We continued with black holes, and reviewed the concept that there are two main types of black holes in the Universe: Supermassive black holes at the centers of galaxies, and stellar-mass black holes that result from the evolution of a massive star. Since black holes do not emit radiation (the escape velocity from inside a black hole has to be faster than light, and since nothing travels faster than light, nothing can be emitted from a black hole, the question is how do we know they're there? For supermassive black holes, such as the one at the center of the Milky Way, we can follow the tracks of individual stars as they zip around the black hole, and - knowing the masses of the stars (through spectral classification) and using Kepler's equations, we can calculate the mass of the unseen object. In the case of stellar-mass black holes, we need to find one in a binary system with a 'normal' star, and through the motion of the normal star, we can deduce the mass of the unseen companion.