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.

Best wishes Luboš

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.