Learn Finnish: Kuuluisa tähti taitaa olla itsekäs. - The famous star is probably selfish.

Itsekäs (selfish) is derived from the pronoun itse (self) plus the suffix -käs, which denotes an abundance or an inherent quality. Literally, it refers to someone who has a lot of self or is full of themselves. Because the vowels in itse take front vowel harmony, the suffix uses the ä-form: -käs.

Why is the verb taitaa used here instead of a simple on varmaan (is probably)?

Taitaa is a modal verb meaning to probably do or to seem to. It is always followed by the basic dictionary form of the main verb, which is why we see taitaa olla. While on varmaan means the same thing, taitaa olla is very idiomatic and often implies a deduction, like saying it seems the star is selfish or I guess the star is selfish.

Does tähti mean a celebrity star just like it does in English?

Yes. Tähti literally means a star in the sky, but Finnish uses the exact same metaphor as English to describe a highly famous performer or celebrity.

How would the adjective itsekäs change if the sentence was in a different case, like the elative (from)?

Words ending in -kas / -käs have a stem that changes to -kkaa- / -kkää- when you add most grammatical case endings. Because of consonant gradation, the single k becomes a double kk. For example, from a selfish star would be itsekkäästä tähdestä.