The Emperor's New Mind: Concerning Computers, Minds, and the Laws of Physics (Oxford Landmark Science)

by Roger Penrose

What about the low-entropy nuclear energy in the uranium-235 isotope that is used in nuclear power stations? This did not come originally from the sun (though it may well have passed through the sun at some stage) but from some other star, which exploded many thousands of millions of years ago in a supernova explosion! Actually, the material was collected from many such exploding stars. The material from these stars was spewed into space by the explosion, and some of it eventually collected together (through the agency of the sun) to provide the heavy elements in the earth, including all its uranium-235. Each nucleus, with its low-entropy store of energy, came from the violent nuclear processes which took place in some supernova explosion. The explosion occurred as the aftermath of the gravitational collapse5 of a star that had been too massive to be able to hold itself apart by thermal pressure forces. As the result of that collapse and subsequent explosion, a small core remained – probably in the form of what is known as a neutron star (more about this later!). The star would have originally contracted gravitationally from a diffuse cloud of gas, and much of this original material, including our uranium-235, would have been thrown back into space. However, there was a huge gain in entropy due to gravitational contraction, because of the neutron-star core that remained. Again it was gravity that was ultimately responsible – this time causing the (finally violent) condensa...

Where does uranium 235 come from