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Could someone elaborate more on the use of the word "constituent"? Is the question basically asking why these four elements are the most prevalent in interstellar molecules? If so, why are they the most common?

Derived from a question in Pathways to Astronomy, 4th Edition by Stephen E. Schneider and Thomas T. Arny. It is concept question 3 of Unit 73.

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  • $\begingroup$ They are the easiest and this first elements to be "made"...it's only natural they exist in larger amounts. $\endgroup$ – Zach466920 Jul 28 '15 at 15:50
  • $\begingroup$ Your second question answers your first. Now here's another question for you to think about and research: why isn't helium in the list since it is the 2nd most common element in the universe? No need to respond; just think about it. $\endgroup$ – Bill N Jul 28 '15 at 16:03
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Yes, the question is basically asking which are the most abundant elements, though it's specifically asking about elements that form molecules and this is presumably why the second most abundant element helium is excluded (helium being chemically unreactive).

Big Bang nucleosynthesis produced mainly hydrogen and helium, with small amounts of lithium and beryllium but nothing else. However in stars three helium nuclei can fuse to produce carbon. Once carbon is created it produces nitrogen and oxygen in the CNO cycle, so as stars burn, the amount of carbon, nitrogen and oxygen keep building up. When stars shed their outer layers into space (gently in the case of red giants or rather more violently in the case of supernovae!) the most common nuclei puffed out into dust clouds are hydrogen, helium, carbon, nitrogen and oxygen.

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  • $\begingroup$ Your answer was very easy to understand, thank you. So basically, the answer is that N, C, H and O are the most common constituents because helium is chemically nonreactive? $\endgroup$ – Jay Jul 28 '15 at 16:46

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