Why is there still mostly hydrogen in pop I and II stars?

  • #1
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Main Question or Discussion Point

I've heard in the past something along the lines of "every atom in your body came from an exploding supernova". Yet, I can't see how that could be true when there is still such an abundance of hydrogen. Wouldn't any hydrogen in a star prevent it from reaching supernova stage?

Assuming hydrogen can't come from supernova, my next question is how did it avoid being captured in a star for the first few billion years of the universe? If I had to guess, I'd say that it is spread so thin that it simply takes billions of years for gravity to weakly pull it into a forming star. And, despite it being so thin, the volume is so enormous that there is enough of it to form stars when it finally does coalesce.

I've been thinking about this for a while, but in the process of typing it out, I feel I may have answered my own question. I guess maybe I just misheard the original quote, or it was exaggerated. Still maybe there is something going on that I'm missing, and it really is possible for hydrogen to survive to supernova stage.
 

Answers and Replies

  • #2
Chalnoth
Science Advisor
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I've heard in the past something along the lines of "every atom in your body came from an exploding supernova". Yet, I can't see how that could be true when there is still such an abundance of hydrogen. Wouldn't any hydrogen in a star prevent it from reaching supernova stage?
There's still hydrogen in the outer layers. Basically, very heavy stars end up with an iron core (which is inert) and a sequence of progressively lighter elements moving outward, with nuclear fusion going on at the boundary of every layer. Once the iron core gets to the point that it can no longer provide enough pressure to support its weight, it collapses and goes supernova.

That said, I don't think it's necessarily the case that the hydrogen came from supernovae. But the rest of the atoms did.
 
  • #3
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Indeed most of the atoms (and thus most of the hydrogen atoms) in the Universe have never been in a star or in a supernova. All heavy elements are produced in stars (but carbon, for example, is mainly produced in the atmospheres of AGB stars, not in supernovae). The intergalactic medium holds the vast majority of the mass in the universe, but slowly this matter all collapses into galaxies, stars, planets, people and so on. So, most of your hydrogen is not necessarily processed through stars.
 
  • #4
Chronos
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Primordial abundance is the short answer. Most of the progenitor mass for newly forming stars is still predominately hydrogen. Dying stars polute this mix with metallicity, but, not significant extent. Most of the mass of our galaxy is still locked away in this mix of polluted primordial gas - e.g.,
http://www.sciencedaily.com/releases/2010/05/100526111230.htm
 

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