If God finished the work of Creation, then why are new stars and species still coming into existence?

Neither. It’s proper to speak in two ways of God “creating,” and the scientists and naturalists you mention are using the term differently than does Genesis. Let’s be precise as to what Genesis says: “Thus the heavens and the earth and all their array were completed. Since on the seventh day God was finished with the work he had been doing, he rested on the seventh day from all the work he had undertaken. So God blessed the seventh day and made it holy, because on it he rested from all the work he had done in creation.”

Notice that the passage neither says nor implies that no additional creations would be forthcoming, but simply that the Lord’s initial work of creation (see Gn 1:1-31 for details) had been completed. Common sense tells us that not everything God would create was created at that point. After all, each human soul is a later creation, since the soul is created directly by God at the moment of conception–it does not derive from the souls of the parents.

But God creates in another sense. In Genesis 1 the Lord brought into being, ex nihilo, all matter that would ever exist. Over the ensuing eons, he created the various parts of the physical universe out of that primordial matter. Stars, comets, and nebulae, for example, have formed, disintegrated, and re-formed (many times, presumably) from the same matter. The same is true of plant and animal life on earth in all its myriad forms.

We realize that that cow standing in a pasture has come into existence only recently. The cow is truly one of God’s creations, though it was created by God indirectly; the constituent elements which make up its body have existed since God’s initial ex nihilo creation eons ago and have been “recycled” countless times prior to making up this particular cow.

A carpenter rightly may say he has “made” a chair although, of course, he fashioned it from preexisting wood which came a preexisting tree. God is like that carpenter as he creates new stars or new human beings, yet he is unlike him because he, unlike the carpenter, also created the raw materials with which he works.

With this in mind we can see that Genesis emphasizes the finality of God’s creation of primordial matter and of all the first plants and animals and human beings with which he adorned the newly created earth, while scientists point out (whether they realize it or not) that God continues to create new things in the material realm in the sense that he fashions them from the preexistent matter of his initial creation. Both uses of the idea are correct.