$S_t$ is the random variable representing the risky asset price at time $t$.
M_t is the riskless asset. They are governed by the equations
$\frac{dS_t}{dt}=\mu dt + \sigma dZ_t$ and
$dM_t = rM_t dt$
where $Z_t$ is Brownian motion. If we define the discounted risky asset by $S_t^{*}=S_t/M_t$. How does the process $S_t^{*}$ become governed by
$\frac{dS_t^{*}}{dt}=(\mu-r) dt + \sigma dZ_t$ ?
I cannot see why you subtract $rdt$.