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I'm reviewing nucleogenenesis and came across the table below. I like it because it actually labels the s and r-process elements, but I became confused as to why the neutron capture processes begin at A = 38. I thought anything with an atomic number greater than 26 has to be formed by neutron capture? What is my confusion here? Thanks in advance!

enter image description here

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  • $\begingroup$ Can you provide a source/credit for this figure? $\endgroup$ Mar 24 at 22:50
  • $\begingroup$ @ Peter. I cannot. I see some sources in the figure but that's it. It's the only periodic table I've ever found that explicitly mentions the r and s processes, but I think it may be wrong. I showed it to another astronomy student and he also thought it must be wrong. It was just a google image. I usually wouldn't pay any mind without a source, but then I started questioning whether I had the concepts mixed up. $\endgroup$
    – Astroturf
    Mar 24 at 23:44
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    $\begingroup$ Well, I do find the picture on the web at meta-synthesis.com/webbook/35_pt/… $\endgroup$
    – B--rian
    Mar 25 at 9:04
  • $\begingroup$ I think you mean "Z = 38". (A is conventionally atomic mass, which is the sum of protons and neutrons; Z is atomic number, which is the number of protons.) $\endgroup$ Mar 31 at 10:45
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If you go back to the sourced article in your graphics by Jennifer Johnson it has a more verbose colour legend. For the grey part it explicitly states

very radioactive elements. Nothing left from stars.

and her figure caption more verbosely

Each element in this periodic table is color-coded by the relative contribution of nucleosynthesis sources, scaled to the time of Solar System formation. Only elements that occur naturally in the Solar System are shown; artificially made elements and elements produced only through radioactive decay of long-lived nuclei are shown in gray. The data plotted in this figure are available in table S1.

And that pretty much sums it up:

The colour gives the source of the elements as found on Earth.

Thus the colour does NOT indicate how the elements can be formed in general, but the origin of the isotops as found on Earth (or present on formation of Earth, thus still long after nucleosynthesis in their progenitor stars). The lifetime of the longest isotope of technetium is 4.2 million years, and of promethium of only 17 years. Thus basically all of those grey-coloured elements are decayed long ago and none of the naturally occuring specimen on Earth are anymore of primordeal origin but result of fission of higher-order elements.

EDIT to add: As to the 2nd question in your posting which is quite distinct from the title

"why the neutron capture processes begin at A = 38. I thought anything with an atomic number greater than 26 has to be formed by neutron capture?"

Those elements can actually be formed by fusion of the lighter elements in the core of the supernova due to the high density and temperature (basically similar to how we can create heavier elements in the lab, colliding heavy ions) - but that fusion will consume energy instead of releasing energy.

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