The bibliography style `astron' has its own ideas about which fields in an entry are required, optional or ignored. Astronomical journals make an effort of `economic' printing, which means that they leave everything out that is not absolutely necessary.
The classes of the fields in each entry are tabulated in appendix B. One thing I will give away here: in the `author-year' citation system it is obvious that author and year information is required for every entry. Remember that `required' means that BibTeX issues a warning if the field is empty, i.e., if the information is not available in your database.bib file.