- A halo is a part of a galaxy
- The milky way is a kind of galaxy
- So, a halo is part of the milky way

- A halo is part of the milky way
- A halo contains mostly old stars
- So, the halo is the part of the milky way that contains mostly old stars

Hence, the halo is the part of the milky way that contains mostly old stars
Question: Which region of the Milky Way contains a population that consists mostly of old stars?
Context: - A halo contains mostly old stars 
- A halo is a part of a galaxy 
- The milky way is a kind of galaxy