Heat is kinetic energy that agitate particles. Particles loose kinetic energy by transferring it to other surrounding particles. Space is not a void where particles are absent, but there are very few particles in empty space. When you have an exhaust port on earth, it is filled with air. The heat will be evacuated by being transferred to the gas particles. The gas will expend because particles with a lot of kinetic energy have more collisions with other particles. You are then able to vent heat successfully. The Death Star is in space. Why put an exhaust on the Death Star. It won't work. It would have been more efficient to surround the reactor of the Death Star with water. Water is very abundant and since the Death Star is in space, you don't really have to care about the mass of that water. The entire plot of the IV episode falls apart the minute you bring physics in the picture. Gato Villano