A number of literature studies have been done on this subject over the years. The average number of papers published per year is about 3, with one of them being 'first authored' by the astronomer and the rest 'co-authored'. First authoring means your name comes first and you are the originator of the idea or discovery who does most of the writing and research. Co-authoring means your name appears somewhere in the list of authors and usually means your work load was significantly less. Curiously, when astronomers get tenure, or a stable long term work arrangement, their publication rate often nose dives below 3 papers per year. Young astronomers, and those working under short term contracts publish at rates often higher than 3 papers per year, and do so for a long long time. Some argue that the worst thing that can happen to your research career is to get tenure, but this doesn't stand up to rigorous testing as I note in another question.