Learn Hardcore Norwegian: Hun peker på himmelen.

på himmelen: “on the sky,” used when you point at the sky.
i himmelen: “in heaven” or “in the sky,” often with a religious or poetic sense (“in heaven”).

hun is the third-person singular feminine pronoun (“she”). In standard Bokmål you use hun, but in many Western and Trøndersk dialects you’ll hear ho for the same meaning.

Why is the preposition på used after peker? Could we use til instead?

Norwegian uses the combination peke på noe to mean “point at something.” til is less common here; peke til can imply movement toward something, but when indicating the target of your finger, you almost always say peke på.

Why is himmel in its definite form (himmelen)? Is it necessary?

Norwegian often uses the definite form for unique or well-known things. himmelen means “the sky.” Saying på himmel (without -en) sounds unnatural, as if you meant one sky among many.