Chris Peterson wrote:
That 2000 km/s relative motion between gas clouds is certainly enough to reduce the collision interval between particles at mean free distances of tens of kilometers sufficiently to create shock fronts.
http://en.wikipedia.org/wiki/Shock_waves_in_astrophysics wrote:
<<A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium (solid, liquid, gas or plasma). Shock waves are characterized by an abrupt, nearly discontinuous change in the characteristics of the medium. Across a shock there is always an extremely rapid rise in pressure, temperature and density of the flow. A shock wave travels through most media at a higher speed than an ordinary wave [and] is treated as a discontinuity where entropy increases over a nearly infinitesimal region.
Shock waves are common in astrophysical environments:
- The bow shock of a solar system (the existence of our solar system's bow shock has been disputed recently).
Supernova remnants driving a shock through the interstellar medium (ISM).
Shocks traveling through a massive star as it explodes in a core collapse supernova.
Shocks in interstellar gas, caused by a collision between molecular clouds or by a gravitational collapse of a cloud.
Accretion shocks at the edge of galaxy clusters.
Because of the low ambient density, most astronomical shocks are collisionless. This means that the shocks are not formed by two-body Coulomb collisions, since the mean free path for these collisions is too large, often exceeding the size of the system. It is widely accepted that the mechanism driving these shocks consists of plasma instabilities, that operate on the scale of plasma skin depth(; i.e., 
,) which is typically much shorter than the mean free path(; i.e., 
).
It is known that collisionless shocks are associated with extremely high energy particles, although it has not been definitively established if the high energy photons observed are emitted by protons, electrons or both. The energetic particles are in general believed to be accelerated by the Fermi acceleration mechanism. It is usually agreed that shocks caused by supernova remnants expanding in the interstellar medium accelerate the cosmic rays measured above the Earth's atmosphere.
Shock waves in stellar environments, such as shocks inside a core collapse supernova explosion often become radiation mediated shocks. Such shocks are formed by photons colliding with the electrons of the matter, and the downstream of these shocks is dominated by radiation energy density rather than thermal energy of matter.
An important type of astrophysical shock is the relativistic shock, in which the shock velocity is a non-negligible fraction of the speed of light. These shocks are unique to astrophysical environments, and can be either collisionless or radiation mediated. Relativistic shocks are theoretically expected in gamma ray bursts, active galactic nucleus jets and in some types of supernova explosions.>>
[quote="Chris Peterson"]
That 2000 km/s relative motion between gas clouds is certainly enough to reduce the collision interval between particles at mean free distances of tens of kilometers sufficiently to create shock fronts.[/quote][quote=" http://en.wikipedia.org/wiki/Shock_waves_in_astrophysics"]
<<A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium (solid, liquid, gas or plasma). Shock waves are characterized by an abrupt, nearly discontinuous change in the characteristics of the medium. Across a shock there is always an extremely rapid rise in pressure, temperature and density of the flow. A shock wave travels through most media at a higher speed than an ordinary wave [and] is treated as a discontinuity where entropy increases over a nearly infinitesimal region.
Shock waves are common in astrophysical environments:
[b][list] The bow shock of a solar system (the existence of our solar system's bow shock has been disputed recently).
Supernova remnants driving a shock through the interstellar medium (ISM).
Shocks traveling through a massive star as it explodes in a core collapse supernova.
Shocks in interstellar gas, caused by a collision between molecular clouds or by a gravitational collapse of a cloud.
Accretion shocks at the edge of galaxy clusters.[/list][/b][b][color=#0000FF]Because of the low ambient density, [u][size=125]most astronomical shocks are collisionless[/size][/u]. This means that the shocks are not formed by two-body Coulomb collisions, since the mean free path for these collisions is too large, often exceeding the size of the system. It is widely accepted that the mechanism driving these shocks consists of plasma instabilities, that operate on the scale of [u]plasma skin depth[/u](; i.e., [img]http://upload.wikimedia.org/math/0/a/3/0a3346b5cee5289a0cfbe1325a9a099d.png[/img],) which is typically much shorter than the mean free path(; i.e., [img]http://upload.wikimedia.org/math/1/b/d/1bda38a66b117817edf9652dc865a5bc.png[/img]).[/color][/b]
It is known that collisionless shocks are associated with extremely high energy particles, although it has not been definitively established if the high energy photons observed are emitted by protons, electrons or both. The energetic particles are in general believed to be accelerated by the Fermi acceleration mechanism. It is usually agreed that shocks caused by supernova remnants expanding in the interstellar medium accelerate the cosmic rays measured above the Earth's atmosphere.
Shock waves in stellar environments, such as shocks inside a core collapse supernova explosion often become radiation mediated shocks. Such shocks are formed by photons colliding with the electrons of the matter, and the downstream of these shocks is dominated by radiation energy density rather than thermal energy of matter.
An important type of astrophysical shock is the relativistic shock, in which the shock velocity is a non-negligible fraction of the speed of light. These shocks are unique to astrophysical environments, and can be either collisionless or radiation mediated. Relativistic shocks are theoretically expected in gamma ray bursts, active galactic nucleus jets and in some types of supernova explosions.>>[/quote]