Comments by "EebstertheGreat" (@EebstertheGreat) on "What Happens If A Star Explodes Near The Earth?" video.

  1. 4:23 This is a very, very common misconception. Iron-56 has the lowest mass per nucleon but that does not mean it is the most stable nuclide. Fe-56 is relatively proton-rich when compared to Fe-58 and nickel-62, and neutrons are slightly more massive than protons in a vacuum. Thus, although Fe-56 has a greater mass defect per nucleon than Ni-62, it nevertheless has a smaller binding energy per nucleon, making Ni-62 the most stable nuclide (followed by Fe-58 and then Fe-56). However, Ni-62 is not produced in stars, because it would require the intermediate formation zinc-60, which is less stable.
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  2.  @Mr.NothingBurger  I was being a little imprecise. Ni-62 is not normally produced in the s-process of stars. The onion-esque star you see in the video would never fuse nickel-62 or iron-58, though it would eventually fuse iron-56. But practically all elements are formed "in stars," so I was kind of saying that wrong. Nickel-62 and other isotopes of nickel are formed in the r-process in stars. The s-process is the usual fusion seen in low- and medium-mass stars up to and including their death, which is capable of producing nuclides up to Fe-56. Larger nuclei are only produced by other methods such as the r-process, which likely occurs in rare extreme scenarios like core-collapse supernovae (the death of high-mass stars) and white dwarf star collisions. A figure on Wikipedia claims that 70% of nickel on Earth came from exploding white dwarf stars (Type 1a supernovae) and 30% came from exploding massive stars (most other supernovae), but I'm not sure what the source is. Generally-speaking, nuclei heavier than iron-56 tend to come from cataclysmic events like that. I'm not sure why exactly Ni-62 is not produced in most stars but Fe-56 and Ni-56 are. This depends on the particular conditions within the star. Wikipedia claims that "During nucleosynthesis in stars the competition between photodisintegration and alpha capturing causes more 56Ni to be produced than 62Ni (56Fe is produced later in the star's ejection shell as 56Ni decays). The 56Ni is the natural end product of silicon-burning at the end of a supernova's life and is the product of 14 alpha captures in the alpha process which builds more massive elements in steps of 4 nucleons, from carbon. This alpha process in supernovas burning ends here because of the higher energy of zinc-60, which would be produced in the next step, after addition of another "alpha" (or more properly termed, helium nucleus)."
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