* Formally: It's generally considered respectful, especially at the beginning of a semester or in formal settings like office hours for the first time, to address a professor as "Professor [Last Name]" or "Dr. [Last Name]".
* Informally: Some professors explicitly encourage students to use their first names. They might state this in their syllabus or during the first class. Pay close attention to cues from the professor.
* Observing cues: Watch how the professor addresses students and other faculty. If they use first names, it's likely acceptable to do the same. However, if they consistently use titles, it's safer to follow suit.
* When in doubt, ask: If you're unsure, it's always best to err on the side of formality. You can politely ask the professor how they prefer to be addressed during office hours or after class.
In short, there's no single answer. Be observant, respectful, and if necessary, politely inquire.