In a Starfall Realm, you are collecting 5 Silver Stars. But it's not as easy as it sounds. There are guardians, and if awakened, they'll begin hunting you down. When asleep, they can't hunt you down at all. Collecting a Silver Star will put the guardians to sleep for only 100 seconds, making it a little hard to hurry and get the Silver Stars on bigger courses. However, on the golden Starfall Realm in the Great Star Temple, you have only 75 seconds and you are finding Bronze Stars to make the Star Staff appear.
The Guardians Awake
When your in-game timer runs out, you step in Guardian Waters, or your hit by a Starfall Goomba, the guardians awaken. As said earlier, the only way to put the guardians back to sleep is to collect a Silver Star.