Looking at the diagram, since OA and OB are radii of the circle, then OA=OB, therefore this forms an isosceles triangle inside the circle. Also, note that since triangle AOB is isosceles ...

When calculating angles using a circle theorem ... so with the chord they form an isosceles triangle. Angles in a triangle add up to 180°.

And the asterism is part of an even larger asterism known as the the Winter Circle (or Winter Hexagon ... you get smaller ...