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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, that were indeed evident the actual images of galaxy interactions. The term Toomre sequence is also used for newer lists created to demonstrate the same phenomenon.
The Toomres' sequence of eleven galaxy images was introduced at the 1977 conference, The Evolution of Galaxies and Stellar Populations, whose proceedings were published but are not freely available online. The galaxies in the eleven images are: NGC 4038/9, NGC 4676, NGC 7592, NGC 7764A, NGC 6621/2, NGC 3509, NGC 0520, NGC 2623, NGC 3256, NGC 7252, and NGC 3921.