A quasar is the highly energetic core of a remote active
galaxy; quasars are the most luminous objects in the universe, capable
of radiating over a trillion times as much energy as the Sun from a region
little larger than the Solar System. The first quasars were found because
of their radio emission and were called quasistellar radio sources.
But these represent only about 1% of the quasar population. When others
turned up that were radio quiet, the name was changed to quasistellar
object (QSO). In either case, quasar is a contraction of "quasistellar,"
in reference to the fact that the visual appearance is star-like. Following
Maarten Schmidt's discovery that
quasars have very high redshifts, it became
clear that they lie at remote cosmological distances and, therefore, to
appear as bright as they do, must be fantastically luminous. Moreover, some
quasars show marked variability over a period of just a few days, pointing
to an incredibly compact source. Their radio structures often include jets and lobes similar to what we see from radio
galaxies. Indeed, quasars and radio galaxies are simply different aspects
of the same active galactic nucleus phenomenon, all powered ultimately by
the same engines – supermassive
black holes. The nearest quasar is 3C273, at a distance of about 2.5
billion light-years in Virgo (R.A. 12h 29.1m,
Dec. +20° 3.1'), the first of its type to be identified (1963) and the
brightest quasar in apparent magnitude (12.8); it is visible with a good
25-cm telescope under dark skies and therefore qualifies as the most remote
object normally accessible to serious amateurs. 3C273 is unusually luminous
for being, in quasar terms, relatively nearby: most of its brilliant brethren
are found populating the early universe at distances of around 10 billion
light-years. 3C273 also has by far the brightest optical jet known among
Several quasars have been discovered with redshifts greater than 6, which
places them at distances of around 13 billion light-years and a time of
less one billion years after the Big Bang.
Observations of these remote objects thus shed valuable light on conditions
in the early universe.