Arches supermassive star cluster, artwork. This star cluster contains some of the most massive stars known. Astronomers used the HST to look at the Arches cluster to learn more about star formation. It is obscured by dust in visible light but can be seen in infrared. The cluster is massive, and according to standard theories should contain stars up to around 1000 solar masses. However, it was found that the heaviest stars were no more than around 150 solar masses, implying a physical cut-off point in star formation. Why this cut-off exists is as yet unknown. The research was published in March 2005.