Giant star

A giant star ( or just giant) is a star with above-average large diameter and above-average big luminosity, the (HRD ) is located at the same surface temperature within the Hertzsprung -Russell diagram well above the main sequence. Usually giants have a radius between 10 and 100 solar radii with a brightness that between the 10 - and is 1000 times our sun.

There are four types of giants.

• Subgiants, stars of luminosity class IV You are in the HRD between the giant branch and the main sequence.
• (Normal) giants of luminosity class III. They form the giant branch in the HRD.
• Bright giants, stars of luminosity class II can be found in the HRD above the normal giant
• Supergiants, stars of luminosity class I. Because of their even higher luminosity, they are still in the HRD on the bright giants.
• Hypergiants, stars of luminosity class 0

In late spectral classes the radiation maximum of giants in the red spectral range. They are therefore referred to this stage as red giants or red supergiants. Accordingly, we referred giant medium or earlier spectral classes as Yellow or Blue Giant.

Development scenarios

Star from about 0.25 solar masses evolve according to consumption of the total available in the core hydrogen fusion into a giant star. In these stars takes place during most of their lifetime inside a continuous convection, that is, there is a steady heat flow within the core, so that the fusion of the hydrogen can continue for a period of more than 1012 years; a period that is much longer than the current age of the universe. Eventually, but its center will become a radiation core, with the result that the hydrogen in the exhaust core and a combustion of hydrogen in a shell around the core begins. (For stars with a mass greater than 0.16 solar masses, this may lead to an expansion of the shell, but this expansion will never be very large. ) Shortly thereafter, the offer will be fully utilized to hydrogen at such a star and he will collapse into a white dwarf with a helium nucleus.

Is a star more massive than 0.25 solar masses, it will contract when the total hydrogen was consumed in its core by the merger. Hydrogen is then burned in a shell around the helium-rich core to helium, wherein the part of the star expanding outside of the shell and cools. During this period of his development such a star will now belong to the sub- giant branch in the Hertzsprung -Russell diagram. This section includes stellar objects whose luminosity remains approximately constant, but the surface temperature decreases. May be such a star also begin to move in the Hertzsprung -Russell diagram in the field of red giants. At this point, the surface temperature of the star, which typically has reached the stage of red giant here, dramatically expand its radius at approximately remains constant luminosity. The core will contract further, which now leads to a continuous increase in its temperature.

From a star that is located on the main sequence and its mass remains below about 0.5 solar mass, it can be assumed that he will never reach the necessary temperatures that are required for the fusion of helium. From such a star is a hydrogen- burning red giant § 4.1, 6.1 will develop, from which a white dwarf will eventually emerge with a helium nucleus.. Otherwise, if the core temperature reaches a value of approximately 108 K, the helium will begin to fuse, forming carbon and oxygen in the core by the so-called three - alpha - process., § 5.9, Chapter 6, the energy caused by fusion of the helium is generated causes the core expands. This leads to an effect in which the pressure in the vicinity of the hydrogen burning shell is reduced, thereby reducing the energy reduced. The luminosity of the star thus decreases its outer shell runs together again and the star leaves the branch of the red giant.

His further development now depends on its mass. If he is not very solid, it will move in a horizontal branch on the Hertzsprung -Russell diagram, or its position passes through the diagram in loops., Chapter 6 If the star is not heavier than about 8 solar masses, it will after some time helium have been used in the core and it begins helium fusion in a shell around its core. Due to this, its luminosity will then increase again and it rises, now as AGB star, in the asymptotic giant branch of the HR diagram on. After the star has lost most of its mass, its core will remain as an existing carbon and oxygen white dwarf. , § 7.1-7.4.

For those main-sequence stars, whose masses are large enough to finally ignite a carbon fusion, this is from about 8 solar masses is the case, p. 189, different scenarios can occur. These stars are their brightness does not increase significantly after they have left the main sequence, but they will appear red. However, you can also develop into a red supergiant or even a blue supergiant, pp. 33-35. ; Similarly, there is the possibility that a white dwarf is formed from them, which has a core made of oxygen and neon. Another conceivable application is the emergence of a type II supernova, which eventually leaves a neutron star or even a black hole. , § 7.4.4-7.8.

Examples

Known giant stars of different fluorescent color are:

• Alkione ( η Tauri ), a blue-white (B- type) giant, the brightest star in the Pleiades star cluster.
• Thuban ( α Draconis ), a white (A- type) giant in the constellation Draco.
• σ Octantis, a white (F- type) giant, which is the southern counterpart of the North Star.
• Capella, a yellow- white (G- type) giant, the main star in the constellation Auriga.
• Pollux ( β Geminorum ), an orange- colored (K- type) giant of the constellation Gemini.
• Mira ( ο Ceti ), a red (M- type) giant in the constellation Cetus.
• VFTS 102 so far the fastest rotating star, as a giant star in the Large Magellanic Cloud.