The oldest stars and galaxies are made of the exact simple elements predicted by the Big Bang theory.

The Big Bang theory predicts that the early universe was a super-hot soup that could only make the simplest ingredients, mostly hydrogen and helium. When we use telescopes to look at the very oldest stars, we find they are made of exactly these elements in the perfect amounts predicted. Newer stars have other ingredients, but the oldest ones are like time capsules from the very beginning.