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A galaxy's tidal arm (or tidal tail) is a "tail" on a galaxy of gas and stars, in some cases extending lengths greater than the diameter of the galaxy. Such a galaxy is called a tidal-tail galaxy. These are stars pulled out of the galaxy by a nearby galaxy by its galactic tide, a process called tidal stripping. Such a tail can suggest a recent galaxy merger when no other separate galaxy is near enough to have triggered it. Such a merger can instigate star formation, including within the tail: as much as a tenth of such a galaxy's star formation can be within such a tail.
The term tidal stream is used for similar structures drawn by tidal forces from clouds or stellar clusters.
There has been a theory that "ordinary" spiral arms are triggered by tidal interaction with other galaxies, suggesting they could be termed tidal arms.