TASK DEFINITION: You will be given a trivia clue, and the category it belongs to. You should answer with the best answer that belongs in the category and is described by the clue. For simplicity, answers should be in all lower cased letters.
PROBLEM: Category: ASTRONOMY 
Clue: A galactic cloud of dust & gas, its name comes from the Latin for "cloud"

SOLUTION: nebula

PROBLEM: Category: HE WAS IN THAT? 
Clue: As a child Robert Blake played a Mexican boy in this Bogart classic directed by John Huston

SOLUTION: "the treasure of the sierra madre"

PROBLEM: Category: MEDIEVAL TIMES 
Clue: By the 12th century, the Chinese had harnessed magnetism & were using this device

SOLUTION:
compass