He is an expert in computer science.

He is majoring in astronautics.

He is an expert in toxicology.

My major is mechanical engineering.

He is an expert in systems engineering.

I am majoring in clinical psychology.

Astronautics is a field of study related to the exploration of space and the design of spacecraft.

Analytical chemistry is a science that reveals the composition and structure of substances.

He is an expert in paleoanthropology, investigating archaeological sites around the world.

He is an expert in folkloristics and has a lot of knowledge about local traditions.