double integral bounds question

I am to evaluate the double integral over a region s of 1/(x^2 + y^2)^(1/2)

S is the region where x^2 + y^2 is greater or equal to 4

and where x^2 + y^2 is less than or equal to 25

y is greater or equal to 0

I see right away that my coordinates limits for r should be 2 to 5 and that wehave the function of 1/r times r dr dtheta

I can not see why theta goes from 0 to pi.

Once the bounds are set this is a ridiculously easy integral to take I know!!!!

I realize this is a simple concept that I keep misunderstanding. Would someone please let me know how we know that theta is from 0 to pi? Thanks very much, Frostking