# Speed of sound in astrophysics

Why is the speed of sound given so much importance in Astrophysics? For example in gas outflow (and accretion) problems, we often calculate the sonic point (the point at which the outflow speed becomes equal to the speed of sound). Is it simply because speed of sound is a convenient number that can be used to scale down the variables, or is something deep involved?

$$\lambda_J = c_s \left( \frac{\pi}{G\rho} \right)^{1/2}$$
where $c_s$ is the speed of sound in the gas and $\rho$ is the mean density.