For the theory to work, the hole must have a mass of about 100 million solar masses packed into a region not much larger than the Solar System. An accretion disk forms around the hole as matter falls in, and hot gas boils off the disk, forming jets and clouds of radio-emitting gas. The amount of energy generated may dwarf the light output from the galaxy's stars and, for a quasar, outshine the Milky Way by a factor of a thousand. Galaxies are usually grouped into clusters a few million light-years across. The Milky Way belongs to a poor cluster called the Local Group , which contains about thirty member galaxies.
Rich clusters contain thousands of galaxies, which are primarily ellipticals.
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