If you don't have the current Star Hero unlocked yet due to missing tokens, you will have the possibility to make special Star Hero Calls.

The call itself has increasing odds for every unsuccessful attempt you make. This means that if you do not get Star Hero tokens on your first try, the next call will have a higher chance of giving you the tokens.
If your second try doesn’t yield the Star Hero tokens, your third call will have an even higher chance. This goes on for every attempt you make, until you get the reward.

Details of the call and the chances will be visible in the Radio Tent.