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Homework Help: Sun Fusion

  1. Jan 6, 2005 #1
    Alright. Now, I understand that the Sun is 1/2 way through its complete H-He fusion cycle, and I realize that after this fusion cycle, it will pass through all the elements until Fe, (i don't know whether it passes through each element in 7 seconds, or it takes 7 seconds for each element). My question is, if the atoms are getting larger as you go farther down the table, and the Sun would theoretically expand (possibly) to Jupiter's position in that short time, and considering that matter cannot travel faster than the speed of light, how does this work?

    I know, I shouldn't think in classical physics terms, but I can't seem to find an answer. The reason being, is that it takes 8 minutes for light to reach Earth from the Sun, however, the Sun would be expanding far beyond Earth in a matter of seconds. Why?

    P.S. my Physics teacher had told me how far the Sun would expand to, so I'm using his information.
  2. jcsd
  3. Jan 6, 2005 #2


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    Can you explain to me why you think that the sun would expand this far in a matter of seconds? I always thought that the expansion to red giant stage would take place more gradually. Also, I remember reading that a star like the sun would expand enough to envelop the inner planets, but not the outer ones, contrary to what your teacher said. Anyone?
  4. Jan 6, 2005 #3
    Well, I understand that the Sun will undergo fusion reactions until its Fe state, then fission occurs.

    Again, time and sizes have been from my teacher's mouth, not mine. But the question is, if matter cannot exceed the speed of light, and it takes 8 minutes for light to reach the earth, then why is it taking only a matter of seconds for the Sun to pass through all the fusion reactions and expand to envelop the inner planets? Thinking classically, it's not possible. However, on another level of physics it is, and I'm asking why.

    Sorry if I'm going around in circles on this one.
  5. Jan 7, 2005 #4


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    I not one to contradict your teacher, but what he's saying just isn't making sense to me. Nothing with mass, can travel at or faster than the speed of light. Gravity travels at light speed, so should anything happen to the sun, on earth we would feel it 8 mins later, and so on and so forth.
    Our sun will most problably go through as cepheid said, a red giant phase. In a red giant phase, as a star has used up it's outer H-He, it they begins to fuel itself with H from a relatively small shell around it's core. Because fusion is occuring so close to the core (creating heavier elements, like you said stoping at iron) the star's outer levels begins to expand. To expand it uses up energy (beating the sun's gravity), so the star is cooled and redshifted.
    The star becomes larger and larger, cooler and cooler, giving it the name red giant. It will most probably envelop the inner planets (mars might get excluded), though due the gravitational concequences of the red giant phase, the inner planets might be lost in space. This procces does not happen in seconds or minutes, it takes quite an amount of time, but I don't know exactly.

    1 extra thing, remember though the atoms are getting heavier, there are less and less of them, for 1: because they are being fused, and 2: the star is running out of them.
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