|MadSci Network: Physics|
Uranium is made in the explosions of old, massive stars that have reached the end of their fusion cycle. These explosions are one type of supernova.
Stars get their power from fusion, that is, combining the nucleus of lighter atoms to make heavier ones. This only works until they make iron, which is the most stable nucleus. At that point, since they can no longer gain energy from fusion to hold up their outer layers, the stars collapse. This sets off a series of complicated events that results in a lot of extreme reactions between atomic nuclei. One of these is the very rapid absorption of many neutrons by some of the atoms in the star. This leads to unstable, very heavy atoms that then radioactively decay back to stable or (in the case of uranium) semi-stable elements. Most uranium, with an atomic mass of 238, is fairly long-lived, taking some 4.5 billion years before half of it decays. A smaller part of the uranium formed is other atomic masses, chiefly 235, and these last for shorter times. Not only uranium is formed this way; all the elements heavier than bismuth are formed in this way, either directly or as decay products from even heavier elements.
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