Many journals require you to submit references with abbreviated journal names, the style of which may vary from journal to journal.
Fortunately, Jabref provides an easy means of toggling your Bibtex catalogue journal entries between full and abbreviated form (Tools -> Abbreviate journal names).
It's also possible to use your own custom list of journal abbreviations (Options -> Manage journal abbreviations). Since I couldn't find any ready made lists of abbreviations for astronomers, I created this repository. Currently I've only provided an incomplete list of abbreviations in the MNRAS style, but this will grow over time, hopefully with the help of fellow astronomers.
I recommend that you maintain a `long-form' bib file containing all your references with full-length journal names, and only make a copy for conversion to abbreviated form when you are actually submitting a paper. That way you prevent any risk of corrupting your reference catalogue through mishaps.
MNRAS require that journal names be in `initials only' form for frequently cited journals, otherwise use the IAU standard abbreviations.