The proposed scenario of multiple mergers for Arp 220. Each pair of spiral galaxies merges into one, resulting in two merged starburst galaxies. As time goes by (200 million years after merging or longer), these galaxies evolve into post-starburst galaxies, which then merge again, resulting in the current Arp 220 with a pair of post-starburst tidal tails. Credit: Ehime University/NAOJ)