Elements inside a star

  • #1
Hello,

Is there any specific reason that a star is primarily composed of hydrogen and helium and not any other metals?

Thanks.
 

Answers and Replies

  • #2
Bandersnatch
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The reason is those are the elements that the universe is predominantly composed of.

Anything heavier needs to be fused in the stars themselves, and then ejected into the interstellar medium.
That is why only the first generation of stars(aka population III) after big bang was really purely helium+hydrogen. Each subsequent generation does have heavier elements, with increasing concentration.(still a tiny amount compared to H & He)
Read up on "metallicity" on the wiki for more information about the subject.

Weinberg's "First Three Minutes" as well as wiki article on Big Bang nucleosynthesis go into more detail about the creation of elements in the early universe.
 
  • #3
phyzguy
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Because that is what the universe is made of. Stars are formed by gravitational collapse of clouds of gas. In our neighborhood, the gas is made up (by mass) of about 70% hydrogen, about 28% helium, and about 2% everything else. There isn't any significant differentiation in the formation of a star, it just forms from whatever is there.
 
  • #4
Chronos
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Once the primordial hydrogen in the universe is exhausted, we could see some very exotic stars form - perhaps composed primarily of carbon and an admixture of other heavy 'metals'. Of course we are talking about the very remote future. Only a tiny fraction of the hydrogen supply has been 'burnt' to this stage in the life of the universe. You may find this interesting: A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects, http://arxiv.org/abs/astro-ph/9701131.
 
  • #5
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Are heavier elements only formed in a star? The Earth, and the other planets in our system, are composed of Nickel, Iron, Carbon, etc. When/if Sol goes nova, would those elements then be spread about, possibly to be accreted into another body?
 
  • #6
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Yes all heavier elements are formed in stars. If Sol were to nova its heavier elements could supply those elements to other bodies
 
  • #7
phyzguy
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The sun is not massive enough to form elements heavier than Carbon and Oxygen. Heavier elements (iron, nickel, etc.) are formed in core collapse supernovae, which only occur in stars heavier than about 8X more massive than the sun, and in Type 1A supernovae, which are formed in double stars with at least one white dwarf.
 
  • #8
Drakkith
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  • #9
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The sun is not massive enough to form elements heavier than Carbon and Oxygen. Heavier elements (iron, nickel, etc.) are formed in core collapse supernovae, which only occur in stars heavier than about 8X more massive than the sun, and in Type 1A supernovae, which are formed in double stars with at least one white dwarf.
At first blush, those conditions do not sound very common. How likely could it be that the heavier elements were originally formed in the manner in which you describe, but then 'recycled' from other bodies which contained those elements but were destroyed in a nova when the star they orbited went nova?
 
  • #10
Drakkith
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At first blush, those conditions do not sound very common. How likely could it be that the heavier elements were originally formed in the manner in which you describe, but then 'recycled' from other bodies which contained those elements but were destroyed in a nova when the star they orbited went nova?
I'm unsure of what you are asking, but I can say with confidence that supernova nucleosynthesis is a well accepted fact in astrophysics that accurately describes how the elements around us were created in the proportions that we observe.

http://en.wikipedia.org/wiki/Supernova_nucleosynthesis
 
  • #11
Chronos
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You need energy levels not believed possible in ~solar mass stars to create elements heavier than silicon.
 
  • #12
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As I understand it elements up to bismuth can be created in stars via fusion. Elements up to uranium uses the Neutron capture method mentioned above. Elements heavier are believed to be formed in supernova. Its been awhile so could be wrong on that
 
  • #13
Chronos
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Even silicon is a stretch for a solar mass star. Please give a citation that affirms the formation of busmuth.
 
  • #14
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Not sure how far I trust this reference. Its. Been awhile since I last looked into heavy element formation. I am currently seeking something more reputable.

http://en.m.wikipedia.org/wiki/Supernova_nucleosynthesis#section_2

In nuclear fusion processes during stellar nucleosynthesis, the maximum weight for an element fused is that of iron, reaching an isotope with an atomic mass of 56. Prior to a supernova, fusion of elements between silicon and iron occurs only in the largest of stars, in the Silicon burning process. A slow neutron capture process, known as the S-process which also occurs during normal stellar nucleosynthesis can create elements up to bismuth with an atomic mass of approximately 209. However, the s-process occurs primarily in low-mass stars that evolve more slowly. Once the core fails to produce enough energy to support the outer envelope of gasses the star explodes as a supernova producing the bulk of elements beyond iron. Production of elements from iron to uranium occurs within seconds in a supernova explosion. Due to the large amounts of energy released, much higher temperatures and densities are reached than at normal stellar temperatures. These conditions allow for an environment where transuranium elements might be formed

Below is an older thread on this.
https://www.physicsforums.com/showthread.php?t=37551
 
Last edited:
  • #15
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http://arxiv.org/pdf/1212.0211v1.pdf

the above is probably the most reliable but to me its not very clear.

http://suite101.com/article/origin-of-the-chemical-elements-a23458


There are two processes to manufacture elements heavier than iron. They are the slowly occurring s process and the rapidly occurring r process. One at a time, neutrons slam into atomic nuclei and subsequently decay into protons by releasing electrons. Adding a proton forms the next element on the periodic table.

Red giants last long enough for this s process to slowly manufacture elements as heavy as bismuth on the periodic table.

Elements heavier than bismuth can only be manufactured by the r process during a supernova. The successive fusion reactions leave an iron core, which collapses and rebounds. The rebound triggers a Type II supernova explosion that releases as much energy as the Sun does in 10 billion years and a very large number of neutrons. These neutrons rapidly slam into atomic nuclei and decay into protons. This r process manufactures elements heavier than bismuth


I've been unable to find what I would consider an ideal breakdown. However as you can see by several of the links I provided that there is processes for higher that Iron in stars prior to what is produced in supenova
 

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