Supersonic Speeds
The speed of sound plays a fundamental role in compressible gas dynamics: it is the speed at which a small amplitude disturbance propagates through a gas. Sound is actually a pressure oscillation, areas of compression and rarefaction passing through a medium (gas, liquid, or solid). When a body is moving faster than the speed of sound, it compresses the sound waves radiating out from it. These build up into a large pressure front which is the shock wave. As the body passes the observer, the effect of this shock wave is a sudden spike in pressure. In the atmosphere, this is the so-called sonic boom.
Imagine the following thought experiment. Take a platform, and move it at a constant speed through a uniform gaseous medium. Arrange a sound speaker on platform, so that it is continually emitting sound. If the platform is at rest, the sound signal propagates in all directions at the speed of sound. Now, once the body is moving forward with a fixed speed, the signal will propagate relative to the platform. So long as the platform is moving slower than the speed of sound, an observer in front of the platform will always hear the forward propagating signal before the platform itself arrives. However, if the platform moves faster than the speed of sound, an observer in front of the platform will hear absolutely nothing before the platform itself arrives. This thought experiment illustrates how supersonic speeds can play an important role in gas dynamics: they behave quite distinctly from subsonic motions.
A more detailed examination of the problem of a body moving supersonically through a uniform medium reveals that the gas arrives at the observer through a shock wave that propagates supersonically with respect to the background gas. Shock waves are like "sudden news" to the observer: because they are propagating supersonically, the observer does not hear them coming. Under ordinary terrestrial conditions, the thickness of the shock front is set by the distance that a free streaming molecule in the advancing shock travels before it impacts another molecule of unshocked gas. This distance is referred to as the collisional mean-free path. Since this distance is so small in comparison to most terrestrial dimensions, pre-shocked gas impacted by the shock is rapidly compressed and heated, and attains the post-shocked state consistent with conservation of mass, momentum, and energy.
Because the speed of sound on Earth is so large, terrestrial supersonic gas dynamics finds relatively few applications: mainly to explosions and projectiles. However, in astrophysics, supersonic speeds are very commonplace: they occur everywhere from stellar atmospheres to superonovae explosions in interstellar space. For instance, some scientists have argued that the tremendous energy released in supernovae explosions, and transmitted via shock waves to the surrounding interstellar medium, is very effective in heating the interstellar medium gas to temperatures up to millions of degrees Kelvin, where it emits strongly in the x-ray range. The heating of interstellar gas via such shock waves plays a very significant role in the overall energy budget of interstellar gas in the galaxy.
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