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

[DaleSwanson]

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.

 

[Chalnoth]

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.

 

[harcel]

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.

 

[Chronos]

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

 

[sardar]

Your reasoning is largely correct. The reason why there is still mostly hydrogen in pop I and II stars is because hydrogen is the most abundant element in the universe. It is estimated that about 75% of the mass of the universe is made up of hydrogen. This is because during the Big Bang, the universe was filled with incredibly hot and dense plasma, and the simplest and lightest element, hydrogen, was formed in abundance. As the universe expanded and cooled, this hydrogen was able to clump together due to gravity and eventually form stars.

In the early universe, there were no heavy elements like carbon, oxygen, and iron, which are necessary for life and are also formed in supernovae. These elements are created through nuclear fusion in the cores of stars and are then released into the universe when the star dies in a supernova explosion. So, while it is true that every atom in our bodies came from a supernova, the hydrogen that makes up the majority of our mass was already present in the early universe.

As for your question about how hydrogen avoided being captured in stars for the first few billion years of the universe, you are correct in thinking that it was spread out thinly and took a long time for gravity to pull it into forming stars. In addition, the process of star formation is a complex one and not all regions of the universe were equally conducive to star formation. Some areas may have had higher concentrations of heavier elements, making it easier for stars to form, while others may have had lower concentrations, resulting in a slower formation of stars.

In summary, the abundance of hydrogen in pop I and II stars is a result of its abundance in the early universe and the slow process of star formation. While it is possible for hydrogen to survive to supernova stage, it is not the only element present in stars and is not responsible for the majority of their mass. I hope this helps clarify the concept for you.