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

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As the hydrogen shell continues burning, the degenerate main point growshotter and hotter without expanding. Helium, gift a larger nucleithan hydrogen, requires more kinetic energy to fuse, which meanshigher temperatures. In ~ 100 million degrees, helium deserve to be convertedto carbon through the triple-α process.Triple-α Process:Energy generation is the vital to the lifetime of a star. The proton-protonchain and also the CNO cycle room the resource of energy for main sequence stars,but as helium builds up in the core, the star must uncover another source ofenergy or it will certainly die out. Fusion can happen between any nuclei if themotion of the nuclei is fast enough to conquer the electrostatic repulsionbetween the protons. Because that the garbage product of the proton-proton chain orthe CNO cycle, helium, over there is double the lot of electrical charge (twoprotons) so the is twice as difficult to fuse them contrasted to protons.
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In the triple-α process, three He nuclei space fused to type carbon (technically, twoHe fuse an initial to type beryllium (Be), then beryllium and also another the fuse to kind carbon;but this wake up so quick that Be just exists because that a few microseconds).Carbon is the waste product plus power in the form of gamma-ray photons.
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The energy released by the triple-α process continues to warmth thecore elevating its temperature even more. Again, under normalcircumstances, the heater would rise the pressure and also the corewould expand and cool. But, with the core being degenerate, thetemperature goes up however the core does no expand. Highertemperatures means a quicker triple-α rate, which method moreenergy, which means greater temperatures, etc...When the temperature that the main point reaches 300 million degrees, a nearlyexplosive usage of the helium bring away place called the helium flash.During the an extremely short helium flash (a few minutes), the star emits moreenergy than 100 times the calculation of the whole Galaxy. However, thisenergy never reaches the surface yet instead goes right into removing thedegeneracy the the electrons and also expanding the core.For stars much more than 2 solar masses, the triple-α procedure startsbefore the electrons end up being degenerate. And, so, there is no heliumflash, just a gradual change to a main point helium burning an ar surroundedby a hydrogen burning shell.Horizontal Branch Stars:After helium burning begins (either explosively through a flash, orgradually for more heavier stars), the star has actually two resources of energy,hydrogen combination in a shell roughly the core and also helium fusion in thecore. Helium burns into carbon, and also carbon combines through helium tomake oxygen. The core of the star becomes rich in carbon and oxygennuclei, and the star"s surface ar temperature goes up to become ahorizontal branch star.
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Stars v masses greater than or same to the Sun end up being smaller andhotter in ~ a consistent luminosity. Lock evolve to horizontal branch starsby moved throughout the HR diagram at continuous brightness. Short mass stars atabout 10 solar luminosities, high massive stars (10 solar masses) in ~ about200 solar luminosities. An alert that together they evolve, HB stars overcome theinstability strip. Because that a brief time, high mass stars will be Cepheidvariables and also low massive stars will certainly be RR Lyrae stars.
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Asymptotic large Branch Stars:After existing as horizontal branch stars because that a few million years,the helium in the main point of the star is exhausted (now being mostlycarbon and also oxygen nuclei) and also a helium burning shell will developunderneath the hydrogen burning shell. The electrons and also nuclei in the core againbecome degenerate and the star expands and cools to become an asymptoticgiant branch star.
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Most of the power is comes from the hydrogen burn shell, thehelium burning covering is small at this time. However, the hydrogenshell is dumping helium ash ~ above the helium shell. After sometime,enough helium is collected so the the helium covering undergoes anexplosive event called a thermal pulse.
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The thermal pulse is barely i found it at the surface of the star, butserves to increase the fixed of the carbon/oxygen core, so that the sizeand luminosity of the star progressively increases v time. Together the starclimbs the asymptotic large branch, a wind creates in the star"senvelope which blows the external layers right into space. The is in this windthat dust corpuscle (important because that interstellar clouds and also proto-solarsystems) are formed from carbon product dredged increase from the main point by convective currents.
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During this time, a special dust shell blocks the visible light fromthe star such that also though it is 10,000 brighter 보다 the Sun, itis just seen in the IR.To summary the advancement of a mainly core, the following figure showsthe alters in a high massive star end time.
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The stellar wind reasons mass loss for AGB stars. This lose is around10-4 solar masses per year, which method that in 10,000 yearsthe usual star will dissolve, leaving the central, hot core (the centralstar in a planetary nebula). If the star is bigger than 8 solar masses,then the core proceeds to heat. Carbon and also oxygen fuse to kind neon,then magnesium, climate silicon. All developing into burning shells surroundingan iron ash core.

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Iron is unusual in that it is extremely stable and resistant to fusion.The temperature that an iron core have the right to reach 3 billion degrees. When theiron core reaches a vital mass, the collapses, violently, into asupernova explosion.
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