Firehose instability
![Fig. 1. The firehose instability in an N-body simulation of a prolate elliptical galaxy. Time progresses top–down, from upper left to lower right. Initially, the long-to-short axis ratio of the galaxy is 10:1. After the instability has run its course, the axis ratio is approximately 3:1. Note the boxy shape of the final galaxy, similar to the shapes of bars observed in many spiral galaxies.](/Images/godic/202501/13/FirehoseNbody5555.png")
![Fig. 2. Unstable eigenmodes of a one-dimensional (prolate) galaxy. Growth rates are given at the left.](/Images/godic/202501/13/FirehoseEvenModes5556.png")
The firehose instability (or hose-pipe instability) is a dynamical instability of thin or elongated galaxies. The instability causes the galaxy to buckle or bend in a direction perpendicular to its long axis. After the instability has run its course, the galaxy is less elongated (i.e. rounder) than before. Any sufficiently thin stellar system, in which some component of the internal velocity is in the form of random or counter-streaming motions (as opposed to rotation), is subject to the instability.