When speaking of bigness among stars, you have to define your terms. There are very heavy stars. And there are gigantic stars, in terms of sheer physical size. At the top end of the scale, the most massive known star in the sky is R136a1, a star more than 300 times as massive as our sun. Although R136a1 is the most massive known star, it is not the largest, since it only stretches about 30 times the radius of our sun. The largest known star is UY Scuti, a hypergiant with a radius somewhere around 1,700 times larger than the sun. Its mass, however, is only 30 times that of our nearest star.