This research examined lifelong science learning reported by amateur astronomers and birders. One hundred seven adults who reported engaging in an informal (out‐of‐school) science interest were interviewed as part of an ongoing series of studies of lifelong science learners. The goal of the study was to gain insight into how and why amateur astronomers and birders pursue their hobby, how their hobby developed, and what influenced that development. Educators may use this information to design programs that support informal science learning over the lifespan. Informed by self‐determination theory, the characteristics, initial experiences, and interests, influences on continued development of science interests, sources of education, and benefits of engaging in a science hobby were explored. Results showed that most of the participants’ lifelong science interests began in childhood and were influenced by events, resources, and family members. Members of both hobby groups reported that, as a result of their informal science interests, they were more knowledgeable about science, more knowledgeable about how science is done, possessed better observation skills, possessed more environmental awareness, and had opportunities to socialize and network with others with similar interests. The intersection of motivation and social capital derived from hobby engagement and lifelong science learning is discussed. © 2016 Wiley Periodicals, Inc. J Res Sci Teach 54: 412–433, 2017