Roughly 1% of the Sun's output is from the CNO cycle. The original C12 came from the triple alpha process. Three helium atoms collide to form C12 more-or-less

Roughly 1% of the Sun's output is from the CNO cycle. The original C12 came from the triple alpha process. Three helium atoms collide to form C12 more-or-less directly. Technically, two helium atoms combine to form Be8, which combines with a third helium atom to form C12, but the half-life of Be8 is 8e-17 seconds. There was no C12 anywhere in the universe until the first stars began dying. Only after the first generation of stars lived their entire lives, and off-gassed C12 (and other heavy elements) and the next generation of stars formed from the remnants did any stars exist with C12 in them, or planetary disks form around them that contained any elements heavier than lithium. This had fairly significant effects. First of all, the first generation of stars (called Population III stars) could not burn hydrogen via the CNO cycle, which can happen much more rapidly than the proton proton chain. Second, they were much more transparent. Combined, this means that they were much, much less constrained in terms of mass than contemporary stars. They could have been enormous. They could have been large enough that it was energetic enough in their cores that the highest energy gamma rays might preferentially form positron-electron pairs- that was a lower energy state than just being plain old light. This would reduce the temperature in the core, which would cause it to contract. As it contracted, it would heat up again- which would cause more electron position pair production. Normally, fusion in the core of a star is self regulating. As it heats up, it expands, which reduces the rate of fusion. This causes it to cool down, which causes it to shrink. This increases the rate of production. This process stable and self regulating. However, pair-production throws a wrench in all this. Higher temperatures are short circuited into pair-production, which causes it to shrink, which increases the rate of fusion, which increased the rate of pair production, which causes it to shrink more. It's a feedback loop that creates a very small, very very very hot core. And the core collapses. But this doesn't collapse into a black hole or anything, it shrinks until the point where the density of positrons is so great that they annihilate as fast as they are being created. At this point, the feedback loop breaks, and the entire star explodes in an impossibly powerful supernova. There's no warning and no remnant; it's a normal (albeit very large and very bright) star one moment, a ridiculous supernova the next, and nothing but an expanding gas cloud containing a wide variety of heavy elements the next. We have never observed a Population III star, or a pair-production supernova. Just hypothesized about them. It's odd that we've never seen a population III star- why isn't there a small red dwarf kicking around (which can easily have a lifetime of a trillion years) with no elements heavier than helium in it? (lithium is destroyed fairly quickly in a star) We think that this tells us something about star formation; there basically had to have been no small stars in the early universe, just very large ones. It's an open question as to why. You might imagine a planet with life on it who are quite annoyed by this. They're minding their own business, when suddenly their star explodes and they're all dead. But this couldn't have happened. Remember there's no elements heavier than lithium at this point, and only a minuscule quantity of lithium at that. There wasn't enough "stuff" in the star's disk to form planets, and if a small chunk of lithium/lithium hydride managed to coalesce into a planetoid like object, there's not really any chemistry interesting enough to form life that can happen.