Opportunity cost refers to the idea that an individual undergoes an economic or financial loss by not taking part in an opportunity.
For movie stars and professional atheletes, these individuals can earn income from acting and playing sports. If they chose to go to college, they will forgo that income. Even if they go part time and still work, the idea is that the hours they spend in school could have been spent acting or playing sports. Therefore, by going to college, for any number of hours these individuals forgo income, and will therefore experience an opportunity cost.