Question

What do astronomers mean when they say that we are all made of star stuff?

Answer 1

The asronomers mean that we are galaxy and humans are 97% made up of the similar atoms. By looking at the first law of thermodynamics we can see that mass can't be created or destroyed, that is why univese and humans are 97% made up of similar atoms.

And most importnanly most of the elements were made up billion years ago during the big bang. Like, hydrogen, helium. And lithium in our bodies contains a very little trace amount which originated from the big bang.

Thank you, hope it helps