Star events aka (star trial events) give you the event unit based on how many stars you got in the event's maps.
So I you could only 3* 4 levels and 2* 2 levels, then your total is 16 stars, if you reach cap (18 in this example) you get the best prize (which is usually min cost unit)
The worse you do, the worse the prize unit will be

I hope I made myself clear