Which star is the largest known by radius?

UY Scuti is frequently listed among the largest known stars by radius. It is a red supergiant whose size is difficult to measure precisely because it is distant, variable, and surrounded by extended stellar material. Estimates often place its radius at more than 1,700 times the Sun’s radius, though the exact number is uncertain. If placed at the center of the Solar System, it would extend far beyond the orbit of Mars. Objects like UY Scuti help astronomers understand the late life stages of massive stars.