This movie summarizes the advancement of a star like our Sunafter it has actually reached the main sequence. evolution of a low-mass star on HR diagram now in lecture, ns will shot to explain what"s behind this shifts and also countershifts;how the reactions at the center of a star affect itsvisible outermost layers.Stars ~ above the main sequence fuse hydrogen to heliumin their cores.Since low-mass stars procedure their hydrogen relativelyslowly, they remain on the key sequence for a long time.But what happens once they ultimately use up all the hydrogenin your cores?The answer turns out come dependon the specific mass the the star:the greater it is, the much more violent the end.Very short mass: gradual decrease of hydrogen fusionStars which start out v much less massthan the sunlight -- around 0.4 solar masses or much less --have a fully convective interior:
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Hydrogen fuses come helium only in the main core,but the convective activities mix the helium-richproduct throughout the whole interior.At the finish of your main-sequence lifetime,they are uniformly helium-rich:
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As the star runs lower on hydrogen, the rateat which it generates energy gradually declines.Gravity traction the external layers inwards top top the core,but the temperatures never rise high enoughfor any type of other atom reactions to take it place.Slowly, gradually, the star becomes fainter, cooler,and smaller.Eventually, it will shrink to a cold round aboutthe dimension of the Earth: a black dwarf.Of course, this is all just speculation.Stars with such tiny masses take it a long, longtime to run through all their hydrogen fuel.A star of 0.2 solar masses might take a sunshine years to use up every its hydrogen. existing age of universe: approx 15,000,000,000 years red dwarf lifetime: 1,000,000,000,000 yearsThere hasn"t to be time yet for a solitary very low-mass starto use up every its hydrogen, so we can"t check to seeif our models space correct!Sun-like mass: helium burning together a red giantStars favor our sunlight differ from your lower-massbrethren in a chistoricsweetsballroom.comical way: their main cores transfer warm outwardsvia radiation, fairly than by convection.

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The star evolves slightly on the HR diagram, away from the mainsequence.
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This means that together hydrogen fuses come helium in the core, the resulting helium continues to be there.As a result, a helium-rich zone build up in thecenter the the star, and also hydrogen fusesonly in a shell approximately the core:
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Material in the hydrogen-burning shell is trappedbetween the outer layers and also the inert helium core.Pressure and also temperature rise, significantly increasing the rate at which power is generated.During the shell-burning phase, the extra power pushes the external layers the the star really faroutwards, raising the dimension of the photosphere.As the photosphere moves far from the warm inner regions,it cools off.The star begins to relocate to the right in the HR diagram.
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Now, together the hydrogen concentration in the coredecreases, and also the price of fusion likewise decreases,the outer layers press inwards as result of gravity.The temperature and also pressure that the gas in the corerises.Because there"s so small hydrogen left in the core,ordinary combination reactions cannot develop enough energyto push the outer layers back, also at greater temperatures.The mainly thermostat is broken,so the force of gravity pulls the star inwardson the main point harder and also harder.Eventually, the temperature get a chistoricsweetsballroom.comicalpoint of roughly 130 million Kelvin (!).Helium nuclei in the core move so promptly that they are able to conquer their electric repulsion because that each other and also fuse togetherin the triple alpha process:
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Note that the intermediary product producedin action 1, beryllium-8,is very unstable:it will radioactively decay ago into two helium-4 nuclei within much less than 10^(-16) seconds.In order for a third helium-4 nucleus to smashinto the beryllium-8 before it decays,the density of the mainly core need to be very high:roughly 7700 times that of water.The rate of energy generatedby the triple-alpha procedure is even much more sensitive to temperature than that the the CNO cycle: 41 rate of power production = (something) * TQ: If the temperature in the core of a star boosts by 10%, by exactly how much go the power generated by helium blend increase?The minute at which the within core begins to fusehelium to carbon is referred to as the helium flash.After that occurs, the star settles down to anew phase of helium burning. When again,there is a pretty thermostat to regulate theinternal properties.The star moves onto a location in the HR chart sometimes dubbed the horizontal branch.
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At this point, the star consists of a really extended external envelope, largely hydrogen a hydrogen-fusing covering a helium-fusing core
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This helium-burning phase of the star"s life lastsquite a lengthy time.Not as long as the hydrogen-burning main-sequence phase,for 2 reasons:first, not all the hydrogen has been turned into helium.Second, the helium-fusing reactions occur at a higher ratethan the hydrogen-fusing reactions, as result of the highertemperature and also pressure that the innermost core.To a an extremely rough approximation,a star fuses helium for about one-tenth the lengthof that main-sequence lifetime.The Sun, for example,will spend roughly one billion years as a red giant.As the helium fuses into carbon, the carbon progressively accumulatesat the facility of the star. Collisions between carbon-12 and a helium nucleus have the right to createthe stable nucleus the oxygen-16,which rises with the carbon concentration.Eventually, the helium blend is minimal to a shelloutside a central core that inert carbon and oxygen:
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And, when again, during a shell-burning phase, the star move again up and to the right, alongtheasymptotic large branch.
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Will the temperatures ever before rise high enough to ignitethis core of carbon?No, no for stars which begin out with less than around 4 solarmasses of gas.Instead, the helium- and hydrogen-fusing shellsgradually expand outwards.The procedure isn"t always smooth --the combination reactions in both shells often tend to operation in fits and also starts.If the hydrogen-fusing shell doesn"t fuse quicklyenough, because that example,then the helium-fusing shell may run short on fuel,and shut down until more helium becomes available.These helium shell flashescan have solid effects top top the external envelopeof the star ...Evolution of the external envelope: planetary nebulaeWhat wake up to the outer envelope that a Sun-like staras the inner regions switch indigenous fusing hydrogen come fusing helium?Two things: it swells outwards greatly, increasing the star"s diameter by much more than a variable of one hundreds it cools off, dropping from around 6000 levels to listed below 3000 degrees
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When our sun reaches the red gigantic stage, its external layers will increase to gulp down the planetsMercury, Venus, and maybe also Earth.
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Because the outer layers of a red gigantic are for this reason farfrom the star"s center,the gravitational force on castle is really weak.If the pressure of the radiation native the interiorof the star have to increase,it can quickly shove the photosphere outwardsa big distance;or, if the radiation pressure decreases slightly,the outer layers can loss inwards a long waybefore they with equilibrium again.Stars in the red giant phase are an extremely oftensomewhat unstable:their external layers may pulse outwards and also inwardsover timescales of days to years.We can see these pulsations directly:as a star"s photosphere changes in radius and in temperature,the lot of visible irradiate it emits varies.Many red giants are long-term change stars.Some pulse really regularly:
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Thanks to the AAVSO for offering this data. Q: How plenty of months does it take for Mira"s brightness come go through one complete cycle? Q: exactly how much brighter is Mira at maximum than minimum? a. Around 7 time brighter b. Around 40 time brighter c. Around 630 times brighterIf one details pulse is particularly strong, it might push a portion of the external envelope outwardsso hard that the gas paris off into room and never returns;in other words, that may provide the gas a speedbeyond the to escape velocityof the star.This is one method that red giants may lose some of their mass.Even without sudden pulses,most red giants have very solid stellar winds:flows that atoms, ions and also molecules from their external atmosphereinto space.A red giant"s wind may lug as lot as ten millionthsof its mass far into an are each year;the Sun"s solar wind, through contrast, pushes about one hundreds trillionth the its mass away each year.The repercussion of every this mass loss native red giantsis the development a large shell of material speeding outwardsinto room around the star.Many stars lose their whole envelope, exposing theirhot inside regions.
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When the ultraviolet irradiate strikes the atoms in the shell, the excites them come high power states.As the atom drop earlier down to lower levels,they emit light together beautiful planetary nebulae. Q: Why space they dubbed "planetary" nebulae?Some look at like basic spherical shells that gas, such as M57,the Ring Nebula:
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But rather exhibit traces of complicated structure,such together the Cat"s-Eye Nebula:
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MyCn18,
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or the Eskimo Nebula:
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It appears that numerous planetary nebulae have actually a bipolar structure,in which product flows preferentially in oppositedirections native the star:
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It"s no clear exactly what creates the framework withinthese glow clouds that gas.It might be early the results of stellar rotation on product flowingoff the extended envelope.It could be because of magnetic fields.In part cases, it could derive native the orbitof a binary companion roughly the red giant.At the moment, we just don"t know planetary nebulae.But the doesn"t stop civilization from doing your bestto do models that the procedure.Here are a couple of movies mirroring simulations of jetscoming from an old star punch bipolar balloon in the surroundings, created by Vincent Icke. Quicktime movie (low-quality version) Quicktime movie (high-quality version) The bitter end: a white dwarfAfter it has worn down the helium in its inner shell,a star has no means to generate an ext energy:there is no an ext hydrogen no one helium in that is hot, densecentral regions.When the push of radiation stops flowing outwards,the external layers of the star (if any type of are left)fall earlier inwards.The star shrinks and shrinks ~ above itself,becoming denser and denser --but not endlessly.When that reaches a size roughly equal come the Earth"s,collisions in between electrons in that is coreprovide enough pressure to halt the collapse.The star handle down into a last state the equilibriumas a white dwarf:a very, very dense body,with the massive of a star however the dimension of a planet.Q: at its present size, the sunlight has around the same overall density as water. Once it becomes a white dwarf the dimension of the Earth, exactly how much denser will certainly it be?White dwarfs start out really hot, because of the high temperaturesof their inner regions.It bring away that warmth a lengthy time to leak out to the surfaceand radiate away right into space.Because they space so small, white dwarfs emit very littleenergy, regardless of their high temperatures.Being an extremely hot, but very feeble, they loss in thelower-left edge of the HR diagram.

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It take away billions the years for white dwarfs to cool,but eventually they do, movingever down and also to the best on the HR diagram.Their fate is to finish up together cold, dense, lonelychunks the matter: black dwarfs.Some day, our sun will sign up with their ranks.For more information The American combination of variable Star Observers has decades of observations of change stars; you can access it anywhere the Web. Professor Joel Kastner that the Imaging scientific researches Department research studies planetary nebulae -- you might ask that why they look the way they do. Hydrodynamics the Planetary Nebulae is an whistoricsweetsballroom.come-up by Vincent Icke describing exactly how one have the right to use a computer to simulate the development of a planetary nebula.
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