Westerlund 1 is a young super star cluster residing 15,000 light years from Earth, in the constellation Ara. Stars are classified according to their spectral type, surface temperature, and luminosity. While studying and classifying the cluster's constituent stars, astronomers discovered that Westerlund 1 is home to one of the largest stars ever discovered, originally named Westerlund 1-26. It is a red supergiant (although sometimes classified as a hypergiant) with a radius over 1500 times that of our Sun. Most of Westerlund 1's stars are thought to have formed in the same burst of activity, meaning that they have similar ages and compositions. The cluster is around three million years old, relatively young in astronomical terms. Our own Sun is about 4.6 billion years old.