Here's why:

* First generation stars were made entirely of hydrogen and helium. These stars formed directly from the Big Bang, and the only elements present were the lightest ones.

* Our sun contains heavier elements. We know this because we can analyze the light from the sun and see the spectral lines of elements like carbon, oxygen, and iron.

* Heavier elements are created in stars. Stars like our sun fuse hydrogen into helium, and some larger stars fuse heavier elements in their cores. These elements are released into space when stars die, enriching the interstellar medium.

Therefore, our sun must be a second or later generation star, formed from a cloud of gas that already contained heavier elements from previous generations of stars.