You probably don't need to go out of your way to get the highest level 6 Cosmic Star

I finally got the Cosmic Star item to its highest level, level 6 haha. When you merge lower-level stars to create higher ones, you get higher-level chests as a bonus. I was hesitant to open the first level 6 star I completed because I didn't want to waste it lol, but when I made three, they took up too much space (level 5 takes 1 space, level 6 takes 4 spaces). So, I opened one, wondering what good stuff it would give me. In the second picture, the star on the right hasn't been tapped at all, and the star on the left is one that I've tapped a couple of times to get rewards. It seems like the more you tap, the smaller it gets. I tapped it about 6 times and got coins, chests, plants, etc. (The third picture is to show you that chests can appear. I also merged chests to a higher level because of space constraints ;) My impression is that the rewards aren't as great as the effort put in, haha. You just get higher-level chests, but I even tapped a level 4 star, and getting around level 4 is sufficient. Also, the good stuff you get from opening chests (like dragon eggs) requires gems, so I just sell them. Anyway, Merge Dragons is super fun!