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Is there a cosmologist in the house? I've got a basic understanding (with some degree of error) of some simple facts:

  • The Universe is a little over 13 billion years old. Our galaxy is almost that old.
  • Our solar system is roughly 4.6 billion years old.
  • The heavier elements (Carbon, Oxygen, etc..) are only produced by stars.

When our solar system coalesced, the stuff that makes up the planets came from a previous generation of stars. (Meanwhile our Sun is busy making its own slightly-heavier elements, but those aren't ours. Those belong to the next "generation" after our star dies.)

So we're all sitting on a rock whose matter was spewed out of a star (or stars) that preceded this solar system. Got it. Don't need that lesson. But what I want to know is...

How many cycles of coalescing, fusing, doing the nucleosynthesis thing, exploding -- and then repeating the cycle -- have this atoms in this ball of rock I'm standing on been through?

How do you know this? Can you tell by the make-up of the matter in the solar system? Looking around the Orion arm to see remnants of our former home? Wild ass guess?

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5 Answers 5

up vote 7 down vote accepted

A very qualitative way to look at it:

  1. The Earth, and therefore you are formed of the same material that contributes to the metallicity of our Sun
  2. Our Sun is a population I star, which means that it has a relatively high metallicity indicative of having formed after the heavy and short lived star of population II had already had their big blow offs.
  3. The population II stars divide into early and late groups, and all post-date the assumed population III stars.

From this I conclude that a non-trivial number of the nucleons in your body have been part of a few stars. Maybe as many as five. As Georg notes there has been time for the most prolific path to include many stars (dozens?).

Certainly all the carbon, nitrogen, oxygen and trace elements that make up your body have been part of at least one star.

None of these facts shed much light on the average star-membership-history of the nucleons that make up your body.

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This is the track that I was trying to go down, and you and Luboš are starting to shed some light on it. –  Clinton Pierce Jan 24 '11 at 18:23

Its likely a complicated mix, possibly including some virgin material, clouds of which occasionaly are absorbed into the galaxy, and some material that may have multiple recyclings. And not all the material that goes into a star is reacted before its blown off, so some (maybe much) of the hydrogen in the sun could have once been in the outer layers of another star. And even some first generation starts (pop III) that were low mass still haven't completed their lifecycles, so it is just a chaotic mess of gas and dust that gets pushed around by gravity and stellar winds for billions of years, plus some new gas that has recently fallen into the Milky Way all mixed up, but not so well mixed up as to be of uniform composition.

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The "generations" of the stars are surely not as sharply separated as you suggest. It's true that the percentage of heavier elements - or "metallicity" - is increasing as the stars continue in their thermonuclear fusion. But most of the stuff that our Sun burns is still Hydrogen - and it's the same "initial generation" Hydrogen as any previous stars were using. Stars are being born continuously and the metallicity is usually a little bit higher than for the previous star.

In the era of "nucleosynthesis" that ended about 3 minutes after the Big Bang, mostly light elements were created in a thermal equilibrium at huge, "nuclear" temperatures that existed at those times. A good theory of nucleosynthesis predicts that most of the elements in the Universe should be hydrogen, with a smaller amount of helium and some trace amounts of lithium and other elements. The observations confirm that the predictions are pretty much accurate.

Heavier elements were created in previous stars. But it is not correct to suggest that the Sun is "almost entirely" built out of a recycled material. Quite on the contrary: it's more accurate to say that the Sun is mostly built from the hydrogen that was created in the first minutes after the Big Bang - and it is also "contaminated" by metals and other heavier elements from the previous stars. Those impurities are important for our life - and industry - but they're not important for the ability of the Sun to burn.

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Of course, current hydrogen and other light elements arising from the big bang may have made up part of the mass of one or more stars along the way... –  dmckee Jan 24 '11 at 18:15
And where are all those burned out stars? –  Anixx Feb 7 '11 at 17:44
It depends which stars. The remains of those that went supernova were distributed across vast distances in the skies, for example: almost all the material explodes away. Others could have been swallowed by larger objects, e.g. the black hole at the galactic center. But I think that there haven't been too many stars that are already dead. The lifetime of an average star is not far from the current age of the Universe. –  Luboš Motl Feb 7 '11 at 21:03
@Anixx To add to what Luboš said about the loss of heavy stars (more than ~9 solar masses), those medium mass stars that have died formed white dwarfs, and have been cooling ever since. Though they are initially very hot white dwarfs are very small and consequently have low luminosity, which means that we can only see them close up. None-the-less, the wikipedia article says that one count found eight in the nearest hundred stars. So, the answer is "They are all around us." –  dmckee Feb 13 '11 at 23:06
"Others could have been swallowed by larger objects, e.g. the black hole at the galactic center." the BHs mostly formed from gas/dust clouds. "The lifetime of an average star is not far from the current age of the Universe." now, yes. But first stars had much shorter liftime otherwise how could they explode before Sun formed? –  Anixx Feb 14 '11 at 2:09

First of all -- check the metallicity, then the nucleocosmochronology. That gives the rough idea about the topic. And also that the topic is a currently developing area in cosmology.

It seems that the concise and experimentally confirmed theory of formation of first stars as well as theory of reionization are required to answer your question. And there no such theory (yet).

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There are three general populations of stars, based on their metallicity, which is the abundance of heavy elements on their atmosphere. That abundance is an indicator of the abundance of heavy elements of the initial cloud that gave birth to the star.

So we have Population I stars that are metal-rich, Population II that are metal-poor and Population III that are metal-free. Population III stars are the first stars that formed in our universe and they must have been very massive and sort lived. Population II are the second generation of stars that created the heavier elements. Finally, Population I stars are the most recently created stars that also have heavier elements that were created in the Population II stars. Our Sun is a Population I star, so in a sense the Sun is a third generation Star. Taking into account the age of the universe and the average life-time of the more massive stars that form the heavier elements in supernovas, then the assumption that the Sun has used the material of at least two previous stars is reasonable.

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