Its radius was calculated to be around 965 solar radii[1] (6.72 × 108 km, 4.48 au), making it one of the largest stars discovered so far.

This corresponds to a volume 899 million times bigger than the Sun.

If placed at the center of the Solar System, the photosphere of Westerlund 1-20 would almost reach the orbit of Jupiter.

The star is classified as a luminous cool supergiant emitting most of its energy in the infrared spectrum.

It is therefore likely that its morphology was affected by either the intracluster medium or the cluster wind of Westerlund 1.