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I Star formation and heavy elements

  1. Jan 12, 2018 #1
    Hello. First post here so hi all.
    My question(s) is regarding the formation of solar masses by accretion of gases and dust. From what I understand, stars are formed by large clouds of gasses and dust particles pulling together and with enough gravity, (magnetic fields), and time. A fusion process begins. Afterwards, planetary bodies, dwarf planets, moons, etc will form (if there is sufficient gas and matter to do so). So everything in a Solar system is comprised of the matter that it originally started *barring anything from outside said system being added to the chemical make up.

    Gravity being an effect of mass (to me) means that higher mass equals higher gravity. Simply; Larger bodies attract with greater ability then smaller bodies.

    My understanding is that this should be true of elemental density as well. Heavier elements have higher atomic mass and (should) have greater gravitation attraction then lighter elements. So here’s the thing that is confusing me with stars, solar system formation, and accretion process. Our planet has vast amounts of heavier elements e.g. iron. Most of the planets and moons seemed to be comprised of similar matter but If gravity pulls gases and matter towards the center of a forming star, how is it that stars are mostly formed of the smallest elements? Hydrogen & helium but mostly hydrogen? Shouldn’t the heaviest elements (have) pulled to the center of a (developing) gravity well with the lighter elements swirling in the outer layers of rotating cloud? If (as seems to be said a lot these days) the fusion of iron by a star marks the end of a stars (due to iron’s energy absorption) ability to push back against gravitation collapse with consistent/equal energy generation, then how do stars not suffer from rapid fusion decay/energy loss due to already containing heavier metallic elements from the initial formation?
  2. jcsd
  3. Jan 12, 2018 #2
    Because in the begining of the universe (or in the another words when the first stars formed) there was mostly hydrogen and helium. So In this sense mostly all of the stars contains hydrogen and helium. (If you think more you'll see that our star has a lifetime of 10 billion years). So in this sense I guess its normal to think that the stars that we see around is made by mostly hydorgen because in early times there was only hydrogen and mostly helium and just not enough iron or heavy metals. Of course stars contains heavy elements but not that much respect to the hydrogen due to this reason.

    For more information you can find this site useful

    Last edited: Jan 12, 2018
  4. Jan 12, 2018 #3
    Hello. Thank you for the link (interesting info) to types of stars. It answered some but also raised another. If iron is a "fusion killer", how can some metallic containing stars continue to keep from collapsing within short periods of time? (millions of years vice billions). The article in the link makes sense that first stars formed within the time from the universal starting point (big bang) to when hydrogen/helium/light elements started to form approximately 340,000-390,000 after the big bang. So yes, first population III stars at that point can only be made up of the elements around them. Our sun is a population I star, so shouldn't it have large amount of heavier elements?

    I suppose the real problem I have is disinformation or misunderstanding of "why and when" iron can have devastating effects on a stars energy production?
    Once iron is fused inside a star, it will suffer gravitational collapse within a short period of time.
    So is my understanding of the statement in error?

    A. If iron is already present in small traces during protostar formation, its not an issue.
    B. If a star has fused all the lighter elements and starts to fuse iron, that's the beginning of the end.
  5. Jan 12, 2018 #4
    Both A and B are true I think. There are couple reason why I think like that, First reason is even there's some amount of iron in the star, there's also large amount of hydrogen which goes under fusion and produces large amount of energy so at this begining stages nothing happens. But when the star consumes most of the fuel and heavy elements start to produce (like iron), then star starts to die cause there's not much enough energy to keep the star in gravitaitonal balance.

    A quote

    "When a star is fusing iron in its core, it’s still giving off insane amounts of energy. The helium, hydrogen, carbon, oxygen, and silicon are still there in the star in different shells. Hydrogen is at the surface, still fusing to helium; a little further down, helium fusing to carbon and oxygen; further down we have silicon until the core, where silicon fuses to iron. This is why the star still exists and doesn’t spontaneously explode the moment the first iron atom pops into existence".

    At this point, the energy process is just no longer exothermic but endothermic. Iron cannot be fused into anything heavier because of the insane amounts of energy and force required to fuse iron atoms. The atomic structure of iron is very stable, more so than most other elements. I’m not saying all other elements are radioactive or unstable, just that iron is slightly more stable than the previous elements.

    Stars this massive can turn into several things; it depends on how heavy it is. They can explode into supernova, collapse into various types of neutron stars, or even form a black hole. The iron in the star’s core isn’t the reason why the star went supernova, its overall mass made it explode. But, the iron in its core caused it to die."


    If you are asking why not much higher then the current value, well I dont know why, simply maybe there wasn't enough heavy metal in the nebula or maybe some of them created planets (a small ratio compared to sun's mass but still..)

    Hope this answers your question about it
  6. Jan 12, 2018 #5
    Our Sun and similar stars formed from gas and dust that was still primarily made up of hydrogen and helium.
    There would have been also some amount of heavier elements, those having been cooked up in earlier stars that ended their life as as supernovae.
    I don't think the term 'a large amount' is really appropriate, since in total, elements heavier than helium amount to around 2%.
    Most of it being still fairly light elements, Carbon, Oxygen, Nitrogen.
    The truely heavier elements may seem like a lot because the Earth and other rocky planets and objects exist,
    but these comprise still only a tiny amount of the whole content of the solar system.
    98% of the solar system is hydrogen and helium in the Sun.
    Last edited: Jan 12, 2018
  7. Jan 12, 2018 #6

    jim mcnamara

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    Metallicity is the term astronomers use. It is confusing. In this context it means: the content in a star of every element on the table of elements above hydrogen and helium.


    It is a measure of the "age" of a star, as well as whether the star was formed out of "recycled" matter or formed very early on in the age of the universe. Population I stars are newbies, made from "old" hydrogen and helium and newer heavier elements that came from a supernova. Population II stars are "oldies" formed a long time ago when there was not much in the way of heavier elements. Let's ignore Population II stars for now.

    Next. Iron can be in a star and NOT stop fusion. It may have gotten there from the remnants of a supernova because it is a population I star, for example.
    What the "iron thing" is about is that fusion of iron into heavier element cannot occur in normal stars like our sun. So when lots of iron builds up the star becomes increasingly hotter, and if the star is big enough it becomes a supernova - then iron gets fused into elements like gold.

    So. We have elements in us humans and in the Earth's crust that came from a supernova, which is kind of cool. And our sun does have iron in it and and other heavier elements, too, just in incredibly small amounts. It seems happy to continue fusion.
  8. Jan 12, 2018 #7
    Thank you (again). This has helped my understanding a lot and I appreciate the time and patience you have taken to explain it. The evolution of stars has fascinated me for years and the various stages they go through are interestingly complex yet simple when looked at from a novice POV. The final stages of a stars life are extremely curious as they can (as you said in the previous response) become a number of different and altogether extraordinary objects. Stars that with potential to become Super/Hypernovae, destructive power to the Nth degree plus new material for future stars and planets. But the truly massive stars which can become; Black Hole's, Quasar's, magnetar's, neutron stars, or even just a stellar core.
    My person favorite is the magnetar {Babcock's star specifically}. My interest in the sun lead me to websites like SOHO, SDO, and NASA. These lead to wanting to know more about astronomy and astrophysics. The question I asked was a result of knowing slightly more and starting to question the deeper mechanics of it.
    Thank you again.


    Now if they can just figure out what is causing GRB's ((unknown source) assumed to be from the birth of black holes or neutron star collisions).
  9. Jan 12, 2018 #8

    Hello and thank you for the response.

    As I am also happy that it (the Sun) continues to fuse. :)
    {There is no fusion 'Halfinum'}
    So (If I am understanding you correctly sir) it isn't necessarily the content OR the fusion process of iron. It (end of a stars life) is dependent of the physical size/fusion ability of any given star and the element that's fused isn't the factor. The characteristics of the star that come into play more so then what elements it is producing via nuclear fusion.

    Larger stars then ours can produce iron without ill effect, smaller stars may never achieve fusing iron due to the limitations of size and mass.

    Essentially, there isn't any "death metal" specific to every star, there are only the limits of the individual star to fuse heavier elements until they cannot fuse any higher.

  10. Jan 12, 2018 #9

    jim mcnamara

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  11. Jan 12, 2018 #10


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    There is another relevant factor and that is the bigger a star, the faster its reactions are and the shorter its lifetime, before it actually goes Nova (producing heavy elements). The early biggest stars, that formed a supernova will have supplied the Universe's heavy elements relatively quickly, compared with the more modest sized stars that can be composed of this 'second hand' material in nebulae resulting from supernovae.
  12. Jan 12, 2018 #11


    Staff: Mentor

    You have to be careful about what exactly you are comparing.

    If you have a single atom of hydrogen and a single atom of iron, then yes, the iron will have stronger gravity than the hydrogen. Or if you have, say, ##10^{45}## atoms of hydrogen and ##10^{45}## atoms of iron, the iron will have stronger gravity than the hydrogen. But that's not the right comparison to be making for the scenario you are concerned with. See below.

    Because there are vastly more atoms of hydrogen than of heavy elements. For example, the Earth has roughly ##10^{45}## atoms, mostly of heavier elements like iron. But the Sun has roughly ##10^{58}## atoms (actually nuclei, since the Sun is too hot for atoms to exist, it's a plasma), mostly of hydrogen. That's why the Sun is so much more massive than the Earth and is the primary source of gravity in the solar system. The same was true of the cloud of matter that originally formed the solar system; yes, iron atoms are heavier than hydrogen atoms, but there are so many more hydrogen atoms that the latter are what count as far as gravity is concerned.
  13. Jan 13, 2018 #12

    stefan r

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    Suppose you started with a 0.1 solar mass ball or pure iron. Then you dump hydrogen on it. The iron would stay in the center. The hydrogen layer would increase in density until fusion started. Then it would puff directly into something similar to a red giant star. Fusion would occur at or very near the boundary between the iron and the hydrogen layers. The energy from the fusion will blow material out. The density of hydrogen would drop and the fusion would slow down. As hydrogen reached the surface it would continue fusing. A solar mass star with an iron inner core would burn hydrogen much faster than the sun. "Fusion slowing down" means burning hydrogen thousands of times as fast as the sun instead of tens of thousands.

    The only places that have a stellar mass ball of iron is the core of huge stars. Large stars collapse into neutron stars or black holes. When stars form the gas and dust is mixed. The core of large stars fuse to become iron but started out with the same mix as other stars.

    Building a star is not within NASA's budget. It is only hypothetical.
  14. Jan 15, 2018 #13


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    Moderator's note: A number of off topic posts have been deleted.
  15. Jan 15, 2018 #14
    Yes, that is something I had originally considered. The state of different elements existing as plasma, it would simplify (as well as complicate) many questions of how a star works.

    I decided against thinking that about stars (nuclear fusion process) operating this way as scientist can determine what (elements) stars are fusing based on their radiated color/light. So (attempting) to think along these rationals, stars internally have both conditions for complete atomic nuclei of elements AND matter which has become plasma(?). There could be both at different layers of stars. Much in the same way that the planet Jupiter is a Gas Giant but internally is solid. Different states of matter at different layers. The idea that Jupiter has enough pressure to compress hydrogen into a metal-like material at its core for me raised the question of what EXACTLY is the physical state of atomic and sub-atomic matter in a stars core?
    (This isn’t a question for response, only one other thing I question and hope to understand one day).

    Thank you for replying and correcting my usage of element/mass and the gravitational comparisons.

    I appreciate the knowledge, “boot to the head”.

    Respectfully & Gratefully,
  16. Jan 15, 2018 #15
    They cannot make a (fusioning) star but a donut (Tokamak)? Sure. Order us up that Magnetically-bottled ring please.

    Thank you for the input, I appreciate it. Yourself and others here have provided some truly insightful information and some links to even more to help me understand solar process’s.

    Respectfully & Gratefully,
  17. Jan 16, 2018 #16

    stefan r

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    We only see radiation from the photosphere. A thin outer layer. Fusion only occurs in the core. Many stars have convection zones but most do not reach all the way from the surface to the core. Convection does not transport a pure sample of a deeper layer.

    You can get the temperature and pressure required to fuse elements in a lab on earth. Stars go through "dredge ups" later in their life cycle. From the elements/isotopes that emerge at various stages we can learn a lot about what happened and when.

    Degenerate matter. Electron degenerate matter should be in the core of Jupiter, most of a white dwarf, and the core of red giant stars.

    The sun is plasma all the way down.
  18. Mar 31, 2018 #17
    You were wondering why all the heavy elements didn't all sink to the center of the sun, and instead made the inner planets? Well, previous supernovae infused the interstellar medium ( clouds of hydrogen gas) with the heavy elements. These elements permeated the solar nebula so they were distributed throughout the solar system. As the sun and planets formed, these heavier elements indeed migrated toward the inner part of the solar system due to the gravity of the sun, and these are what formed the inner planets, so your intuition was correct!
  19. Apr 2, 2018 #18
    No, they did not.
    Silicon, a lighter element, is abundant in inner planets. Argon, a heavier element, is not.
  20. Apr 2, 2018 #19

    stefan r

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    Silicate rock is denser than argon gas at room temperature. In a distillation apparatus oxygen (O2) and argon separate from nitrogen as a group. Argon is separated from oxygen in a second more difficult step. Chemically pure argon is produced by burning something because separating O2 from Ar by distillation is painful. Argon will liquefy after methane and carbon monoxide but slightly before nitrogen.

    I had the impression that a new star blows gas away. Much of Earth's iron was already in the inner solar system.
  21. Apr 2, 2018 #20
    And low pressure. Like that of solar nebula.
    Sun blew away what was gas, whether light elements (hydrogen and helium) or heavy elements (xenon), including stuff which is not gas at room temperature and atmospheric pressure but is vapour at room temperature and space vacuum (water). What was solid dust grains remained - whether it was heavy elements (uranium) or light elements (lithium, which becomes chemically bonded to silicate rocks).
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