Despite policy changes and targeted outreach, issues of under-representation persist for women and minorities in science fields. Previous research has explored extrinsic motivational factors for underrepresented groups in both formal educational settings. However, little research has investigated intrinsic motivation within informal educational experiences like active participation in free choice learning activities like a science-based hobby. Interviews were conducted with 17 astronomy hobbyists who self-identified as under-represented individuals (race, ethnicity and gender). Respondents' hobby experiences and motivational aspects emerged from analysis of the descriptive accounts provided by the interviews. Findings suggested there was a positive relationship between the degree of hobby participation and number of challenges presented in various phases of hobby growth. Additionally, disparities in sampled minorities' access to authentic Science, Technology, Engineering, and Mathematics (STEM) experiences served as a factor influencing their hobby motivation and subsequent development. Other factors that influenced minority engagement included differential participation in hobby clubs, lack of access to mentors, and the delay of active hobby participation until adulthood. Findings suggest science hobbies as an avenue to motivate underrepresented groups in science interests. This information may prove useful in guiding next steps regarding policy and research in the under-representation of women and minorities in STEM hobbies and STEM careers alike.