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An open string is a one-dimensional fundamental object in string theory that has two end-points, and therefore is topologically equivalent to a line interval. Such an open string can execute one of infinitely many vibrational patterns. In some cases, the lowest-energy vibration is identified as a tachyon, and it can undergo tachyon condensation. One of the other vibrational patterns may identify the open string with the photon and so on.
The oldest superstring theory containing open strings was type I string theory. However, the developments in string theory in the 1990s have shown that the open strings should always be thought of as ending on a new type of objects called the D-branes, and the spectrum of possibilities for open strings has increased a lot.
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