Each main event in the career will give you up to 5 stars - 3 for podium finishes (1st = 3, 2nd = 2, 3rd = 1). Each race will also have two additional requirements you need to fulfil - this can be as simple as getting perfect nitro and drifting, or not wrecking for the duration of the event.
If you don't get all 5 stars in one go, you can just redo the event. It's also not necessary to complete an entire event - you just have to collect 50 stars overall, and it'll pop.