Consider the following diagram,
The above figure depicts the initial line segment OA, which turns through half of a circle.
Theorem: The angle corresponding to a straight line is 180 degrees.
Observe that due to this turn through half of a circle forms a straight line AB.
Therefore, the theorem suggests that the angle AOB will be 180 degrees.
Thus, there are 180 degrees in an angle that turns through half of a circle.