What evidence do we have that the halo population of stars are older than other stars in the galaxy?

a. They have a smaller proportion of heavy elements.
b. They are farther away. They have fewer planets.
c. They move slower than other stars in the galaxy.
d. They have higher masses than other stars in the galaxy.

a.They have a smaller proportion of heavy elements.

Explanation:

As a  star grows old , its metal content becomes low . Our sun is Population I star which means it is very new . Its metal content is 1.4 % . By the term metal we mean any element heavier than helium . Population II stars are older than population I stars . They will have lesser content of metal . Population III are oldest star group which have metal content as less as .015 % .

one "light year" is the distance that light travels, through vacuum, in one julian year.   that's 365.25 days, of 24 hours each.