Sentences Structures

Question
The Eng Lang and English team have a question for you which is causing us some consternation, and we would be very grateful if you could help us out.
Here is the sentence:
       The force of gravity on each star is the same and points towards the other star.
Our question is: Is this a compound sentence with an implied subject in the second clause (with the subject 'the force of gravity' elipsed in the second clause), OR is this a simple sentence with a compound predicate?
Eg as there is no subject here: 'and points towards the other star', technically does this make it a simple sentence?
Grammarly and AI tell us that the sentence is simple with a compound predicate, but Sara Thorne (1997) 'Mastering Advanced English Language' says it's a compound sentence.
If you have the time to answer and explain the reasoning, we would be very keen to hear from you.
(from Judith and the teaching team)

Kate's response
Many thanks for your wonderful question. It’s caused both Izzy Burke and I to have an interesting conversation over a point of grammar — a rare treat!
 Certainly, one way of analysing these examples is the following:
 As you point out, in a compound sentence, there are two (or more) coordinated clauses with overt subjects; e.g.

1. Fred swills his soup, and he gobbles his food.

This account argues (as ChatGPT has) that an independent clause should be complete and able to stand by itself.
So something like the following two coordinated imperatives would also form a compound sentence because both clauses can stand alone — the missing subject of the second clause here is of course part of the grammar of imperatives:
          2. Do not swill your soup and do not gobble your food.
So this account of compound sentences would then force us to recognise another category, the compound predicate that has two (or more) verbs sharing one subject; e.g.
          3. Fred swills his soup and gobbles his food.
It’s interesting that neither Izzy nor I have ever used this label compound predicate before, and (as far as we can tell) there’s no linguistic textbook that does either, at least none of the usual suspects (though references to “compound predicate” abound on the internet). This would account for Sara Thorne’s analysis.
As you know, coordinators always allow ellipsis of the subject of the clause they introduce if it’s co-referential with that of the preceding linked clause — and this really is the preferred construction (from the view point of information flow).
And if the subjects and the auxiliaries are identical, ellipsis of both is normal:
          4.  Fred has swilled his soup and (Fred has) gobbled his food.                               
And let’s not go into what happens when the object is ellipted:
           5. Fred likes (soup), and Mary hates, soup.
(Though this might sound a bit contrived!).
Anyway, to our minds it does add an extra layer of complexity to view examples involving ellipsis differently — and to introduce another label “compound predicate” to cover examples like: “Fred swills his soup and gobbles his food” (or your example “The force of gravity on each star is the same and [it] points towards the other star”).
Fortunately, this kind of detail doesn’t come up in the Study Design. But I’m glad you raised it — it’s something that Izzy and I didn’t think of when we put together Chapter 5 in the textbooks.
 
Thank you for contacting us, and for giving us the chance to talk grammar!