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Question about Star Main Sequence & Red Giants

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bbrock

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When a star consumes it's hydrogen and helium it leaves the main sequence and begins to swell into a red giant. Such as Antaries, Betelgeuse etc. <br /><br />Why would a star swell when it begins running out of hydrogen? It begins to get cooler. I would think it would get smaller. <br /><br /> Then the story goes that it finally consumes all of it's fuel and then crashes in on it self to become a white dwarf and a planitary nebula or blow up as a supernova and forms a neutron star or a black hole. It's the red giant part that I don't understand. <br /><br />Bill
 
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dragon04

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The short answer is that when Helium fusion begins, the star can't find equilibrium at its hydrogen burning diameter, so it expands. Helium fusion causes higher outward pressures.<br /><br />Here's a good article:<br /><br />http://en.wikipedia.org/wiki/Stellar_evolution <div class="Discussion_UserSignature"> <em>"2012.. Year of the Dragon!! Get on the Dragon Wagon!".</em> </div>
 
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bbrock

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This answers it. Thank you so much. I have never seen this explained anywhere before. <br /><br />Clipped from that reference Article. -----------<br /><br />Maturity<br />After millions to billions of years, depending on its initial mass, a star has exhausted all the hydrogen in its core. Larger and hotter stars consume their hydrogen much more rapidly than cooler and less massive ones. Once the core's ready supply of hydrogen is gone, nuclear processes there cease.<br /><br />Without the outward pressure generated by these reactions to counteract the force of gravity, the outer layers of the star begin to collapse inward on the core. The temperature and pressure increase as during formation of the protostar, but now to even higher levels, until helium fusion begins at core temperatures of around 100 million kelvins.<br /><br />The very hot core causes the outer layers of the star to expand enormously; the star becomes as much as 100 times larger than it was during its main sequence lifetime. It is now a red giant, and the helium burning phase lasts for a few million years. Almost all red giants are variable.<br /><br />What happens next depends, once more, on the star's mass.<br /><br />--------------------------------------<br /><br />Thanks<br />Clear Skies<br />Bill
 
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thalion

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As this is something that puzzled me when I first learned about it, this is a subject close to my heart.<br /><br />The reason stars swell and cool when they become red giants is:<br /><br />1.) Once the inner core runs out of hydrogen, it contracts and heats up, but does not become hot enough to start fusing the helium "ash" just yet.<br /><br />2.) The contracting core heats up the surrounding hydrogen envelope, which is where fusion starts once again--basically creating a hydrogen-burning "outer core" around an inert, hot helium "inner core".<br /><br />3.) The new layer of fusion burns a larger volume of hydrogen than the original core did, which progressively increases the star's luminosity. The increased radiation pressure makes the star grow in size.<br /><br />4.) Even though the star is brighter, its surface temperature can't quite "catch up" with its luminosity, because of it's larger size and volume; this makes the star cooler than it was on the main sequence, even though in sum it's brighter.
 
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bbrock

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OK<br /><br />So on initial contraction, the hydrogren surrounding the core begins fusion. But at some point soon after, helium would need to begin fusion. The remaining hydrogen would be burning up at a furious rate. The star is now swelling to enormous size. That takes a lot of energy. <br /><br />When does helium begin fusion. <br /><br />
 
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Saiph

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Star forms from gas cloud, compacts and heats up.<br /><br />Core gets hot and compact enough for hydrogen fusion. Star stops contracting.<br /><br />Core runs out of hydrogen, begins contracting, heating up. <br /><br />Envelope of star (non-core regions) is expanding...because the core is contractign and heating up. At this point the star isn't powered by fusion, but waste heat and gravitational energy from the core collapse.<br /><br />Core compacts, begins helium fusion (may even "flash" if star is smaller).<br /><br />Helium fusion stops contraction of core, also pumps energy into hydrogen shell around core. <br /><br />Hydrogen shell <i>also</i> begins fusing, has a larger surface area, and is closer to the suns surface.<br /><br />These two sources provide more energy, per second, than the initial hydrogen burning. The star swells out to am even larger radius. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>
 
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thalion

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Initially, the helium core simply contracts and heats while the hydrogen-burning shell expands. Eventually though, the (now degenerate) helium core reaches the temperature necessary for fusion. In stars of a few solar masses or less, helium fusion begins explosively, with pretty much the entire core "lighting up" at once. This releases a huge amount of energy, but not enough to blow the star up. In fact, lighting the helium core stabilizes the star, allowing it to shrink and heat up in a relatively brief, "clump giant" phase where it burns helium in its core and hydrogen in a surrounding. <br /><br />Inevitably, the helium core becomes choked with "ash" once again, only this time it's carbon and oxygen, with maybe some neon. The core collapses again, forming a degenerate center surrounded by a helium-fusing shell, and *another* hydrogen-fusing shell. This causes the star to swell up again to dimensions much larger than it was on the earlier red giant branch. Ultimately, failure to light the carbon-oxygen core causes the unstable interior to blow off the surrounding shells, creating a planetary nebula.<br /><br />For stars bigger than a few solar masses, helium fusion starts smoothly and evenly--there's no helium flash--and fusion progresses upwards towards various other elements. In the end though, gravity always wins.
 
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newtonian

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BBrock - Please note that this is the standard model for "average" main sequence stars.<br /><br />It does not take into account how different main sequence stars can be.<br /><br />One of the differences is magnetic fields and properties.<br /><br />Scientific American ran an article on one recently discovered resulting difference: magnetars.<br /><br />The model other posters above are citing assumes zero mixing of core areas with outer layers before the main sequence star enters red giant phase.<br /><br />If a star, like our sun, has sufficient motion from core to surface driven by unusually powerful magnetic dynamos involving ions in motion, then some mixing will occur from core to surface - and this will throw off more than just the timing of entry into red giant phase.<br /><br />The discovery of extreme differences in stellar magnetic fields is new - much more remains to be discovered besides the existence of magnetars.<br /><br />Could it be that is not only earth that is unusual in properties? Could is also be that our sun is unusual in properties?<br /><br />Is our sun's corona unique in being so much hotter than the solar surface due to the influence of magnetic dynamoes deep within the sun?<br /><br />I submit that our sun will enter red giant phase much later than assumed, with considerably less remaining mass than assumed because of fusing a larger percentage of outer layer hydrogen before red giant phase than assumed.<br /><br />This is just an educated guess, btw. More study and research, with more discovery, needs to occur yet.
 
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bbrock

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You guys are good. <br /><br />Doe's our sun - in fact - have sufficient mixing from core to surface due to magnetic fields. Actually that is a different topic. My original questions has been analyzed in many forms and permutations --- as it always seems to be with the nature of things. <br /> <br />Much Thanks<br />Bill
 
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abhinavkumar_iitr05

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What actually happens is that in the euilibrium state the internal pressure developed by the nuclear fusion of the hydrogen to helium balances the gravitational collapse thus the star shines for a long time.Now since the size of the core is smaller than the periphery thus the fuels in the center is consumed earlier but the hydrogen outside the core still burns.Since the source of the energy is much closer to the outer shell the shell expands thus giving the view that whole star expand.But it is actually the outer shell which expand.This makes the star more luminous but reduces its effective temperature thus making the star to appear red.<br /><br />The core in the meanwhile goes on shrinking & when the temperature becomes sufficiently high to start helium carbon cycle it appears to be a white dwarf.<br /><br />Here I am posting a link for more information.The links are<br /><br /> ONE <br /><br /> TWO <br /><br /> THREE
 
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bbrock

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Very excellent articles! I'm printing these out for future reference. Much Thanks<br /><br />Clear Skies<br />Bill
 
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newtonian

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BBrock - On stellar mixing from core to surface - I don't know - it was a suggestion.<br /><br />I do know our sun has very strong magnetic fields, though I doubt as strong as the stars that become magnetars.<br /><br />I was simply citing the fact that stars can be the same in one category, as in main sequence, and still be very different in other categories - notably magnetic field dynamo strenths and properites and hence corona temperatures and proterties, etc., etc. <br /><br />We still have much too learn!<br /><br />There are other factors that will change the timing of change from main sequence to Red Giant phase - notably collisions and mergers. A brown dwarf with the right mass, speed and trajectory can actually rejuvenate an old main sequence star - see Scientific American article on When Stars Collide.
 
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