le>Stellar Evolution: Red Giants
Red gigantic Evolution:Readings: Schneider & Arny: Unit 64
A star"s advancement after the red giant phase counts on that is mass.For stars better than 1 solar mass, but less than 2 solar masses, thehydrogen burning shell eats its way outward leaving behind more heliumash. Together the helium piles up, the main point becomes an ext massive andcontracts. The contraction heats the core as that becomes much more dense. The density of the core increases to the point where the electrons and helium nucleibecome degenerate. The core begins to act much more like a liquid 보다 a gas,and it becomes incompressible and further convulsion stops.

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As the hydrogen shell continues burning, the degenerate core growshotter and also hotter without expanding. Helium, being a larger nucleithan hydrogen, requires much more kinetic energy to fuse, which meanshigher temperatures. In ~ 100 million degrees, helium have the right to be convertedto carbon v the triple-α process.Triple-α Process:Energy generation is the key to the life time of a star. The proton-protonchain and also the CNO cycle room the resource of power for key sequence stars,but together helium builds up in the core, the star must discover another source ofenergy or it will die out. Combination can happen between any type of nuclei if themotion the the nuclei is fast enough to get rid of the electrostatic repulsionbetween the protons. For the garbage product of the proton-proton chain orthe CNO cycle, helium, there is double the lot of electric charge (twoprotons) so that is twice as difficult to fuse them contrasted to protons.
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In the triple-α process, 3 He nuclei room fused to form carbon (technically, twoHe fuse an initial to type beryllium (Be), climate beryllium and another he fuse to form carbon;but this wake up so rapid that Be only exists because that a few microseconds).Carbon is the waste product plus power in the type of gamma-ray photons.
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The power released by the triple-α process continues to warmth thecore elevating its temperature even more. Again, under normalcircumstances, the heating would boost the pressure and also the corewould expand and also cool. But, v the main point being degenerate, thetemperature goes up yet the main point does not expand. Highertemperatures method a quicker triple-α rate, which method moreenergy, i beg your pardon means higher temperatures, etc...When the temperature the the main point reaches 300 million degrees, a nearlyexplosive usage of the helium takes place called the helium flash.During the really short helium speed (a couple of minutes), the star emits moreenergy 보다 100 time the calculation of the totality Galaxy. However, thisenergy never reaches the surface however instead goes right into removing thedegeneracy of the electrons and expanding the core.For stars an ext than 2 solar masses, the triple-α process startsbefore the electrons come to be degenerate. And, so, there is no heliumflash, simply a gradual change to a main point helium burning an ar surroundedby a hydrogen burn shell.Horizontal Branch Stars:After helium burning starts (either explosively with a flash, orgradually for more heavier stars), the star has two sources of energy,hydrogen blend in a shell around the core and helium blend in thecore. Helium burns right into carbon, and also carbon combines with helium tomake oxygen. The core of the star becomes wealthy in carbon and oxygennuclei, and also the star"s surface ar temperature goes up to come to be ahorizontal branch star.
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Stars with masses greater than or same to the Sun become smaller andhotter in ~ a consistent luminosity. Lock evolve come horizontal branch starsby moved throughout the HR diagram at constant brightness. Short mass stars atabout 10 solar luminosities, high mass stars (10 solar masses) in ~ about200 solar luminosities. Notice that as they evolve, HB stars cross theinstability strip. For a quick time, high mass stars will be Cepheidvariables and also low mass stars will certainly be RR Lyrae stars.
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Asymptotic large Branch Stars:After existing together horizontal branch stars for a few million years,the helium in the main point of the star is worn down (now gift mostlycarbon and also oxygen nuclei) and a helium burning covering will developunderneath the hydrogen burning shell. The electrons and also nuclei in the core againbecome degenerate and the star expands and cools to come to be an asymptoticgiant branch star.
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Most of the energy is coming from the hydrogen burn shell, thehelium burning shell is small at this time. However, the hydrogenshell is dumping helium ash top top the helium shell. After ~ sometime,enough helium is built up so the the helium covering undergoes anexplosive event dubbed a heat pulse.
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The thermal pulse is barely i found it at the surface ar of the star, butserves to rise the fixed of the carbon/oxygen core, so the the sizeand luminosity the the star progressively increases through time. Together the starclimbs the asymptotic huge branch, a wind develops in the star"senvelope i m sorry blows the external layers right into space. That is in this windthat dust corpuscle (important because that interstellar clouds and proto-solarsystems) are created from carbon material dredged increase from the main point by convective currents.
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During this time, a thick dust covering blocks the visible light fromthe star such that also though the is 10,000 brighter 보다 the Sun, itis only seen in the IR.To summarize the evolution of a mainly core, the following figure showsthe alters in a high fixed star over time.
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The stellar wind causes mass loss because that AGB stars. This loss is around10-4 solar masses every year, which method that in 10,000 yearsthe usual star will certainly dissolve, leaving the central, warm core (the centralstar in a planetary nebula). If the star is larger than 8 solar masses,then the core proceeds to heat. Carbon and also oxygen fuse to form neon,then magnesium, climate silicon. All forming into burning shells surroundingan iron ash core.

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Iron is unusual in that it is incredibly stable and resistant to fusion.The temperature of an stole core deserve to reach 3 billion degrees. Once theiron main point reaches a an essential mass, that collapses, violently, into asupernova explosion.
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