The style file astron.sty is a LaTeX style file.
It defines the \cite and \cite* commands such that
\cite{label} is replaced by `(Author, year)'
in the running text, and \cite*{label} by
`(year)'.
It also defines the thebibliography environment. It causes the text `References' to be produced as the heading of the list of references in `article s', and `Bibliography' as the heading in `report s' and `book s'
The thebibliography environment is defined in terms of the
LaTeX `list' environment, in which the items are preceded
by `\bibitem'.
All parameters related to the `list' environment, such as
`itemsep', `leftmargin', etc., are set at the
\begin{thebibliography} call.
To summarize in brief: the BibTeX style astron.bst takes care
of the typographical details of each item in the bibliography;
the LaTeX style astron.sty takes care of the global
typographical appearance of the bibliography.