The aim of this research study was to explore learning, i.e., changes in the knowledge structures of three in-service science teachers for the subject of active galaxies, through group activity conducted as part of a teacher training program. Qualitative methods were used in this study consisting of creating visual representations of the teachers' knowledge structures by analyzing texts, drawings, and data obtained through observations of and interviews with the teachers. The results show that new information acquired through conversations was incorporated into the teachers' preexisting knowledge structures through elaboration and organization. In the case of a teacher with a cognitive conflict, acceptance of the new information depended on the teacher's level of satisfaction with the explanations given. The main influence factor for the modification of knowledge structures was the teachers' orientation to science teaching. These findings suggest that in-service teacher training based on group activities can effectively enhance teachers' confidence in content knowledge (CK) of recent updates in scientific research. However, some format modifications are needed to guarantee efficient learning, such as the addition of a CK representation stage and discussion items based on the preactivity knowledge structure selected through expert review.