What can old stars teach us about the birth of our galaxy? | Anna Frebel | Big Think

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ANNA FREBEL

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TRANSCRIPT: It has always been really important for us humans to understand what our past is. On Earth we do for example genealogy. We ask our parents about our grandparents and so forth to learn about the history. And astronomers do the same thing about the universe. So we are asking the question how did everything begin? How did everything start to evolve? How did everything fall into place so that Earth could form at some point together with our Sun and later as humans we could start to emerge.

And so the work that stellar archaeologists are doing is to try asking and answering the question how did the chemical evolution of the universe begin which means how did all the chemical elements, how did they form and how were they produced. And we know that they are produced in stars and supernova explosions. All the elements of the periodic table are created in stars and supernova explosions. And we specifically have means with these old stars to reconstruct how each of these atoms, for example, iron atoms or carbon atoms or calcium, how this was all created for the first time in the very first stars about 13.5 billion years ago.

But how do astronomers actually know how old stars are? Well it’s actually not that easy because we can’t just go – first of all we can’t go there and measure an age. And actually age dating in general with various astronomical techniques is very, very complicated. But we can use a different fact to our advantage. As I’ve said the elements are created in stars and supernova explosions and there were actually no heavy elements heavier than hydrogen, helium and lithium present at the very earliest times soon after the Big Bang. But with time and with time I mean over the billions of years since then all the elements were created successively in stars. And so their content has built up over time.

Today the universe contains a whole two percent of all these heavy elements that we know in the periodic tables, everything except hydrogen and helium. And we use those facts because we just look backwards and we search for the stars that have the smallest amounts of all of these heavy elements. So we look for example for the most iron poor stars. We like to use iron as our reference element. And that takes us back to this very early time when there simply wasn’t that much iron in the universe. So in order to determine that a star is old and hence interesting we need to do a chemical composition.
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