
#1
Jul809, 09:34 AM

P: 38

how is it known that iron is formed in stars and not anything heavier? And how is it known that elements like uranium arent in fact formed in the cores of stars prior to supernova? answers much appreciated




#2
Jul809, 11:08 AM

Mentor
P: 15,620

Neither of those statements are true. (Hence the "how is it known" part is superfluous)




#3
Jul809, 11:52 AM

P: 528





#4
Jul809, 12:50 PM

Mentor
P: 15,620

question about origins of elements
Maybe I am having trouble parsing all those negatives.




#5
Jul809, 01:36 PM

P: 38





#6
Jul809, 01:59 PM

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one can measure spectrum of stars and supernovae and find out which and how much elements there are.. how do we know that the sun is made up of mainly hydrogen? Well we look at and analyse the solar spectra...




#7
Jul809, 02:12 PM

P: 38





#8
Jul809, 02:15 PM

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Clearly, one should analyse other stellar spectra as well... DUH! 



#9
Jul809, 02:47 PM

P: 68

Elements heavier than iron do form in the cores of stars, otherwise where would they come from? I think the idea is that no selfsustaining endothermic fusion reaction will create elements heavier than iron. So, we have to wait until the core collapses and dumps copious quantities of external energy into the existing iron and other heavy elements there, before exploding.
We do the same thing in the laboratory by slamming say gold nuclei into tungsten targets. But exploding stars do it on a much grander scale. 



#11
Jul809, 02:52 PM

P: 68





#12
Jul809, 02:54 PM

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#13
Jul809, 03:27 PM

P: 68

What I was saying is that nuclear fusion reactions can be selfsustaining using any fuel with nuclei less than the mass of iron. Nuclear fusion using elements heavier than iron can take place, but will not be selfsustaining because they actually require that energy be input. I think the terminology is that stars fuse elements exothermically (I said endo in the other post incorrectly), and releasing energy. So stars with iron fuel cannot keep generating energy by creating elements heavier than iron. When a star goes supernova, some of the material is slammed into other material at energies great enough to cause elements heavier than iron to form. A sort of grand particle accelerator. Sound correct, or am I totally in the toilet? 



#14
Jul809, 03:29 PM

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#15
Jul909, 02:05 PM

P: 38

so is the whole idea that stars fuse only up to iron during their lifetimes backed up by stellar spectra observations? Or does that idea only come from the fact that anything heavier than irons not a self sustaning reaction.




#16
Jul909, 02:21 PM

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So the story goes as follows: lead is the endpoint of self sustaining thermonuclear reactions, theory statement. YET observations showed that stars DO contain heavier elements, and also elements heavier than Iron was found here on earth so.. So it was then realized that there are more mechanisms of element generation in stars, namely the sprocess. And also one realized that one can have the rprocess in supernovae. So one now had more theories, and those also made some sense! Of course, there are still some small unsolved problems but in total we have clear understanding on how the elements are formed in stars, it is a fine precision and delicate science. Expert litterature: http://www.amazon.com/NuclearPhysic...7167255&sr=81 http://www.amazon.com/CauldronsCosm...ref=pd_sim_b_1 Have fun 



#17
Jul1009, 03:45 AM

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PF Gold
P: 9,183

Actually, nickel is currently believed to be the heaviest element that can be formed end of the fusion cycle in ordinary stars. See
http://helios.gsfc.nasa.gov/nucleo.html, and http://en.wikipedia.org/wiki/Silicon_burning_process Nucleosynthesis cannot account for elements heavier than Ni56 by current theories. The process would actually consume more energy than it produces, as already noted. Note also that once a star starts burning silicon, it is on its last legs. This process can only be sustained for a very short time before core collapse occurs. 


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