Why is Alpha Sagittarii so designated? Usually the alpha star in a constellation is one of the brightest stars in the constellation, or else it has some special significance that gave it that designation, an example of the latter being Alpha Draconis, whose special significance is that it was formerly a north pole star.

But Alpha Sagittari is far from being the brightest star in Sagittarius, and I know of no special significance that would have given it the alpha designation.