Active Concentration of 106 to 1012 stars, dust and gas, that are gravitationally bound. Our galaxy contains ~2 × 1011 stars. There are four main types of galaxies: • Elliptical • Lenticular • Spiral • Irregular Click on Term to Read More characterized by very rapid (day to day) variability in total Basic property used to characterize stars, luminosity is defined as the total energy radiated by a star each second. An object’s luminosity is often compared to that of the Sun (Lsun = 4 × 1033 ergs/s = 3.9 × 1026 Watts). Luminosity has the same units as power (energy per Click on Term to Read More, no emission lines, strong nonthermal radiation, and starlike appearance. It is apparently a radio galaxy aligned so that we are looking down the jet into the Core of an atom, where nearly the entire mass and all positive charge is concentrated. It consists of protons and neutrons. Click on Term to Read More.