Why is the isotope Nitrogen-14 formed preferentially to Nitrogen-15 in the CNO-cycle in stars? Most of the universe's nitrogen is formed in larger, main sequence stars using the CNO Cycle, right?
But I cannot find a good, specific explanation as to why $^{14}$N, with both an odd number of neutrons and protons, is formed preferentially to $^{15}$N?
 A: Both $^{14}$N and $^{15}$N are produced as part of the CNO cycle during the hydrogen-burning main sequence phase of stars more massive than the Sun. However $^{15}$N reacts rapidly with protons to (re)form $^{12}$C and an alpha particle, whereas the much slower $^{14}$N$(p,\gamma){}^{15}$O reaction allows $^{14}$N abundances to build up and dominate when the CNO cycle reaches an equilibrium.
Details:
The addition of protons to $^{14}$N
$$ p + {}^{14}{\rm N} \rightarrow  {}^{15}{\rm O} + \gamma$$
is the slowest reaction in the CNO cycle and hence at equilibrium there is a build up of $^{14}$N.
As to why this is the slowest reaction in the cycle; it is likely because:

*

*(i) Of the other proton addition reactions in the cycle, adding a proton to a carbon nucleus has a lower Coulomb barrier so is faster.


*(ii) The beta decay reactions, although governed by the weak interaction, are not subject to the high Coulomb barriers of the proton addition reactions and so they are faster.


*(iii) That leaves
$$ p + {}^{15}{\rm N} \rightarrow {}^{12}{\rm C} + \alpha$$
which is a faster reaction than adding a proton to $^{14}$N because whilst the latter is a "radiative capture" reaction involving an electromagnetic transition resulting in a gamma ray (see Brune & Davids 2015), the former is a more rapid (by 4 orders of magnitude) strong force interaction.  This will also be the reason why $^{15}{\rm N}(p,\alpha){}^{12}$C is totally dominant over $^{15}{\rm N}(p,\gamma){}^{16}$O and as a result allows the $^{12}$C to be regenerated; and means that there is a CNO cycle at all!
