subtraction

The passage around this formula

Not every element past iron comes from an explosion. Most of the nuclei between strontium and lead that are made by neutron capture rather than fusion are built one careful step at a time, inside ordinary, long-lived stars, by the slow neutron-capture process — the s-process. The name describes the pace: neutron captures happen far apart in time compared to the beta-decay half-lives of the nuclei absorbing them, so each newly captured neutron has time to beta-decay, if it is going to, before the next one arrives. That condition can be written as a simple inequality between two competing timescales: $\tau_{\beta^-} \ll \tau_n \qquad \text{(s-process)}$. where $\tau_n$ is the mean waiting time between successive neutron captures…

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