The Toomre sequence is an ordered list of imaged regions of the celestial sphere, the list assembled to show the stages of galaxy mergers. It begins with instances of interacting galaxy-pairs, followed by some peculiar galaxies that suggest a merger in progress, and ends with a single galaxy. The list was assembled by astrophysicists Alar Toomre and Juri Toomre in the 1970s while exploring the mechanics of galaxy mergers through simulation, giving them the opportunity to compare the sequence with snapshots of their simulations. Their simulation-generated images showed features, such as tidal tails, observed in observed the images of galaxy interactions. The term Toomre sequence is also likely used for newer lists created to demonstrate the same phenomenon.