The odd couple: quasars and black holes
  Quasars emit more energy than any other objects in the universe, yet are not
much bigger than the solar system. We are almost certain that quasars are
powered by giant black holes of up to $10^{10}$ times the mass of the Sun, and
that black holes of between $10^6$ and $10^{10}$ solar masses---dead
quasars---are present at the centers of most galaxies. Our own galaxy contains
a black hole of $4.3\times10^6$ solar masses. The mass of the central black
hole appears to be closely related to other properties of its host galaxy, such
as the total mass in stars, but the origin of this relation and the role that
black holes play in the formation of galaxies are still mysteries.
