3C 273 is a quasar or quasi-stellar object (QSO) in Virgo. Originally discovered as a radio source in 1963, Maarten Schmidt examined the spectrum of 3C 273 and realized that it was strongly redshifted, receding from us at 16 percent of the speed of light. He realized it was unlikely to be a star and theorized that it was the bright nucleus of a distant galaxy. Today it is thought that 3C 273 lies at a distance of 2.5 billion light-years, making it one of the closest quasars to us. Its luminosity is more than 2 trillion times brighter than the Sun and 100 times that of a large galaxy. It can vary in brightness from magnitude 11.7 to 13.2, but it is the brightest quasar visible, and the farthest object that is easily visible in amateur telescopes. 3C 273's size, however, is only 1 light-year in diameter. This compares to the Milky Way Galaxy's diameter of 100,000 light-years. 3C 273 is also believed to have a mass of nearly 1 billion solar masses. 3C 273 was the first quasar to be identified.