Red giant
A red giant is a star of great size and therefore a celestial body of high luminosity compared to a main sequence star of the same surface temperature (a so-called red dwarf ). Examples of this are some first-magnitude stars that already appear red with free eyes, e.g. B. Aldebaran in the constellation Taurus and the yellow-red glowing arcturus in the constellation Bear Guardian .
Red giants are “aging” stars of the order of magnitude of the mass of the sun , in whose core the “ hydrogen burning ” (4 protons to 1 helium core) has ceased due to a lack of supplies. Gravity then gains the upper hand and they contract until the pressure, density and temperature are sufficient to fuse helium to carbon in their center . The fusion of hydrogen to helium now takes place outside the hot core in what is known as shell burning and the stars expand to around a hundred times their original size until a state of equilibrium between outwardly directed radiation and inwardly directed gravity pressure is restored . Due to their now much larger surface, the temperature drops there and the stars mostly appear reddish. With the onset of helium burning , sun-like stars leave the main sequence in the Hertzsprung-Russell diagram and are now located as red giants on the asymptotic giant branch . After millions more years, when their fuel supplies run out, they end up as white dwarfs .
features
Red giants mostly belong to the spectral classes K and M, whose surface temperatures, according to Schmidt-Kaler (1982), are 3330 (spectral class M5) to 4750 (spectral class K0) Kelvin . They show one of the spectral classes R, N or S relatively seldom, for which Schmidt-Kaler (1982) gives a temperature range of 1900 to 5400 K. With these low temperatures compared to the sun (whose surface temperature is 5780 K), the maximum of their black body radiation is in the red or orange color range.
Due to their expansion and the associated large surface, the amount of light emitted and thus the luminosity of red giants is very high, so that we are dealing with stars of great absolute brightness . In the visual range, the absolute brightness of red giants of the spectral classes K and M according to Schmidt-Kaler (1982) is −0.4 to 0.7 magnitudes, which means that the sun (which shines with 4.8 magnitudes) is about 100- Exceed times. Schmidt-Kaler (1982) gives values of −2.6 to 0.4 magnitudes for the luminosity of the red giants of classes K and M integrated over the entire spectrum (the so-called bolometric brightness ), which equates to 4.7 Magnitudes up to 1000 times greater. The high luminosity inevitably means that red giants can be seen from a very great distance compared to hotter main sequence stars. They are particularly well represented among the bright stars that are visible to the naked eye.
Because of their low surface temperature and high luminosity, red giants are in the upper right area of the Hertzsprung-Russell diagram .
As explained in detail in the article Star Surface , red giants in particular often have extensive photosphere . Physical variables such as surface temperature, surface gravity or radius therefore require particularly careful definition for these stars.
Development of red giants to asymptotic giant branches
Sun-like starting stars
Red giants emerge from low-mass main sequence stars at the end of their evolution. In detail, what happens depends primarily on the mass, but also on the chemical composition of the original star. In order to work out the principles of this late phase of stellar evolution, let us first take the evolutionary path of a sun-like star (1 solar mass, 68% mass fraction for hydrogen , 30% for helium , 2% for other elements) in the Hertzsprung-Russel diagram (HRD) as an example Models shown by Schaller and colleagues (1992) and Charbonnel and colleagues (1996).
Even in the main sequence stage, the conversion of hydrogen to helium leads to an increase in luminosity in the core. This process reduces the number of particles (4 protons and 4 electrons produce 1 helium nucleus and 2 electrons), while the average atomic mass increases (from 0.5 to 1.33 atomic mass units ). The reduction in the number of particles in the core automatically results in a higher mass density. At the boundary between the core and the inactive hydrogen shell there is a temperature and pressure equilibrium and thus the same particle density on both sides. Since the number of particles in the core decreases, this equilibrium can only be maintained by compressing the mass. The central temperature is directly proportional to the atomic mass (see star structure ), so that the temperature in the core increases with the atomic mass. But this also increases nuclear energy production and thus luminosity. According to the models, the radiation output of the sun has increased by around 35% since the beginning of the main sequence stage around 4.5 billion years ago.
In the course of time, the core of the hydrogen supply runs out (in the model discussed here after around 9.5 billion years), and thus energy production also dries up there. As a result, gravity gains the upper hand over gas pressure + radiation pressure ; the core condenses in turn. The temperature continues to rise accordingly, so that nuclear fusion to form helium can begin in the previously inactive hydrogen envelope. By the time the hydrogen in the core runs out (point A in HRD), the star's luminosity has already increased to around twice the solar value.
The burning of the hydrogen shell drives the shell of the star outwards, which means that it cools down despite the central temperature rising further. Since the luminosity depends very strongly on the surface temperature, it does not increase any further despite the ever higher core temperature. At this stage the star is a yellow sub-giant (spectral type G, luminosity class IV), which moves in the HRD parallel to the temperature axis from left to right. Its radius grows about twice as much.
With decreasing surface temperature ranging hydrogen convection deeper into the star down (again, see stellar structure) until it finally hits the hydrogen-burning zone. For the first time, nuclear reaction products (helium produced by the hydrogen-burning shell) can get into the photosphere (point B in the HRD). This stage is reached after about 10.7 billion years. At around 1.2 billion years, the subgiant phase is eight times shorter than the main sequence existence.
From point B on, the rest of the process accelerates considerably. The hydrogen shell burning increases the mass of the helium nucleus, which means that the effects of the decreasing number of particles, the increasing atomic mass and the gravitational pressure take hold. In only about 600 million years, the star moves to point C in the HRD, where the interaction of high luminosity and low surface gravity now also increases the mass loss caused by stellar winds drastically. The luminosity is now about 35 times the solar value, the radius about 10 solar radii. The star has become a red giant (spectral type K, luminosity class III). With a mass loss of around 10 −10 solar masses per year, the stellar wind is already 10,000 times as strong as that of the sun, but not yet sufficient to decisively influence the structure of the star in a short time.
From point C the star only needs about 50 million years to reach a maximum luminosity for the first time. The distance covered in the HRD is called the first giant branch. In terms of the radiated power, the red giant now surpasses the sun by about 1500 times and in radius by 120 times. It now has the M spectral type. With several 10 −8 solar masses per year, its mass loss is so great that the star loses a significant part of its mass in the course of further development. At around 700 kg cm −3, the central density is so high that the nucleus has largely degenerated like a white dwarf .
Because of this enormous density (and the high central temperature), helium burning can now begin. This means that a new energy source is available at the core, which allows the temperature to rise further. Since the energy yield of the helium burning depends extremely strongly on the temperature (of its 30th power), a process that builds up extremely quickly, which is known as a helium flash , starts. If the core temperature is high enough, its degeneration is reversed. In this way, however, the gas pressure prevailing there becomes temperature-dependent again, which results in violent expansion. The shell of the star is able to intercept this. There is no supernova explosion, but at least the outermost, cool layers are shed. The expansion allows the core to cool down, which ultimately results in a stable state with smooth nuclear fusion.
As the outermost layers repel, the star becomes smaller again and hotter on the surface. The M giant becomes a K giant again with around 47 solar luminosities and 12 solar radii (point D in the HRD).
The red giant spends its helium burning with relatively constant luminosity and surface temperature. Only when this energy source is running low does the star in the HRD move to the top right again. Since this path is not identical to the first giant branch, but is shifted towards slightly higher surface temperatures, it is given its own name as the asymptotic giant branch.
Again, the reason for the increase in luminosity lies in the decreasing number of particles (3 helium nuclei transform into 1 carbon nucleus), the increasing atomic mass (from 1.33 to 1.85 atomic mass units) and gravitational pressure. After about 120 million years the helium in the core is used up (only 1/80 of the duration of the main sequence stage), point E in the HRD is reached (about 120 times the solar luminosity and 23 times the solar radius). After another 40 million years, the star has exceeded the first power maximum, it now has around 2500 solar luminosities and 160 solar radii. In its center there is an inactive core made of carbon and oxygen, again compressed to the point of degeneration (the latter is produced by the addition of another helium nucleus to the carbon), surrounded by a helium-burning shell, which is connected to the hydrogen-burning shell further outside. As will be shown in the next section, the luminosity increases a little further on the asymptotic giant branch.
Two further figures are intended to illustrate the accelerating development over time. In order to be able to make the short stage of the red giant graphically visible, the luminosity and mass are not plotted against the age of the star, but the remaining time until the start of the white dwarf stage. The time is represented logarithmically, i. H. from left to right the individual development phases become shorter and shorter. After a leisurely increase in luminosity in the main row stage and a stable release of energy in the sub-giant phase, there follows a rapid, dramatic increase to the first giant branch. The helium flash triggers a drop in luminosity, which is followed by a second, but short phase of stability. Finally, there is a rapid ascent on the asymptotic giant branch.
The star's mass remains stable for a long time; the stellar wind only intervenes to change its structure when it is near the first luminosity maximum. By the time the helium began to burn, the red giant had lost more than 10% of its original mass. After climbing the asymptotic giant branch, it has lost 30% of its original mass.
Influence of mass on stellar evolution
Of all the variables of state, mass has by far the greatest influence on stellar evolution. As an example, the path in the HRD of a star with a chemical composition similar to that of the Sun, but with 1.7 solar masses, is compared to the scenario just sketched of a star with 1 solar mass. Points A to E correspond to the same stages of development as discussed here.
In the main sequence stage, the more massive star is by far more luminous than the low-mass star, the difference is about a factor of 10. Accordingly, the hydrogen supply in the core has dried up much more quickly despite the greater initial mass; after about 1.6 instead of 9.5 billion years. As with a star with 1 solar mass, its existence in the main sequence is already accompanied by a significant increase in luminosity.
The subsequent subgiant phase is particularly short, it only lasts about 40 million instead of 1.2 billion years. It is now characterized by a remarkable decrease in luminosity, but this can be explained by the relatively strong cooling of the star's surface. While the surface temperature of the low-mass star between points A and B drops by about 700 K, it drops by about 2000 K for the more massive star. The luminosity, however, depends sensitively on the surface temperature, namely its fourth power.
The further development is z. Sometimes surprisingly analogous, the more massive star in particular largely loses its enormous luminosity advantage on the logarithmic scale. Its ascent to the top of the first giant branch happens quickly - in about 80 instead of 600 million years - but then with about 2200 solar luminosities it only exceeds the low-mass star by a factor of 1.5. Despite different initial masses and thus luminosity and surface temperatures, the development paths of stars with less than around 2.5 solar masses come very close to one another in their late phase. There is an agglomeration of the red giants in the HRD, even if the more massive ones are slightly hotter than the less massive ones. Only above about 2.5 solar masses do the initial luminosity differences between stars of different initial masses also remain in the red giant stage, but this is then increasingly assigned to luminosity class II, the bright giants.
Even after the helium flash, the paths of the less massive stars remain close together. The star with originally 1.7 solar masses, with around 86 solar luminosities, is just a factor of 2 ahead of the star with 1 solar mass. As a result, the duration of the central helium burning is only slightly shortened, to 80 instead of 120 million years. The ascent to the asymptotic giant branch, on the other hand, takes place again fairly quickly, namely within 15 instead of 40 million years. With 2700 solar luminosities, the luminosity advantage compared to the star with 1 solar mass has now almost completely disappeared.
Since the star with originally 1.7 solar masses as a red giant is hardly more luminous than the one with initially 1 solar mass, one cannot expect a significantly higher loss of mass. The models used here even predict a lower loss of mass for the more massive branch up to the ascent to the asymptotic giant branch. While the lower mass has already lost around 0.3 solar masses (30% of its original mass) by this phase, the more massive one has only lost around 0.15 solar masses (approx. 10% of its original mass).
While the transition from the giants to the bright giants is fluid towards higher stellar masses, the smallest starting mass of red giants is clearly defined by the age of the universe and the duration of the main sequence phase. Stars with less than 0.8 solar masses have not yet had the opportunity to leave the main sequence. An even lower mass in red giants can only be achieved by strong stellar winds.
Influence of chemical composition on star evolution
Finally, the role of chemical composition should be shown using an example. The sun-like star with a share of 2% of elements heavier than helium is now compared with a star of the same mass, but only a share of 0.1% of "heavy" elements.
The "low-metal" star (with the small proportion of "heavy" elements) is significantly more luminous on the main sequence, about a factor of 3. The less "heavy" elements the star matter contains, the more transparent it is, the weaker those that retain energy are Spectral lines . At the same time, the "low-metal" star is around 1000 K hotter on the surface. Spectral lines occur mainly in the blue and ultraviolet . So this area of the spectrum benefits most from the increased transparency.
Due to the shift towards higher luminosity and temperature, the star in the HRD appears to be among the main sequence of stars of sun-like composition. In the past, this was mistakenly interpreted as a luminosity deficit, as a result of which such stars were called " subdwarfs " and assigned their own luminosity class VI.
As expected, the actually greater luminosity shortens the main sequence stage - according to the models by about a third. After about 6 billion years, the hydrogen in the core is used up. The luminosity difference is also retained in the sub-giant phase. This means that this phase of life is also shorter for the “metal-poor” star - by half at around 600 million years.
At around 200 million years, the path to the first giant branch is even around two thirds shorter than for stars with a sun-like abundance of elements. At the same time, however, the "low-metal" star also loses its luminosity advantage, and so the duration of the helium burn of 80 million years is again only about a third shorter. The ascent to the asymptotic giant branch, however, takes place again quickly, at around 20 million years it only takes half the time compared to a sun-like star.
The higher surface temperature is retained, at least in part. As a red giant, the “low-metal” star is still around 500–600 K hotter. The giant branches of globular clusters , which are characterized by a particularly low proportion of "heavy" elements (mostly only a few 0.01%), are bluer than in open star clusters , the chemical composition of which is comparable to that of the sun.
Asteroseismology
Using asteroseismology , it is possible to study the status of the red giant. The convection in the outer atmosphere stimulates vibrations that are reflected by the photosphere and density leaps in the star and lead to the formation of a complex pattern of standing waves in the atmosphere of the red giant. The peaks in density arise at the edges of the hydrogen and helium burning zones, where heavy elements arise as a result of the fusion . An analysis of minimal brightness variations from the data sets from the Kepler mission resulted in a classification according to hydrogen or helium-burning red giants. This allows the models of stellar evolution of stars of medium mass to be better verified with regard to mass losses and the minimum mass required to ignite the helium flame. Asteroseismology also enables the investigation of the course of rotation within the stars. It turns out that the rotation time of the nuclei of red giants is considerably shorter than that of their extended atmospheres. The rapid rotation leads to greater mixing in the interior of the stars, and thus more fuel is available for thermonuclear reactions , which changes the lifespan of these stars.
Development of red giants to white dwarfs
As a result of their expansion, the outer gas layers have a very low density and are only weakly bound by the star's gravity . Therefore, in the course of its red giant stage, a strong stellar wind develops , through which the outer gas layers are completely repelled; they then surround him for some time as a planetary nebula . Red giants with a mass of less than eight solar masses subsequently shrink to form white dwarfs . At more than eight solar masses , further fusion processes begin at the end of the helium burn until the red giant explodes as a supernova .
Examples
| Surname |
Mass ( M ☉ ) |
Radius ( R ☉ ) |
Luminosity ( L ☉ ) |
|---|---|---|---|
| Aldebaran (α Tau A) | 2.5 | 44.2 | 156 |
| Arcture (α Boo) | 1.5 | 25.7 | 210 |
| Gacrux (γ Cru) | 3 | 84 | 1,500 |
| La Superba (Y CVn) | 3 | 215 | 4,400 |
| Menkar (α Cet) | 3 | 84 | 1,800 |
| Mira (ο Cet A) | 1.2 | 400 | 8,400 |
See also
literature
- C. Charbonnel, G. Meynet, A. Maeder, D. Schaerer: Grids of stellar models. VI. Horizontal branch and early asymptotic giant branch for low mass stars (Z = 0.020, 0.001). In: Astronomy and Astrophysics Supplement Series. Volume 115, 1996, pp. 339-344.
- Norbert Langer: Life and Death of the Stars. CH Beck'sche Verlagsbuchhandlung, Munich 1995, ISBN 3-406-39720-4 .
- G. Schaller, D. Schaerer, G. Meynet, A. Maeder: New grids of stellar models from 0.8 to 120 solar masses at Z = 0.020 and Z = 0.001. In: Astronomy and Astrophysics Supplement Series. Volume 96, 1992, pp. 269-331.
- Theodor Schmidt-Kaler : Physical Parameters of Stars. In: K. Schaifers, HH Voigt (Ed.): Landolt-Börnstein New Series. Vol. 2b, Springer, New York 1982.
- Klaus Werner, Thomas Rauch: The rebirth of the red giants. In: Stars and Space. 46 (2), 2007, pp. 36-44. ISSN 0039-1263
Web links
- What is a red giant? from the alpha-Centauri television series(approx. 15 minutes). First broadcast on July 7, 2002.
- Echoes from the depths of a red giant - Astronomy Picture of the Day of April 8, 2011.
Individual evidence
- ↑ Timothy R. Bedding et al. a .: Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars . In: Nature . tape 471 , no. 4 , 2011, p. 608–611 , doi : 10.1038 / nature09935 .
- ^ MP Di Mauro et al. a .: Internal rotation of red giants by asteroseismology . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1212.4758 .