A lot of other comments already covered some of the constructive feedback I was going to give, but I'd also say that the pacing could probably be improved by limiting the amount of cores needed from 10 down to something like 3, or increasing the rate that you would acquire them by having multiple boss stars appear together after you've collected a few cores already.
Also, having unavoidable attacks is a tricky kind of mechanic, because it punishes the player for not playing well enough in the past, rather than for how they are doing in the present. Pairing the helper stars with some kind of skill or ability to dodge/defend the attacks might be a better way to balance the amount of control the player has. I saw in another comment you mentioned wanting to include other abilities/drops. That sounds like a good opportunity to provide this kind of mechanic if the game gets expanded in the future.
Overall, I like the general idea though. Made me think of something like Slither.io with the constant collection and goal of growing larger