Nuclear Reactions in Stars Without Hydrogen (Salpeter, 1951)

The second paper that we will be considering is the paper “Nuclear reactions in Stars without Hydrogen” by E.E. Salpeter from 1951 (Astrophysical Journal). Oh don’t you just love these shorty, pithy and to the point papers! I know I do.

In this paper the triple-alpha process is elucidated, and interestingly discussion of heavy atom synthesis is also discussed. Salpeter didn’t get everything correct (we now know) but 1951 was a pretty good year for him. He also published a very important pair of papers with Bethe, who is a towering figure in this area.

Since m,any of your questions have been posted (and very good questions they were as well), I’ll throw out one or two more things for you to ponder. How does the synthesis of carbon-12, Fred Hoyle and the anthropic principle relate to all of this. What the heck is the anthropic principle anyway?

The discussion for this paper will be on Wednesday 16th September.


8 Responses to “Nuclear Reactions in Stars Without Hydrogen (Salpeter, 1951)”

  1. Daniel Begay Says:

    It’s amazing to see how many of men and women, who would eventually make huge contribution to science, had to make some type of escape from the Nazis.

    The first thing that stuck out to me was Salpeter making a reference to the carbon-nitrogen cycle, because it being an early version of the CNO cycle (which didn’t involve a stable isotope of oxygen).

    My question is stems from beginning of the last giant paragraph. After the star has used up its He supply and the core temperature increased, it begins to break down lighter nuclei in He and H. These He and H are then used to make even heavier nuclei, up to an atomic weight 40 to 60. During this dissociation, what determined which “lighter nuclei” are broken down into He and H. Would it just be the reverse of the nuclear reaction discussed in the paper? If so, what is keeping the newly made He and H from beginning the He4–> Be8 —> C12 reaction again?

  2. Kevin Greenwood Says:

    I too thought that the last paragraph is where this paper really picked up. Of course the first half of the paper makes a significant contribution in building a bridge over the “pit of death” where stable nuclei cannot be generated by the processes covered so far in the semester. Saltpeter does offer up some explanation as far as the relative abundances of the smaller nuclei by mentioning that Coulombic repulsion becomes a bigger factor in step with nucleus size. What stuck out to me is that besides the first and last paragraphs, there are no references in the meat of this paper. His main argument for the triple alpha process hinges on the abundance of beryllium-8, which he states is abundant in roughly 1 parts per 10 billion, but doesn’t provide any evidence as to how he got that number. I don’t doubt the process, it would just be helpful to show how he got some of these numbers.
    One thing I would like to go over from the third paragraph is what happens at 2 billion Kelvin. Saltpeter states that at this temperature, protons and alpha particles can start flying off nuclei and recombine with other nuclei to make all the varieties of elements up to masses of 40 or 60. Also at this temperature, the Urca process (cool backstory to that name which I won’t go in to) starts kicking off energy in the form of neutrinos and the star cools off and contracts, as soon as the stable elements are formed and no longer are giving off energy. Does that mean that these two processes are occurring simultaneously, and that the only thing keeping the star from collapsing in on itself is the energy given off by the formation of heavier nuclei?

  3. Tony J Says:

    So is this paper not in agreement with the other as far as the origin of elements or is it apples and oranges? It seems to show a more logical solution to “why the relative abundance of elements?” even proposes formation up to uranium- four at a time, alpha particle to alpha particle, nucleus to nucleus.

    Lastly I am not familiar with subatomic particles. How are neutrinos formed and what is decaying them or forming them? Are they really dark matter (or is the internet full of it)? Where do they go?

    • profhurst Says:

      I don’t fully understand the question, but yes it does agree, we need several mechanisms to get from the simpler to the heavier elements. Perhaps give a little more though it writing up and laying out your argument/insight.

      As for neutrinos, yes they are a standard member of the particle zoo as a quick wikipedia search will reveal. Energy can create them and they are needed to fully balance certain atomic reactions.

  4. Hunter Burgin Says:

    My question for this paper also deals with E.E. Salpeter’s speculative final paragraph. Salpeter mentions that as the star exhausts its Helium supply and becomes composed primarily of carbon, oxygen, and neon (by way of the triple alpha process) it contracts gravitationally and increases in temperature to 10^9K. He also goes on to say that because of the high temperature, collisions between carbon-12 nuclei can occur which will yield magnesium-24 nuclei. At these high temperatures he also briefly mentions the importance of lighter nuclei disassociating into helium and hydrogen nuclei. This remark made me wonder if, with the reemergence of protons and alpha particles, previous reactions seen in stars such as the CNO cycle (due to the presence of protons) and the triple alpha process (due to the presence of helium nuclei) could begin again. It would be my guess that the current temperature of the star would be the deciding factor as to which energetic process would be most favorable.

  5. Adam Settimo Says:

    Ahh, so we finally get to the Triple Alpha Process. I’ve been wondering when we would come to this, and it seems to be in 1951; though, we still haven’t heard anything about nickel-56
    Ok, so these are my initial questions from my first read. I’m sure I will have more later. From the last paragraph of the second page, I feel that there are a lot of things that are confusing me. Is the paper saying that this is the only energy production occurring in a star of this size. If these processes are a matter of relationships of temperature and pressure wouldn’t the same processes happening in smaller stars be possible at shallower depths (not in the core) of these stars? I realize that the stars the paper is referring to have converted the majority of their hydrogen to helium, but there was hydrogen at some point during the stars life.
    Also, the age of these stars keeps being stated to be less than 10^9 years, does the size of the star contribute to a smaller life span, or a faster consumption/conversion of hydrogen to alpha particles?

  6. Porter Marsh Says:

    The paper uses a B8 star as an example and says that the internal temperature is 2*10^7 K but that the CN cycle requires temperatures of 2*10^8 K. There are “a few million years” between the exhaustion of the H in the star and the internal temperatures reaching 2*10^8K. My question is about how the star continues existing for a few million years with no fuel. The paper says that gravitational energy is the only energy in the star for the millions of years before the triple alpha process becomes viable. How does gravity explain the continued existence of a star for millions of years with no fuel?

  7. Adam Settimo Says:

    Ok, some more questions…

    When the hydrogen supply has been exhausted from the CN(O) cycle, and the star undergoes gravitational contraction, is there a specific event that starts this? I.E. A specific ratio of hydrogen to helium? or specific density? or is there some event(s) not explained?
    Also, 10 ppb of beryllium-8 at any given time seems like a very small amount to be floating around, is this a limiting factor or a bottle neck to the formation of other heavier nuclei? If so, is this a slow reaction, or is the formation of carbon-12 almost instant as the beryllium-8 is formed?

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