# 11.5.1.1: The Star Conditions

The speed of sound at the critical condition can also be a good reference velocity. The speed of sound at that velocity is

$c^{*} = \sqrt{k\,R\,T^{*}} \label{shock:eq:starSpeedSound}$

In the same manner, an additional Mach number can be defined as

$M^{*} = \dfrac{U }{ c^{*}} \label{shock:eq:starMach}$