Quasars
When quasars were discovered in 1963, no one realized they
were looking at objects that harboured the biggest black holes in the Universe.
After all, they look just like faint stars. Astronomers soon worked out
that quasars are billions of light years away, and to be visible at that
distance, they must be immensely bright. In fact, they are not stars at
all, but the glaring hearts of remote galaxies - star cities in such turmoil
that the Milky Way's activity looks tame. The only way so much enregy can
be concentrated into such a small region is by the gravity exerted by a
truely massive black hole. The brillant light is an accretion disc of gas
spiralling into the hole. Astronomers can weigh the black hole in a quasar
by measuring the spped of orbiting stars or gas: the higher the speed,
the heavier the hole. The record is a black hole weighing in at 100 billion
Suns - as massive as the entire Milky Way.
Breakneck Speeds
This is the heart of a quasar - a glaring accretion disc
made up of gas, torn-up stars, and dirty dust whirls at breakneck speed
around a supermassive black hole, weighing in at billions of Suns. this
activity powers jets that shoot into space at almost the speed of light.
Gas bowl
A quasar's brillant light comes from the hot gas at the
centre of the accretion disc. The expansion of this gas, together with
the forces of gravity and rotation, push the two faces of the disc apart,
creating a bowl shape. Powerful magnetic fields speed up atomic particles
in the gas and force them away as a pair of jets.
Active Family
Quasars have cousins in the radio galaxies and blazars:
all three are often called "active galaxies". In fact, they may be one
and the same. What we see depends on the angle at which we are viewing
them: whether we are seeing the accretion disc and jet face-on, edge-on,
or at an angle to us.
Discovery of Quasars
In 1963, Dutch-American astronomer Maarten Schmidt was
puzzling over a "star" called 3C 273 that gave out light and radio waves.
When he analyzed the light, it didn't make sense. Then he realized: 3C
273 is farther away than most galaxies and the expansion of the Universe
stretches the wavelength of its light. 3C 273 was called a "quasi-stellar
radio source", soon shortened to "quasar".
The first of thousands of quarsars to be discovered, 3C
273 is one of the nearest quasars, just 2 bilion light years form us. The
average quasar is no bigger than our Solar System, yet brighter than a
trillion Suns.
Black holes need food
For all its awesome might, the accretion disc is not
a permanent feature. Eventually, all its gas and dust will spiral into
the black hole. Deprived of its food supply, the black hole lurks starved
and unsuspected, and the surrounding galaxy looks entirely normal. But
if more stars and gas come too close, the hole "wakes up" and feeds voraciously:
the quasar is rekindled.
Click the button below to view
the Quasar
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