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  #16  
Old 01-27-2010, 02:07 PM
horst horst is offline
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it is not so clear cut to me. there are star forming regions inside gas clouds. never heard
of novas around thus places. where comes the iron from. maybe the solar system is special in some way.
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  #17  
Old 01-27-2010, 06:17 PM
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G O R T G O R T is offline
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Quote:
Originally Posted by horst View Post
it is not so clear cut to me. there are star forming regions inside gas clouds. never heard
of novas around thus places. where comes the iron from. maybe the solar system is special in some way.
The iron is already in the gas clouds from previous population 2 and particularly population 3 (the earliest) supernovae. They need not be particularly local as the expanding gas from novae become widespread and mixed.

The metallicity of our sun is quite typical for a population 1 star in this region. High metallicity stars tend to be found in the dense areas near the galactic plane, like ours. Low metallicity stars or older population 2 stars tend to be found farther out in the "halo" above and below the galactic plane.
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  #18  
Old 01-30-2010, 04:26 AM
Mike Dubbeld Mike Dubbeld is offline
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Its the story of stellar fusion. If you have a star with sufficient enough mass and thereby gravity, you can fuse hydrogen into helium and helium into elements with higher atomic numbers all the way up to and including iron. But IRON WON'T FUSE. Thats the rule for our universe. To get 2 protons to stick to each other you have to give them very high kinetic energy (temperature). This is because each proton has a positive charge and 2 like charges repel like 2 north or 2 south poles of a magnet. As soon as 2 protons get close enough, they stick together by the strong force which is about 100 times stronger than the positive electric force that causes them to repel. The strong force is like velcro. It causes protons to stick real tight when you get within a very short distance. But it doesn’t work at all at far distances like the electric force. Now if you try to get 2 helium atoms to stick together you need a lot more energy because 2 protons + 2 protons coming together has a lot more repulsive force than just 2 protons coming together so you have to really heat them up to get to them to stick together. And so it goes fusing helium into carbon and oxygen etc. By the time you get to 2 iron nuclei you are up to 26 protons in each nucleus repelling each other. As it turns out that is simply too much electric force and you can’t get enough mass in a star with enough gravity to cause 2 nuclei with 26 protons each to get close enough to stick together.

If a star has enough mass and continues to be crushed down under the weight of gravity a threshold is exceeded that causes the star to become a type 2 supernova. In this explosion temperatures are reached that allow the fusing of nuclei of atoms that have 26 protons to fuse together. And that is why that all elements above the atomic number of iron are relatively rare.

There are elements that have a higher atomic number than iron 26 but they got there as being debris of a previous generation of stars that went supernova.
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