Astronomers have estimated that the universe could contain 40 to 50 billion galaxies. A black hole is the theoretical end-product of the total gravitational collapse of a massive star or group of stars. Crushed even smaller than an incredibly dense neutron star, such a body may become so dense that not even light can escape its gravitational field. It has been suggested that black holes may be detectable in proximity to normal stars when they draw matter away from their visible neighbors. Strong sources of X rays in our galaxy and beyond may also indicate the presence of black holes.
Quasars emit tremendous amounts of light and microwave radiation.
Although a number of images show collisions between pairs of galaxies, which could trigger the birth of quasars, some pictures reveal apparently normal, undisturbed galaxies possessing quasars. Quasars are among the most baffling objects in the universe because of their small size and prodigious energy output. Quasars are not much bigger than Earth's solar system but pour out 100 to 1,000 times as much light as an entire galaxy containing a hundred billion stars. A quasar detected in March 2000 with a redshift of 5.8 is 12 billion light-years from Earth and is the most distant object ever observed to date. Redshift is the amount by which light from a distant object is shifted toward the red end of the spectrum by the expansion of the universe.