The aim of this study was to identify the mental models of fifth and eighth grade students regarding the concepts of space, the universe, and galaxy. The study was performed using a cross-sectional approach, which is a screening-based study model. This study involved two different groups, which were fifth grade and eighth grade students. We investigated the mental models regarding the concepts of space, the universe, and galaxy among these two groups, and evaluated how these concepts differed between fifth grade and eighth grade students. The study was performed with 116 fifth and eighth grade students from a secondary school in a provincial center in northern Turkey. During the study, students were asked three open-ended questions requiring them to explain the concepts of space, the universe, and galaxy. The study data were analyzed using descriptive analysis. The study results indicated that the level of scientific understanding regarding the concepts of space, the universe, and galaxy was low among both fifth and eighth grade students, although eighth grade students exhibited a slightly higher level of knowledge about these concepts
