Complex sentence: According to standard Lie group theory, SU (2) is then a principal U (1)-bundle over the left coset space SU (2)/U (1) which is diffeomorphic to the 2-sphere.
Split into simpler sentences: According to standard Lie group theory, SU (2) is then a principal U (1)-bundle over the left coset space SU (2)/U (1). One can show that SU (2)/U (1) is diffeomorphic to the 2-sphere.