Cluster structure of the Borromean nucleus ⁹Be and its astrophysical relevance

The study of cluster structures in light nuclei has become a central direction in modern nuclear physics. Experimental and theoretical investigations have revealed that many light systems, such as ⁶He, ⁸Be, ¹²C, and others, exhibit pronounced cluster configurations that strongly influence reaction mechanisms. Particularly striking is the α-cluster structure of ¹²C and its excited Hoyle state, which plays a key role in astrophysical nucleosynthesis. Among light nuclei, the stable yet weakly bound ⁹Be nucleus is of special interest due to its Borromean character and competing cluster configurations, typically described as (α+α+n) or (⁵He+α). These structural features manifest themselves in decay channels, scattering processes, and transfer reactions, and they directly affect the cross sections relevant for astrophysical scenarios such as the triple-α process and the r-process. The paper provides a comprehensive overview of theoretical and experimental approaches to nuclear clustering, emphasizes recent advances, and examines outstanding questions concerning the structure of ⁹Be and its significance in nuclear reactions and nucleosynthesis.