Explanation

Let's consider cosmic jet propagation. Cosmic jet is well found from galactic nuclei, proto stellar objects, and so on as the steeply collimated high speed flow from the source.

	[Initial Condition] Placing the exit of supersonic (Mach number is 6.0) gas at -1 < x < 1 & z = 0 into the rest gas, we will follow succeeding evolution. In the figure, color shows density, and arrows show velocity.
	[Evolved Stage] Jet compresses the rest gas at its front and propagates with making arc shaped bow shock. Density is decreasing from Red, to Orange, Olive, Green, Aqua, Blue, Perple, and Black.