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View Full Version : Why is the universe still mostly hydrogen?



samsara15
2006-Jan-29, 02:16 AM
The universe is roughly 12.5 billions years old. Most of the mass (originally hydrogen) in it, has, at one time of the other, due to gravitational condensation, been part of a star. Stars convert most, if not all, of their hydrogen into helium, or more complex elements, in the course of their lifetimes. Hyrdogen is disappearing rapidly. So why is there still so much hydrogen remaining?

Fr. Wayne
2006-Jan-29, 02:37 AM
If you could suspend gravity for a moment and you were able to take ALL of the stars and round them up in a row, how long would the line be?

I am just guessing but I'd say form here (earth) to the center of our Milky Way. Not very far. Space is vast.

samsara15
2006-Jan-29, 02:55 AM
In other words, you posit that most of the mass of the universe has NOT ever been part of a star, so far?

stu
2006-Jan-29, 03:45 AM
In other words, you posit that most of the mass of the universe has NOT ever been part of a star, so far?

Right. Most of the matter in the universe has not yet been incorporated into stars. Besides that, stars don't convert all of their mass into heavier elements. In a sun-like star, only the inner ~10% is hot enough and under enough pressure to fuse hydrogen. In later stages, some of the outer layers will be able to do this due to partial collapse, but I believe it's around 50% of the Sun will remain "primordial" hydrogen. In more massive stars, I think it's even more.

So, my understanding is that although the ISM will get enriched with each generation of stars, there's still a large quantity of hydrogen that remains hydrogen. And then there's all the gas in the outer regions of galaxies that hasn't birthed a star, and there's the gas between galaxies.

Ken G
2006-Jan-29, 09:44 PM
And another answer to this is, 13.7 billion years is really not all that long! A lot of the mass that has made it to a star, like our own Sun, has simply not had enough time yet to be converted to carbon. Come back in another 13.7 billion years and I think you'll find that you have a lot more stars that don't have much hydrogen. In 100 billion years, maybe hydrogen will no longer be the dominant composition for mass in stars (counting old white dwarfs as stars). But as stu points out, you still will have most of the gas in the space in galaxy clusters, and it still won't be found in stars even then, so you'll still have mostly hydrogen for your "normal" matter.