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Nuclear statistical equilibrium (NSE) is a balance between nuclei that are unstable and emitting particles versus those absorbing the particles, the balance such that the ratios between nuclides (i.e., specific nuclear species) is preserved over time. The state is analogous to the chemical equilibrium of compounds, but the equilibrium is between nuclei. The equilibrium condition's balance of nuclear species can be modeled by an equation somewhat analogous to the Saha equation.
NSE is of interest regarding extreme (combination of extreme temperature and density) circumstances such as the early universe, and regarding nuclear reactions within stars and within supernovae. It is a key to the primordial abundances formed in the early universe. Within massive stars, it is considered a factor once silicon is created and photodisintegration becomes significant from the gamma rays produced. When production of gamma rays of sufficient energy begins, some equilibrium is approached per some relaxation time. The times spent approaching and essentially within equilibrium-condition affect the resulting abundances of the produced elements. In supernovae, the brief interval of essentially NSE explains element abundances evident from spectrography of the light curves and in supernova remnants.
The term neutron-rich NSE (n-NSE) refers to NSE where neutrons make a substantial number of the particles.