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Question about star formation

  1. Oct 23, 2010 #1
    Hello everyone;
    I am a new member and have little formal hard science ed, but have read many pop science books by Sagan, Hawking, and others. In light of this fact please try to formulate answers with this in mind... NO MATH SPAMMING ALLOWED lol

    My question has to do with the chemical/molecular makeup of a star and the gas clouds they form from. If I understand correctly (never a sure thing :uhh:), stars are composed mainly of hydrogen. I would be interested in knowing what percentage of a star is actually hydrogen and what other molecules are present.

    The reason for this question stems from a Discovery Science show I watched a while back that mentioned that when stars begin forming lead in their cores that it begins a rapid reaction that leads to the "death" of that star. This led me today to wonder about the earth's molten metal core, and THAT led me to wonder why the sun/other stars did not have a similar core.

    If I understand correctly, heavier elements would fall more quickly towards the largest gravity well in an area than the lighter elements, and we would end up with stars that have significant amounts of heavy elements near the core when they form from gas clouds, but that would mean that the hydrogen fusion reaction that drives stars would be contaminated with heavier elements from the beginning of a it's life.

    What I am missing/misunderstanding? Thanks in advance for your replies/answers.

    Last edited: Oct 23, 2010
  2. jcsd
  3. Oct 23, 2010 #2


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    In the neighborhood of the sun, the gas that stars form from is about 75% hydrogen and about 25% helium, by weight. Everything else is under 1%, with the most plentiful element after helium being oxygen at just under 1%. It appears that the make-up of the interstellar gas in other parts of the galaxy and other galaxies of about the same age is about the same.

    A couple of comments. First, it's actually the formation of iron, not lead, that leads to the end of energy production and causes massive stars to explode. This is because iron is the most stable nucleus and fusion beyond iron actually costs energy instead of producing it. Lead is only produced after the star explodes. Note that a star like the sun is not heavy enough to produce either iron or lead.

    Second, as discussed above, the sun doesn't have enough heavy elements to have a metallic core. Third, since a star is a gas rather than a solid or liquid like a planet, the heavy elements don't fall toward the center, or at least not as easily.
  4. Oct 23, 2010 #3
    Thank you for the info. To me, this begs th question of how the earth ended up with so much iron/nichel and the sun didn't (see my response to your second point).

    Thanks for the correction on the lead comment and the explanation for why it happens. Although you response does clear up part of my question, it doesn't adress the main issue I was trying to resolve. Why didn't the heavy elements gravitate (pun intended) toward the center of the largest area of mass? Why aren't there more heavy elements in the sun?
  5. Oct 23, 2010 #4


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    Here's a simplified story. Promise me you will look up "formation of solar system" and "inner planets" in Wki. Really try to understand.

    This is an artificially simplified story. Imagine that the gas cloud is (for example) say 75% hydrogen and say 23% helium and say 2% heavier stuff.

    It starts to contract and condense. The sun gets the hypothetical "standard" 75-23-2 mix. No problem. It's core is mainly H, with some impurity, so it can fuse like a normal star, make energy and not collapse. All stars have some impurity in the core.

    The outer gas giant planets form too. They get get the standard 75-23-2 because they are in the COLD and the gas doesn't boil away and escape.

    Also some smaller bodies in the outer system might preferentially condense involatile compounds: like oxides carbonates of silicon, , aluminum, iron etc because they don't ever get enough GRAVITY to hold onto the more volatiles. They would get the standard 75-23-2 except that they can't hold on to some of the stuff.

    BUT IN THE INNER planetary system where it is HOT there is a HUGE PREFERENCE for involatile compounds, and metals. Even though the mother gas-cloud is 75-23 H and He. These rocky inner planetesimals (the coalescing lumps) are too hot to hold onto that stuff. They collect comparatively little H and He even though those elements are abundant around them. They turn out mostly made out of the 2%. of other stuff, like Earth is.

    But in the sun, which has the standard 75-23-2 composition, that 2% is just an almost irrelevant impurity.

    (Footnote, what makes a massive star explode at the end of its life is not the fact that it has some iron in its core. It is the fact that ITS CORE HAS RUN OUT OF FUEL. It has very little besides iron by that time, in the inner core where it is hot enough to sustain fusion reaction. So the explosion occurs because FUSION STOPS and the steady wholesome warmth of fusion no longer supports the outer layers. So collapse must occur and when it happens it tends to be violent. You can read Wippy-kidia about supernovas. Its not the iron *per se* but rather that fusion stops when useless iron crowds the stuff that would be profitable to fuse out of the central place where it is hot enough to cause fusion.)
    Last edited: Oct 23, 2010
  6. Oct 24, 2010 #5

    D H

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    The Sun formed before the planets did. The radiation and solar wind from our proto-sun blew most of the volatile compounds out of the inner system. The terrestrial planets formed from the heavier, less volatile stuff left behind.

    Because it gaseous. Look at it this way: CO2 is considerably more dense than the nitrogen and oxygen that forms most of our atmosphere. Yet the bottom few meters of the atmosphere is not a suffocating layer of CO2. The CO2 concentration in the atmosphere is more or less constant from sea level to the top Everest (and higher).
  7. Oct 24, 2010 #6
    well if you look at the asteroid belt today you can see that it is actually breaking up rather than coalescing. so things had to be different in the beginning when it first formed.

    there was a gas disc slowing things down and there was a lot of radiation heating things up

  8. Oct 24, 2010 #7

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    Some collisions between asteroids result in a single asteroid, some result in a bunch of little rocks. It is both coalescing and breaking up.

    Not really. That the asteroid belt is both coalescing and breaking up is pretty much what the asteroid belt has been doing since it first formed. Some of those collisions result in bits of dust that eventually infall to the Sun, some result in orbits that are no longer stable largely thanks to Jupiter. As a result, the asteroid belt has been slowly losing mass. However, there never was a missing planet between Mars and Jupiter.
  9. Oct 24, 2010 #8
    Thank you all for the input, and I do understand that gas is very different than solids... I have a basic understanding of thermodynamics. I have read the wiki articles on star and planetary formation before, and will read them again after this post just to refresh my memory and make sure I'm not forgetting/mixing up something critical to understanding that might answer my question.
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