UY Scuti is a red supergiant, and is currently considered to be the largest star by diameter (as of 2017) but Westerlund 1-26 may be larger at 2,544 solar radii (3,543,242,496 km), but is likely smaller at 1,530 solar radii (2,128,942,520 km), VV Cephei A might be larger or the same size at 1,900 solar radii (2,644,800,000 km).
Though it was discovered long ago, we only recently discovered it's tremendous size! In 2012, it, along with KW Sagittarii and AH Scorpii were determined to be at least 1,000 times wider than the Sun. We eventually found out that it was 1,708 ± 192 (2,378,875,072 ± 267,414,528 km) times wider than the Sun.
|Spectral Type||M4 Ia|