As the title says. Thank you!
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Stars (and supernovae, which come from stars) are the only place where we know nuclear fusion can take place. Any element that is more complex than hydrogen is a product of nuclear fusion; therefore, the atoms that make us up, like carbon, oxygen, and nitrogen (and more), had to have been created in stars or supernovae. Thus, we are made of “star-stuff.”
Models of the time after the big bang suggest that matter slowly “condensed” into atoms, but at the beginning it could only form hydrogen.
All elements smaller than iron could form in the core of stars. The heavier the element the bigger and older the star would have to be.
Elements heavier than iron only form in supernovae, when a huge star explodes and smashes elements into each other.
So the existence of heavy elements on earth is proof that the sun is a later generation star, that formed from the debris of stars that died before.
Since we are entirely formed from elements found on earth (by eating/breating, or by your mother eating/breathing while you’re still in the womb) we must be made from former stars.
Our bodies rely on relatively heavy elements like iron and zinc to function. We know that when the universe started, there was only energy. We know that as the universe cooled over hundreds of thousands of years, energy started to colaesce into particles of matter. We know that couldn’t have created heavy elements. We know stars work on nuclear fusion. We know fusion creates heavier elements out of lighter ones. Analysis of the light stars produce and measures of their mass and diameter tell us that as stars run out of hydrogen to fuse, they start fusing heavier stuff to result in even heavier products. We know that when fusion finally fails, stars go supernova. We know that stars going supernova scatters material from their cores out into space. We know that there aren’t any natural processes that creates nuclear fusion other than stellar fusion. We know there are no natural process that create the heavier elements we’re made of other than stellar fusion.
Therefore, since we know things like iron and zinc can only be produced in the core of stars, and we know we’re made of iron and zinc and other things like that, we know that what we’re made of came from stars.
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If you are referring to the idea that “we are stardust” – much of this stems from the fact that heavier elements (heavier than hydrogen and helium) are produced by fusion in stars.
So far as we know, the source of these heavier elements are from stars exploding and the resulting stardust being recycled into new stars / planets / systems.
If you are referring to the idea [of life being seeded here from other worlds – called panspermia](https://en.wikipedia.org/wiki/Panspermia). It is really not that serious an idea as we only know of life on our own planet.
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