The field of Astronomy does not have a consensus citation style. When writing for a course assignment, you should consult your professor for guidance on selecting a style.
Professional astronomers are typically required to adopt the citation style used by the journal publishing their research article. In the absence of a required citation style, consider adopting one used for journals published by the American Astronomical Society (AAS).
You might also consider using one of the widely used citation styles (i.e. Chicago, APA, MLA) described in detail on the Bates Library "Citing Sources" guide.