Notes: Giga (abbreviated G) is the international term for 1,000,000,000 or 109. In the U.S. this is called a billion – almost everywhere else it is a milliard. We use G to avoid confusion. M is for million. The universe is about 13G years old. eV are electron-volts; and electron has a mass-energy (E=mc2, remember) of 0.511 MeV. rad = radiation, CBR= cosmic background radiation, QSO=quasars, BH=Black hole, Msun= mass of our Sun. A.U. is astronomical units (Sun-Earth distance, = 8 light-minutes); pc is parsecs: 2.36 light-years.