Usually galaxies are rather tame and benign, but a few, perhaps
about 10,000 of the billions, are called active galaxies.
The most spectacular of these are quasars (quasi-stellar
radio sources), which are violent explosive galaxies emitting
vast amounts of radiation into the universe. It is unknown at
present what processes lead to the formation of quasars;
some believe they occur when an enormous black hole forms
at the center of a galaxy and starts pulling in large amounts of
matter.