Red giant
A red giant is a star of great extent and thus compared to a main-sequence star of the same surface temperature ( a so-called red dwarf) a celestial body of high luminosity. Examples include Aldebaran in the constellation Taurus and Arcturus in the constellation Bootes.
Features
Red Giant mostly belong to the spectral classes K and M, the surface temperatures are according to Schmidt - Kaler (1982 ) at 3330 ( spectral type M5) to 4750 ( spectral type K0) Kelvin. Relatively rarely have a spectral classes of R, N, or S, for which Schmidt- Kaler (1982 ) indicates a temperature range 1900-5400 K. With this in comparison to the Sun ( the surface temperature of 5780 K is ) low temperatures, the maximum of their black-body radiation in the red or orange color range.
Due to their expansion and associated large surface the emitted amount of light and therefore the luminosity red giant is very high, so that it is big star absolute brightness. In the visual range, the absolute magnitudes are red giants of spectral types K and M according to Schmidt- Kaler (1982 ) at -0.4 to 0.7 mag, making it (which shines with magnitude 4.8 ) outperforms the sun by about 100 times. For integrated over the entire spectrum luminosity are ( the so-called bolometric magnitude ) Red giants of classes K and M Schmidt- Kaler (1982 ) values from -2.6 to 0.4 mag at what the luminosity of the Sun 4.7 magnitude exceeds by up to 1000 times. The large luminosity fundamental implication is that red giants in comparison to cool main-sequence stars can be seen from a great distance. Especially under the bright, visible with the naked eye stars, they are particularly well represented.
Because of their low surface temperature and high luminosity red giants are located in the upper right of the Hertzsprung- Russell diagram.
As is described in detail in the article stellar surface, often have straight red giants on extended photosphere. Physical parameters such as surface temperature, surface gravity or radius therefore require these stars particularly careful definition.
Development of red giant to the Asymptotic Giant
Sun-like star output
Red Giants go from low-mass main sequence stars at the end of its development out. In detail, the events depends mainly on the mass, but also the chemical composition of the original star. In order to work out the principles of this late phase of stellar evolution, is an example first, the development path of a sun-like star (1 solar mass, 68 % mass fraction of hydrogen, 30 % helium, 2 % for other elements ) in the Hertzsprung -Russell diagram (HRD ) to the models of the Schaller and colleagues (1992) and Charbonnel and colleagues ( 1996) showed.
Already in the main sequence stage, the conversion of hydrogen into helium in the core leads to a luminosity increase. Through this process, the number of particles is reduced (from 4 protons and 4 electrons go one helium nucleus and two electrons out ) simultaneously increases the average atomic mass (from 0.5 to 1.33 atomic mass units). The reduction in the number of particles in the core automatically attracts a higher mass density by itself. At the boundary between the core and inactive hydrogen envelope prevail namely temperature and pressure equilibrium and thus on both sides each have the same number density. As in the core but decreases the number of particles, this balance can be maintained there only by a compression of the mass. The central temperature is directly proportional to the atomic mass ( see stellar structure ), so that the atomic mass and the temperature in the core rises accordingly. But this is also a growing nuclear energy production and thus the luminosity. The radiation power of the sun is, according to the models grown since the beginning of the main sequence phase about 4.5 billion years ago to about 35%.
Over time, the hydrogen fuel in the core is depleted ( in the model discussed here, after about 9.5 billion years ago ), and thus dried up there, and the energy production. This wins the gravity than the gas pressure, the upper hand; the core condenses again. Accordingly, the temperature continues to rise, so that in the previously inactive hydrogen envelope fusion can use to helium. At the time of the drying up of hydrogen in the core (point A in HRD), the luminosity of the star is already grown to about 2 times the solar value.
The hydrogen shell burning drives the envelope of the star outward, making them cool down, despite a rising central temperature. Since the luminosity strongly depends on the surface temperature, this does not take first, despite the ever-increasing core temperature continues to rise. The star is at this stage a yellow supergiant ( spectral type G, luminosity class IV), which moves in the HRD parallel to the temperature axis from left to right. Thereby increasing its radius of approximately two times.
With decreasing surface temperature of the hydrogen convection reaches deeper into the star down ( again, see stellar structure ) until it finally hits the hydrogen- burning zone. This allows the first nuclear reaction products ( produced from the hydrogen burning shell helium) in the photosphere access point (point B in the HRD ). This stage is reached after about 10.7 billion years ago. The subgiant phase is about 1.2 billion years ago, eight times shorter than the main sequence life.
From point B on the other events accelerated considerably. Through the hydrogen shell burning increases the mass of the helium nucleus, so that the effects of the decreasing number of particles, the increasing atomic mass and gravitational pressure fall more and more. In only about 600 million years ago, the star moves to point C in the HRD, where the combination of high luminosity and low surface gravity can now also increase the mass loss by stellar wind drastically. The luminosity is now about 35 times the solar value, the radius of about 10 solar radii. The star has become a red giant ( spectral type K, luminosity class III). With a mass loss of about 10-10 solar masses per year, the stellar wind is already 10,000 times as strong as that of the sun, but still not sufficient to affect in a short time, the structure of the star key.
From point C of the star requires only about 50 million years to reach a maximum luminosity for the first time. The case covered in the HRD way is called the first giant branch. With regard to the radiated power of the red giant, the Sun now exceeds by about 1,500 times, at radius around the 120 -fold. He now has the spectral type M. His weight loss is so strong that the star in the course of further development loses a significant part of its mass with several 10-8 solar masses per year. The central density is so high that the core as a white dwarf is largely degenerate with about 700 kg cm -3.
Because of this enormous density (and the high central temperature ) can now use the helium burning. This is at the core of a new energy source available, which allows the temperature to rise further. Since the energy yield of helium burning but extremely strongly dependent on the temperature (of the 30th potency), comes a very rapidly -reinforcing process in motion, which is called the helium flash. If the core temperature is sufficiently high, the degeneracy is lifted. This, however, prevailing there gas pressure is again dependent on temperature, which causes a violent expansion into itself. The envelope of the star but is able to trap these. It's not a supernova explosion, but at least to a dropping of the outermost layers cool. The expansion to cool the core, thereby eventually a stable state is reached with quiet expiring nuclear fusion.
With the push-off of the outermost layers of the star is again smaller, and at the hot surface. A K- giant is back with about 47 solar luminosities and 12 solar radii (point D in the HRD ) from the M- giant.
The helium burning spends the red giant with relatively constant luminosity and surface temperature. Only when this source of energy is running low, the star in the HRD moves back to the right. Because this path is not identical with the first giant branch, but shifted to slightly higher surface temperatures down, it gets its own name as Asymptotic Giant Branch.
Again, the cause is for the luminosity increase in the decreasing number of particles ( three helium nuclei transform into one carbon nucleus to ), the increasing atomic mass ( from 1.33 to 1.85 atomic mass units) and gravitational pressure. After about 120 million years ago, the helium is exhausted in the core ( only 1/ 80 of the duration of the main sequence phase ), the point E in the HRD is reached ( approximately 120 times the solar luminosity and 23 times the solar radius ). After a further 40 million years the star has surpassed the first power maximum, he now has about 2500 solar luminosities and 160 solar radii. In its center there is now an inactive, again compressed to degenerate core of carbon and oxygen (the latter goes by the addition of another helium nucleus out to the carbon ) surrounded by a helium- burning shell, the further out subsequent to the hydrogen- burning shell. As shown in the next section, rises on the Asymptotic Giant Branch luminosity on a little further.
Two other figures are intended to illustrate the accelerating development over time again. In order to make the short stage of red giant graphically visible luminosity and mass are plotted not against the age of the star, but the time remaining until the start of the stage of the white dwarf. Is the time represented logarithmically, i.e., from left to right of each development phase are getting shorter. After a leisurely luminosity increase in the main sequence stage and a stable energy supply in the sub- giant phase is followed by a rapid, dramatic increase in the First Giant Branch. The helium - flash fires almost from a luminosity crash, which is followed by a second, but brief period of stability. Finally, a rapid ascent of the asymptotic giant branch.
The mass of the star remains stable over long periods, only close to the first luminosity maximum is applied to the stellar wind is a structure-changing. Until the onset of helium burning of the red giant has lost more than 10 % of its original mass. After the ascent on the asymptotic giant branch, he has lost 30% of the original mass.
Influence of the mass on the stellar evolution
Of all the state variables of the mass has the strongest influence on stellar evolution by far. As an example, compared here the trail in the HRD of a sun-like star chemical composition but with 1.7 solar masses, the just- outlined scenario of a star with 1 solar mass. The points A to E correspond to the same stages of development as discussed here.
In the main sequence stage is the more massive star is far more luminous than the low-mass, the difference is about a factor of 10 Accordingly, the hydrogen supply dries up much faster in the core despite larger initial mass; after about 1.6 instead of 9.5 billion years. Just like a star with 1 solar mass main sequence, the existence is already accompanied by a significant increase in the luminosity.
The subsequent sub- giant phase is particularly short, it comprises only about 40 million instead of 1.2 billion years. She is now characterized by a remarkable luminosity drop, which can be explained by the relatively strong cooling of the stellar surface but. While in the low mass star between the points A and B, the surface temperature decreases to 700 K, it falls in the more massive by about 2000K. However, the luminosity depends sensitively on the surface temperature, namely its fourth power.
The further development is partly surprising analogy, the more massive star lost in particular its initial enormous luminosity projection on the logarithmic scale largely a. His rise to the top of the first giant branch is happening quickly too - in about 80 instead of 600 million years ago - but then with about 2200 solar luminosities, it surpasses the low-mass star, only about a factor of 1.5. Despite different starting compositions and thus luminosities and surface temperatures, the development paths of the stars strong approach having less than about 2.5 solar masses in its late phase to each other. There will be a conglomeration of red giants in the HRD, even if the more massive are slightly hotter than the less massive. Only above about 2.5 solar masses, the initial luminosity differences between stars of different initial mass is retained even in the stage of red giant, this but more and more of the luminosity class II bright giants, is then assigned to.
Even after the helium flash the ways of the lower-mass stars remain close together. The star with 1.7 solar masses has originally with about 86 solar luminosities only a lead of almost a factor of 2 compared to the star with 1 solar mass. This has the consequence that the length of the central firing helium is only slightly reduced to 80 instead of 120 million years. The climb to the Asymptotic Giant takes place, however, again pretty quickly, ie within 15 instead of 40 million years ago. By 2700 solar luminosities the luminosity advantage over the star with 1 solar mass is now almost entirely disappeared.
Since the star with 1.7 solar masses as originally red giant is hardly more luminous than the one with initially one solar mass, one may also expect a much higher mass loss. The models used here say for the more massive to the ascent to the Asymptotic Giant even a lower mass loss ahead. While the lower mass up to that stage about 0.3 solar masses - 30% of its initial mass - has lost, the more massive are only about 0.15 solar masses - not 10 % of its original mass - has been lost.
While at higher stellar masses towards the transition from the giant to the bright giant is flowing, the smallest initial mass red giant 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 had not even the opportunity to leave the main sequence. An even lower mass at the red giant can be set only by violent stellar wind.
Influence of the chemical composition of the stellar evolution
Finally, the role of the chemical composition is shown in an example. The solar-type star with a share of 2% of elements heavier than helium is compared with a star of the same mass, but only a proportion of 0.1 % of "heavy" elements.
The " metal-poor " stars ( with the small proportion of "heavy " elements ) is on the main sequence significantly more luminous, about a factor of 3, the less "heavy" elements contains the stellar matter, the more transparent it is, the weaker the energy are restrained spectral lines. At the same time, the " metal-poor " rating on the surface to about 1000 K hotter. Spectral lines occur primarily in the blue and ultraviolet. This region of the spectrum thus benefited most from the increased transparency.
With the shift toward higher luminosity and temperature of the star in the HRD appears to get under the main sequence of stars of solar-like composition. This was earlier mistakenly interpreted as a luminosity deficit, which such stars " subdwarfs " and called them their own luminosity class VI has been assigned.
The truth in greater luminosity shortened as expected, the main sequence stage - according to the models by about a third. After about 6 billion years, the hydrogen is exhausted in the core. Even in the sub- giant phase of the luminosity difference remains unchanged. Thus falls for the " metal-poor " star also this period of life is shorter - with about 600 million years by half.
The path to the First Giant Branch is about 200 million years even by about two-thirds shorter than in stars with solar-like elements frequency. But doing so they also lost the " metal-poor " star 's luminosity projection, and the duration of the helium Brennes 80 million years ago is again reduced by only about one-third. The climb to the Asymptotic Giant takes place, however, again quickly, it requires about 20 million years ago, only half the time compared to a sun- like star.
Is maintained at least in part to the higher surface temperature. As a red giant, the " metal-poor " star still by about 500-600 K hotter. The giant branches of globular clusters, which are characterized by a particularly low proportion of "heavy " elements (mostly only some 0.01 %), are bluer than in open clusters whose chemical composition is similar to the sun.
Asteroseismology
By means of asteroseismology, it is possible to assess the status of the red giant. The convection in the outer atmosphere excites vibrations that are reflected from the photosphere and density jumps in the star and lead to the formation of a complex pattern of standing waves in the atmosphere of the red giant. The Dichtespünge occur at the edges of the hydrogen-and helium- burning zones where heavy elements arise as a result of the merger. An analysis of minimum brightness variations from the records of the Kepler mission succeeded a classification according to hydrogen- or helium- burning red giant. Thus, the models of stellar evolution of intermediate-mass stars can better verify regarding mass loss and the minimum mass required for ignition of helium burning. The asteroseismology also allows the investigation of the rotation curve within the star. It turns out that the period of rotation of the cores of red giants is considerably shorter than that of their extended atmospheres. The rapid rotation leads to increase mixing in the interior of the star and is thus more fuel for thermonuclear reactions available, thereby extending the life of these stars is changed.
Development of red giant to the white dwarf
As a result of its expansion, the outer gas layers have a very low density, and are only weakly bound by the gravity of the star. Therefore, developing in the course of its red giant stage, a strong stellar wind, through which the outer gas layers are completely repelled; They then surrounded him for some time as a planetary nebula. Red giants weighing less than eight solar masses shrink in consequence to white dwarfs. For more than eight solar masses at the end of helium burning put a further fusion processes until the red giant explodes as a supernova.