We used a mathematical model of crystal growth of [1] for the description of an eutectic pattern [2]. But the stationary problem gives physically unrealizable solutions (fig. 1) for values of system parameters corresponding to real experiments. Now known models [1,3-7] of crystal growth, used for a research of the interface stability of directed crystallization, not take into account of an external temperature field too. As examples of application of these models the regimes of crystal growth are usually used. The purpose of this work - to construct a model of a binary melt crystallization taking into account an external temperature field. Within the framework of this model we deduce analytical dependence of period of the eutectic structure on parameters of the system. We demonstrate, that for real parameters of the system, the parameters of the external temperature field weakly influence period of the eutectic pattern. This outcome is observed in experiments. We also explain the reason of joint emerging eutectic and dendrite of a structure for want of growth of eutectic crystals. We also explain the reason of joint both eutectic growth and dendrite growth under eutectic growth.