= Deuterium quasi-equilibrium in proton-proton burning
{title2=$n_2\simeq\lambda_{11}n_1/(2\lambda_{21})$}
<deuterium> <proton> capture is much faster than the weak reaction that produces <deuterium>. Its abundance therefore adjusts so that $\lambda_{21}n_2n_1\simeq\lambda_{11}n_1^2/2$. Substituting this balance eliminates the fast intermediate from the slower <proton> and <helium-3> evolution equations. This is a <quasi-steady approximation>, not an assumption that <hydrogen> itself is in nuclear equilibrium.
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