Limb darkening is an effect where the middle part of a star (as well as the Sun) looks brighter than the edge, or limb.[1]
↑Roun, Daniel (2003). "Limb Darkening". In Gargaud, Muriel; Amils, Ricardo; Quintanilla, José Cernicharo; Cleaves, Henderson James; Irvine, William M.; Pinti, Daniele L.; Viso, Michel (eds.). Encyclopedia of Astrobiology. Berlin, Heidelberg: Springer. pp. 925–926.