There are many specialist print and online resources for Astronomy in Cambridge. These pages give you an overview of the resources and research support that libraries can offer.
If you're looking for a specific item, the easiest way to track it down is by using the library catalogue, iDiscover. Type a few words from the article's title into the search box. If the article is available online (and many of them are) then you should be able to get to it directly by clicking 'Full text available'. If it isn't online, then iDiscover will tell you where copies of the print journal can be found.
If you're looking for articles on a given topic, add a few keywords to the search box on iDiscover. Be careful with your search terms, and try synonyms if you're not getting the results you hoped for.
You can see the whole list of journals the university subscribes to on the ejournals website. Other useful ways to search for journal articles include Google Scholar, Project Muse and JStor.
Specialist print resources and library help is available via the Institute of Astronomy Library.
There are many other libraries in Cambridge which may also hold useful collections for your subject.