You call someone that studies metal a metallurgist.
Hydrologist
normally .you call one .a letterologist.
Probably a bovinologist.
Podologist pod/o- (Foot) -logist (one who studies)
A marine biologist.
One who studies the stars is called a(n)... Astrologist
Some one who studies snakes.
Climatologist .
You'd call them a sociologist if they study society and group behavior or a psychologist when it involves just one subject.
A physicist studies motion, forces, and energy to explain the way things work.
Heliologist
A person who studies ancient weapons might be a military historian, or might be an archaeologist. There is more than one reason to study ancient weapons.