The traditional Christian view on this is that God completed the work of creation in six days (whether you take that literally or metaphorically). However he is still in the business of sustaining his creation. The Catechism of the Catholic Church says in 301:
With creation, God does not abandon his creatures to themselves. He not only gives them being and existence, but also, and at every moment, upholds and sustains them in being, enables them to act and brings them to their final end. Recognizing this utter dependence with respect to the Creator is a source of wisdom and freedom, of joy and confidence:
In other words, without God's continual work, nothing could continue to exist. It's a separate kind of work from the work of Creation.
As Ryan Frame says, the discovery of new stars does not mean that they were created recently. Also the birth of new stars (or animals, or people for that matter) doesn't imply new creation. God had set in motion at the time of creation the mechanisms by which they are created (coalescing matter clouds, sexual reproduction). That was the work of creation. The bringing-to-birth is the work of sustainment.
Additional references to show that this view is not limited to Catholics: