Phenomena of red giant and the journey of star

                    Stages of  star towards the formation of a Red Giant 

What is a Star ?

 A star is an astronomical object consisting of a luminous spheroid  of plasma held together by its own gravity. A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily hydrogen , along with helium and trace amounts of heavier elements .When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion , releasing energy in  the process. The remainder of the star's interior carries energy away from the core through a combination of radiative and convective heat transfer process. The star's internal pressure prevents it from collapsing further under its own gravity .Stars condense from regions of space of higher matter density, yet those regions are less denser than within a vacuum chamber . These regions - known as molecular clouds consist mostly of hydrogen, with about 23 to 28 percent helium and a few percent heavier elements .The nearest star to Earth is the Sun. Historically , the most prominent stars were  grouped into constellations and asterism. 

Nuclear Fusion

Nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles (neutrons or protons). The difference in mass between the reactants and products is manifested as either the release or the absorption of energy. This difference in mass arises due to difference in atomic binding energy between the nuclei before and after reaction. Fusion is the process that power active or main sequence stars and other high magnitude stars , where large amount of energy are released. A fusion process that produces nuclei lighter than iron-56 or nickel-62 will generally release energy. These elements have relatively small mass per nucleon and large binding energy per nucleon. Fusion of nuclei lighter than these releases energy ( an exothermic process ), while fusion of heavier nuclei results in energy retained by the product nucleons, and the resulting reaction is endothermic. An important fusion process is the stellar nucleosynthesis that powers stars, including the Sun.

What is a Red Giant ?

                                             

 

A red giant is a star that has exhausted the supply hydrogen in its core and has 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 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. , spectral types of K or M , have surface temperature of 3,000-4,000K and radii up to about 200 times the Sun. Stars on the horizontal branch are hotter , with only a small range of luminosities around 75 Solar luminosity . 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. 

Red giants are evolved from main-sequence stars with masses in the range from about 0.3 solar mass to around 8 solar mass. 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 begin fusing hydrogen ( a few million kelvin ) establishes hydrostatic equilibrium. 

Asymptotic giant branch 

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 carbon and oxygen, a shell where helium is undergoing fusion from 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 ( except in the case of carbon stars).

 AGB stage 

The AGB phase is divided into two parts , the early AGB ( E-AGB) and the thermally pulsing AGB ( TP-AGB ). 

• E- AGB
• TP-AGB

E-AGB

During  the E-AGB phase , the main source of energy is helium fusion in a shell around a core consisting mostly of carbon and oxygen. During this phase, the star swells up to giant proportion to become a red giant again. The star radius may become as large as one astronomical unit.

TP-AGB

  

After the helium shell runs out of fuel, the TP-AGB starts. Now the star derives its energy from fusion of  hydrogen in a thin shell, which restricts the inner helium shell to a very thin layer and prevents it fusing stably. However , over periods of 10,000 to 100,000 years , helium from the hydrogen shell burning builds up eventually the helium shell ignites explosively , a process known as a helium shell flash . The luminosity of the shell flash peaks at thousands of times the total luminosity of the star , but decreases exponentially over just a few years . The shell flash causes the star to expand and cool which shuts off the hydrogen shell burning and causes strong convection in the zone between the two shells. When the helium shell burning  nears the base of the hydrogen shell , the increased  temperature reignites hydrogen fusion and the cycle begins again. 

Late Thermal pulse 

The carbon oxygen core is now surrounded by helium with an outer shell of hydrogen . If the helium is reignited a thermal pulse occurs and the star quickly returns to the AGB , becoming a helium-burning , hydrogen deficient stellar object. If the star still has a hydrogen-burning shell when this thermal pulse occurs , it is termed a late thermal pulse. Otherwise it is called a very late thermal pulse. The outer atmosphere of the born again star develops a stellar wind and the star once more follows an evolutionary track . However this period is very brief , lasting only about 200 years before the star again heads toward  the white dwarf stage. Observationally , this late thermal pulse phase appears almost identical to a Wolf-Rayet star in the midst of its own planetary nebula. 

Super-AGB stars 

Stars close to the upper mass limit to still qualify as AGB stars show some peculiar properties and have been dubbed super AGB stars. They have mass above 7 solar mass and up to 9 or 10 solar mass( or more ). They represent a transition to the more massive supergiant stars that undergo full fusion of elements heavier than helium. Super AGB stars develop partially degenerate carbon-oxygen cores that are large enough to ignite carbon in a flash analogous  to the earlier helium flash . The second dredge up is very strong in this mass range and that keeps the core size below the level required for burning neon as occurs in higher mass supergiants . The size of the thermal pulses and third dredge-ups are reduced compared to the lower mass stars, while the frequency of the themal pulses increases dramatically . Some super -AGB stars may explode as an electron capture supernova, but most will end as oxygen-neon white dwarfs.