The carbon in an apple pie was forged inside a star.
Carl Sagan once asked a kitchen question with a cosmic answer: what would it take to make an apple pie from scratch? He did not mean planting an orchard. He meant going all the way down. Cut the pie in half, then in half again, and keep going. After about ninety cuts you would be holding a single atom, and that atom had to come from somewhere.
In Cosmos, Sagan follows the trail backward. The hydrogen was made in the Big Bang. But the carbon in the apples, the calcium in our teeth, the iron in our blood and the nitrogen in our DNA were not there at the start. They were cooked in the interiors of stars and flung into space when those stars died. Later, clouds of that debris pulled together into the Sun, the Earth and, eventually, us. So to make an apple pie from scratch, Sagan concluded, you must first invent the universe.
The idea
We are made of starstuff: the carbon, calcium, iron and nitrogen in our bodies were forged inside stars.
What it means
Sagan means it literally. Every ingredient on your table, and every cell in your hand, has an ancestry older than the Earth. In our words, that fact does not shrink us but gives us a kinship: the universe is not out there somewhere, because we are a piece of it that has learned to ask where it came from.
Caveat
Sagan wrote in 1980. An outside view: astronomers now think collisions of neutron stars forge many of the heaviest elements, such as gold, so supernovae are not the whole story. The hydrogen in your body dates from the Big Bang, not from a star, and "starstuff" is poetry, not a claim that atoms carry any memory of their stars.
