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 isosceles triangle, with the Red Planet as the brightest of the three ...