Spin degeneracy
= Spin degeneracy
{title2=$g_s=2S+1$}
When a single-particle <energy> is independent of its <spin> projection and all projections are available, the <state degeneracy> due to <spin> is $g_s=2S+1$. A spin-half gas has $g_s=2$ if both projections are available, while a fully polarized gas can have one populated <spin> species. A <magnetic field> that resolves <Zeeman splitting> requires counting the separate energies instead of multiplying one common occupancy by $g_s$.