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The Sun's Fire

  1. Mar 20, 2003 #1
    First let's work our way up.

    1. For a fire to start, oxygen has to be present, yes?

    2. There is no oxygen in space(vacuum), yes?

    Since they are surrounded by vacuum, they have absolutely no way to obtain the oxygen to support combustion. Then how is it that the Sun and stars can burn?

    Just a little question too, what is the difference between the Suns and the stars?
  2. jcsd
  3. Mar 20, 2003 #2


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    Stars don't "burn" the way fire does. You are right that fire requires oxygen. Stars generate energy through nuclear fusion - exactly like a large nuclear bomb. The core of the sun is so hot and under such pressure that hydrogen atoms fuse together to form helium, releasing huge quantities of energy, mostly in the form of visible light.

    A sun and a star are pretty much the same thing. Typically (i think) a sun is a star with planets around it.
  4. Mar 20, 2003 #3
    "mostly in the form of visible light"...not really...this is a very small part of it...
  5. Mar 20, 2003 #4
    I have heared that before scientists understood nuclear fusion they believed that the sun must be a burning ball of some fuel such as coal. From this assumption they came to the conlusion that the sun could only have been in existance for so many years before all of this fuel was used up. Does anyone know what this value for the life of the sun was?
  6. Mar 20, 2003 #5
    I don't know when anyone ever calculated it as a ball of fire... but back in the 19th century, a common hypothesis was that it was producing light by gravitational contraction continually heating it up. Kelvin got an age of ~30 million years this way, about ~4 billion too low.
  7. Mar 20, 2003 #6
    Thanks. My physics teacher must have made that bit up. I'm in year 11 at school (15 years old) so he's probably simplified it.
  8. Mar 20, 2003 #7


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    Like russ_watters said, stars are not burning fires. They are collections of matter that release energy from fusion reactions in the core.

    Typically, they are huge collections of mostly hydrogen gas. This is all compressed together by gravity. In the core, the compression is so great that the hydrogen atoms fuse together to form helium. That releases some energy. That is the "fire". Like bogdan said, it's not all visible light.

    This is for stars in their "main sequence" (active part of their "lifetime") of course. "Dead" stars (black holes, neutron stars, white dwarfs) have used up their fuel.

    The kind of fire we are familiar with here on Earth is a different kind of energy release....it's the rapid oxidation (insertion of oxygen) of a combustable material (fuel). That rapid oxidation releases energy (the flames and heat).
  9. Mar 20, 2003 #8


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    I always thought the radiation was centered on the visible spectrum in the blue range. Not sure though.
  10. Mar 20, 2003 #9
    The sun's photosphere is about 5,800K, which peaks at 500nm, which is in the yellow.
  11. Mar 21, 2003 #10
    Why hydrogen being compressed into helium? Why not let's say carbon being compressed into zinc or something like that?

    How is it that hydrogen can change into another element helium under pressure? Can other elements do so too?

    Why won't the heat like "set fire" to the rest of the stores of hydrogen, just like how the hydrogen in my test tube goes 'pop' when i insert a burning splinter?

    Forgive me, i'm not trying to be difficult or anything. I just like to tresh things out, allows me to understand better.
  12. Mar 21, 2003 #11


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    Greetings BP !

    As you may be aware, hydrogen is the first
    element - which means it has one electron in
    the orbital around the atomic nuclei. The atomic nuclei
    of hydrogen is one proton. Hydrogen has isotops -
    one proton+one neutron = deutirium and
    one proton+two neutrons = tritium.
    The next element is helium (with two protons) and
    so on - up Mendeleyev's Element's table.

    Nuclear Fusion is a proccess when atomic nuclei
    join together. For example - deuterium and
    tritium fusion is a proccess during which
    the two neuclei join together and become a helium
    nuclei with one neutron and energy in the form of
    EM wave radiation released.

    Nuclear Fusion is only achived when we have
    atomic nuclei at very high energies - like
    the pressure and energies at the core of the Sun
    (caused by gravity). The reason for this is that
    the nuclei are both charged with a positive electric
    charge (the protons) and like charges repel.
    For the nuclei to fuse they have to have sufficient
    energy to come close enough together, despite
    the electric repulsion, for the nuclear forces to take
    over and combine them.

    Now, atomic nuclei that are fused have lesser energy
    per proton/neutron than the separate nuclei
    prior to the fusion reaction. Here, I'm not talking
    about their kinetic energy, but about the energy of
    the nuclear forces in the nuclei.

    So, we have nuclear fusion in stars' cores.
    Now, the release of energy from the reaction
    is not happenning for all elements. The energy
    gained from each fusion reaction decreases as heavier
    elements are fused together into even heavier
    elements. At a certain point - the 54th element = iron
    (if my memory serves me right...), the reaction
    no longer produces energy. This the reason
    why in addition to hydrogen which comprises
    about 90% of the matter in the Universe, helium and
    iron are the most common elements (the iron is
    produced from dying stars that run out of their
    hydrogen supply, and the helium was created
    with the hydrogen at about a 1 to 10 ratio after
    the BB).

    For "heavier" (more protons) elements the reaction
    draws energy and the opposite reaction - Nuclear
    Fission - which is atomic nuclei "falling apart" into
    "lighter" nuclei, takes place.

    Atomic power plants in fact harness the energy
    released by Nuclear Fission.

    More questions ?

    "Does dice play God ?"

    Live long and prosper.
  13. Mar 21, 2003 #12


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    drag, I'm sure thats a good explanation, but far too long to read.

    Because there is very little of anything else in the Sun. Its virtually all hydrogen. Hydrogen was the only element existing in the early universe. All others were created in stars.

    It doesn't change, you just combine two atoms into one. I'm gonna guess here that you're in 8th or 9th grade (from that experiment you mention), but you've learned a little about atoms, right? A hydrogen atom has 1 proton, a helium atom has two. Combine two hydrogen atoms and you get a helium atom (1+1=2). Simple. And thats why they call it "fusion" - you fuse two atoms together. It also just so happens that doing this releases vast quantities of energy.

    Yes. Pretty much the whole lower half of the periodic table can all do this. Most of those reactions don't produce nearly the energy as hydrogen fusion though. Atoms that are too big won't create stable new atoms (there is a limit to the size of an atom).

    Great experiment. I remember doing it. As you correctly noted in your first post, there is no oxygen in space, so the sun can't explode in that way. And fusion requires a specific temperature/pressure combination to occur. The sun is in a naturally occurring balance. Some stars (larger than the sun) late in life will lose that balance and begin to shrink and cool, which increases the pressure eventually causing the fusion to accelerate and the sun to explode.
    Thats the entire point of this forum.
  14. Mar 21, 2003 #13
    Bubonic - Great questions! And you've gotten some thorough replies. Looks like the only two questions not answered yet (or not correctly) are these two:

    1) What's the difference between the "Sun" and stars?

    Correct answer: "Sun" is the the proper name of one and only one star in the universe. Ours. No other star can properly be called "the Sun." That's it's name. Many stars have their own name, too, like Rigel, and Alpha Centauri.

    "Star" is a generic term. I believe the Greek (?) translation is "Sol" which gives rise to words like our "Solar" system. Technically, other stars with planets around them are called "planetary systems" not "solar systems."

    2) Jack asked: a) Was the Sun ever thought to be a burning ball of coal? b) how long would that have burned?

    Answer: a) Yes. b) About 5,000 years.

    Explanation: In the mid-1800s, during the Industrial Revolution, coal was the most common form of fuel used. As such, it became the next in the long line of suggested solar fuels. In 1848 a young German doctor named Julius Robert Mayer used the annual Solar energy output recently calculated by Herschel and Pouillet to estimate the likely lifetime of the Sun if it burned coal. The answer of 5,000 made it obvious that coal was not the solar fuel or the Sun would have burned up a long time ago.
  15. Mar 22, 2003 #14
    In that case, why won't it like

    *Numbers in the brackets are the proton numbers.

    Possibility 1:
    Hydrogen(1) and Hydrogen(1) combine to form Helium(2), then 2 Heliums(2) get compressed until they form Beryllium(4), then 2 Berylliums(4) get compressed to form Oxygen(8), then 2 Oxygens(8) get compressed to form Sulphur(16), so on and so forth?


    Possibility 2:
    Hydrogen(1) and Hydrogen(1) combine to form Helium(2), then maybe 1 Helium(2) and 1 Hydrogen(1) get compressed to form Lithium(3), the maybe 1 Beryllium(4)(an element that may form in the Sun as speculated in Possibility 1) and 1 Lithium(3) get compressed to to form Nitrogen(7), then maybe Nitrogen(7) gets compressed with Oxygen(8)(an element that may form in the Sun as speculated in Possibility 1) to form Phosphorus(15), so on and so forth?

    What is the difference between fusion reactions and exothermic reactions?

    I'm in Secondary 4, but i don't know how that translates to the Grade system. Yep!

    When the teacher isn't looking, we'd have a 'pop' competition. We'll try and compete to see who can do the loudest 'pop'. Childish really, but when your're bored to death and driven up the wall by long school hours, even the most mediocre drop of fun does wonders.

    I remember reading somewhere that things in space need to "push" something out from behind them to move, so how is it photons can move through space?

    thx :smile:
  16. Mar 22, 2003 #15
    More good questions! :)
    It does! There is helium-helium fusion, and later the CNO cycle that go on. It just so happens that these reactions don't kick in until the star is very old and very compressed. Once you get to iron, fusion is no longer favorable: proton-proton repulsion (like charges and all that) stops the process. Some old stars have huge shells of iron because of this.
    A fusion reaction is one where two atoms merge to create a bigger atom. An exothermic reaction is one that releases heat (energy) -- like many chemical reactions, which alter molecules but not individual atoms.
    Answer one: that's just to accelerate; things that are already moving keep moving without doing anything.
    Answer two: they just do. Photons are one of those things that it's best not to ask many questions about. Until you are ready to learn quantum mechanics, and are used to being constantly confused, that is. :)
  17. Mar 22, 2003 #16


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  18. Mar 23, 2003 #17
    If both possibilites 1&2 happen, then won't the stars also have small amounts of elements like
    Lithium, Beryllium, Boron, Fluorine, Neon, Sodium, Magnesium, Aluminium, Silicon, Phosphorus, Sulphur, Chlorine, Argon, Potassium, Calcium, Scandium, Titanium, Vanadium and Manganese?
    (Basically all the elements from proton number 1 - 26)

    Why does the cycle involve carbon, nitrogen and oxygen, why not any consecutive xyz element below iron? eg. silicon, phosphorus and sulphur.

    Why does the cycle not involve more then 3 elements, wxyz? eg. boron, carbon, nitrogen and oxygen.

    Try me. If you don't, then i will never be able to know if i am or am not ready.:smile:

    For PP chains, nearing the end of the chain, why won't the 2 Helium-3s just fuse to form a Beryllium atom with 2 neutrons? Why must it form a Helium-4 and give out 2 Hydrogen atoms?

    When will gamma rays be released? When will beta-rays be released? When will alpha-particles be released?

    If Carbon-12 can just capture a proton and release gamma-rays, can other elements do so too? eg. Vanadium-51 capturing a proton and releasing gamma-rays.

    In the CNO cycle, when
    Why isn't the isotope called a mass-13 isotope of Carbon, why a mass-13 isotope of Nitrogen? What is the difference between a mass-13 isotope of Carbon and a mass-13 isotope of Nitrogen?

    If there is a PP chain, proton-proton chain, why not a EE chain, electron-electron chain? Or a NN chain, neutron-neutron chain?

    How does a catalyst work?
  19. Mar 23, 2003 #18


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    The "heavy element" shells in a large star can be seen at:


    And heavy element formation has some info at:


    Because Carbon has atomic number of 6 (protons), and the capture of another proton means it isn't Carbon anymore. Nitrogen has 7 protons, and an isotope of any element with 7 protons is a Nitrogen isotope, not Carbon. It is not the mass that defines the element, it is the number of protons. Each sucsessive reaction needed to create (fuse) to heavier elements requires a higher temperature, hence more mass and compression by gravity.
  20. Mar 23, 2003 #19
    Beats me exactly. Complicated reasons. Certain fusion processes are energetically more favorable than others. Some will occur often at stellar temperatures and pressures, some will not. Because of details of atomic structure, certain reactions will be easier than others, certain cycles are self-catalyzing. It's basically chemistry stuff. I'm sure there is some tome on stellar processes or chemistry with detailed explanations as to why those cycles are favored, but I don't know 'em. :)
    That was more a general statement on light... nothin specific here. You could also justify it by thinking of light as a wave; waves always travels at the propagation speed through some material.
    Ask a chemist! :) Depends on the type of catalyst. The basic idea is that for a reaction you often need multiple particles to interact; a catalyst can kind of "stick" to the necessary particles and help bring them into close enough proximity to react. Or it may promote the formation of some intermediate compound/atom necessary for the reaction, and then get released at the end.
  21. Mar 24, 2003 #20
    Well, it's in my syllabus, but the textbook gave a rather vague and unsatisfactory answer. So i tried asking my Chemistry teacher and he also said roughly the same stuff as the textbook, so i decided to ask this here, 'cause maybe someone here might be able to answer my question.
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