclear explosion. Other examples are HM Sagittae and RR Telescopii.

Thorne–Żytkow object

A Thorne–Żytkow object (TŻO or TZO) is a conjectured type of star wherein a red giant or supergiant contains a neutron star at its core, formed from the collision of the giant with the neutron star. Such objects were hypothesized by Kip Thorne and Anna Żytkow in 1977. In 2014, it was discovered that the star HV 2112 was a strong candidate but this has since been called into question.

Udhayanidhi Stalin

Udayanidhi Stalin is an Indian film producer and actor, who has worked on Tamil-language films. He entered the film industry as a film producer and distributor with his production studio, Red Giant Movies, and made films including Kuruvi (2008), Aadhavan (2009) and Manmadan Ambu (2010). He subsequently made his debut as an actor through the romantic comedy, Oru Kal Oru Kannadi (2012), and has since continued producing and starring in his own films.

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