I tend to think the following is incorrect because the 'it' that starts the second sentence refers to a noun which appeared in a previous sentence.
The Milky Way is the name used for our own galaxy. It is 200,000 light years across.
When I went to explain this to my son I became less sure that the use of 'it' should be restricted in this way . I would appreciate some ideas on this .
I have looked at this previous question How to form sentences starting with "it" but it's too specific for my needs. I'm also aware that the list of general references should be consulted (What good reference works on English are available?) but there don't seem to be any of those which are freely available and cover this case.