In today's class we reviewed the structure of the Milky Way, emphasizing the fact that a supermassive black hole resides at the center. The mass of the black hole is about two-and-a-half-million times the mass of our Sun, yet fits in a volume smaller than one tenth the orbit of the Earth around the Sun. That's dense! It transpires that all galaxies in the Universe contain supermassive black holes at their cores. Very distant galaxies, known as 'quasars' (from 'quasi-stellar source') were discovered in the 1960s, and radio observations showed them to power radio jets emanating perpendicularly from their core at nearly the speed of light.