The predominant theory regarding its formation is that the progenitor galaxy was a barred spiral galaxy whose arms had velocities too great to keep the galaxy's coherence and therefore detached.[1] Arp 220 is another unusual galaxy in Serpens. The prototypical ultraluminous infrared galaxy, Arp 220 is somewhat closer than Hoag's Object at 250 million light-years from Earth. It consists of two large spiral galaxies in the process of colliding with their nuclei orbiting at a distance of 1,200 light-years, causing extensive star formation throughout both components.