Of course, when you give star and post it on Newsfeed, peers of receiver can see that. Besides that, your manager (and up the chain) can see Stats about your star giving behavior for current week, last week, and last 30 days. They can also see the details of whom you gave stars.
On your profile which is visible to all your colleagues, Talentrio also shows your 90 day stars given and per week average. This is done to increase visibility and to create an appreciative culture in the organization. Colleagues can not see though any summaries or total of whom and when you gave stars.