This book introduces the mathematical theory of the internal structure of stars and sources of stellar energy, built by methods of the general theory of relativity and reduced to a model of liquid stars. Such a star is uniform internally, with a tiny core in the center that is separated by a collapse surface with a radius corresponding to the mass of the star. Although almost the entire mass of the star is outside the core (the core is not a black hole), the force of gravity approaches infinity on the surface of the core due to the rupture of the inner space of the star's field.