Stars & Uranium

[Kitsune]

What kind of stars made Uranium, Thorium, and other Radioactive materials?

[Gullible Jones]

Stars massive enough to produce supernovas.

It works like this: once you get past iron, fusion reactions are endothermic. In order to generate heavy elements by fusion, you need to slam nuclei together at very high velocities. Supernovas do this, via the shockwave that blows the star apart, so they can generate elements heavier than iron.

[Surlethe]

I believe the very heavy elements are made in supernova explosions, so any star that goes supernova could do it. Fusion of regular star operation doesn't go past iron in any stars IIRC.

True. Iron is the break-even point; fusing higher-mass elements is endothermic, not exothermic. That's why it only happens in a supernova, where there's plenty of spare energy, and only then in trace quantities. To elaborate, a star's core turns from H to He to Li in exponentially shorter periods as it becomes hotter and heavier. The conversion to iron occurs in about a day, after which (so I understand) the star blows up from the sudden release of potential energy from the collapse in the core.

[starslayer]

True. Iron is the break-even point; fusing higher-mass elements is endothermic, not exothermic. That's why it only happens in a supernova, where there's plenty of spare energy, and only then in trace quantities. To elaborate, a star's core turns from H to He to Li in exponentially shorter periods as it becomes hotter and heavier. The conversion to iron occurs in about a day, after which (so I understand) the star blows up from the sudden release of potential energy from the collapse in the core.

Mostly true. The general fusion process in a massive star's core goes something like this: H->He->C->Ne->O->Si->Fe. Lithium, for example, cannot exist in a star, and Be-8 has such a short a half-life that Beryllium is never formed until the nova. Thus, carbon is formed via the triple-alpha process, and various collisions produce even heavier stuff. You end up with lots of Fe-56 (IIRC) in the core. Silicon to iron does take about a day; after that, the core tries to fuse iron, but it can't, because the iron literally becomes incompressible. Thus, as the star's outer layers, and still fusing outer core layers, come crashing down, they create a shcokwave that hits the core. This rebounds, causing an enormous flood of neutrinos, and blowing the star apart.

In the immediate aftermath of the explosion, iron does not fuse with other elements; rather, it acquires lots and lots of extra neutrons, and undergoes various alpha and beta decay stages to form the rest of the periodic table. There are two main ways for this to occur: rapid (the r-method) and slow (the s-method). The r-method produces most of the very heavy stuff like gold and uranium, while the s-method produces the lighter elements, IIRC.