We study an inverse problem for photon transport in an interstellar cloud. In particular, we evaluate the position $x_0$ of a localized source $q\left(x\right)=q_0\delta \left(x-x_0\right)$, inside a nebula (for example, a star). We assume that the photon transport phenomenon is one-dimensional. Since a nebula moves slowly in time, the number of photons $U$ inside the cloud changes slowly in time. For this reason, we consider the so-called quasi-static approximation $u$ to the exact solution $U$. By using semigroup theory, we prove existence and uniqueness...