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Nuclear Fusion in Stars & Element Formation

  1. May 3, 2016 #1
    1. The problem statement, all variables and given/known data
    Ok, these questions are very simple but they are really bugging me and I would greatly appreciate an explanation. Question 1 is "which of the following elements must have been made in stars". The options are hydrogen, helium, carbon, oxygen and iron.
    Question 2 is "which of the following elements may not have been formed in either stars or supernovae". Options are hydrogen, helium, carbon, gold and iron.

    2. Relevant equations

    3. The attempt at a solution
    The answer in the book for Q1 only accepts carbon, oxygen and iron. Stars fuse hydrogen into helium for most of their life so how is this not also made inside stars? The only thing I can think of is that hydrogen and helium were formed in the early universe whilst the other three were not and therefore must be made in stars? I really don't know. Answer for Q2 only accepts hydrogen and helium. I get hydrogen as it is not technically formed in a star (it is the fuel) but why is helium not formed in a star. Or is it the wording where they say 'may' and 'or' meaning that helium is formed in a star but not in a supernova.

    Thanks for any help offered.
  2. jcsd
  3. May 3, 2016 #2


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    If that's the wording, they only want elements that can't have been made otherwise. So H and He don't qualify.
  4. May 4, 2016 #3
    Is that because they are produced else where i.e. shortly after the big bang?
  5. May 4, 2016 #4


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    Obviously. Stars couldn't form unless they had something from which to form.
  6. May 4, 2016 #5
    How is that obvious? Stars are formed from hydrogen not helium aren't they?
  7. May 4, 2016 #6


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    Yes, that's true. But the hydrogen and the helium had to come from somewhere before the first stars evolved. The Big Bang did not create the first stars; therefore you need star stuff before you can have stars.
  8. May 4, 2016 #7


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    Look at the Wikipedia entry for the Big Bang .

    More interesting to me is the Wikipedia entry Big_Bang_nucleosynthesis .

    The beginning of this reads:

    In physical cosmology, Big Bang nucleosynthesis (abbreviated BBN, also known as primordial nucleosynthesis) refers to the production of nuclei other than those of the lightest isotope of hydrogen (hydrogen-1, 1H, having a single proton as a nucleus) during the early phases of the universe. Primordial nucleosynthesis is believed by most cosmologists to have taken place from 10 seconds to 20 minutes after the Big Bang, and is calculated to be responsible for the formation of most of the universe's helium as the isotope helium-4 (4He), along with small amounts of the hydrogen isotope deuterium (2H or D), the helium isotope helium-3 (3He), and a very small amount of the lithium isotope lithium-7 (7Li). In addition to these stable nuclei, two unstable or radioactive isotopes were also produced: the heavy hydrogen isotope tritium (3H or T); and the beryllium isotope beryllium-7 (7Be); but these unstable isotopes later decayed into 3He and 7Li, as above.

    Essentially all of the elements that are heavier than lithium and beryllium were created much later, by stellar nucleosynthesis in evolving and exploding stars.

    (Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc., a non-profit organization.)​

    An additional question might be regarding which of these elements; helium, carbon, gold and/or iron, might (else might not) be formed in a star by fusion. (That is: prior to any explosion type of episode.)
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