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Cen A (NGC 5128) in Context
 HST image of center of Cen A
NASA/HST/E. Schreier
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At the "nearby" distance of 11 million light years, Cen A gives
astronomers an excellent opportunity to study the strange and
fascinating class of galaxies called active galaxies. Active
galaxies are characterized by explosive activity that occurs in
the central regions. The energy output of these central regions,
called Active Galactic Nuclei, or AGNs, can in many cases affect
the appearance of entire galaxies.
In extreme cases, called quasars, an AGN can be a thousand
times brighter than the host galaxy. The output from the Cen A
AGN is slightly less than that of the entire host galaxy, so it
is considered a relatively weak AGN. However, it is much closer
to us than any quasar, so it is special.
The source of this central explosive activity is thought to be
a supermassive black hole with the mass of many millions of
stars. X-rays from AGN's are produced when infalling matter is
heated to temperatures of millions of degrees as it approaches
the gravitational event horizon. X-ray and gamma-ray
observatories provide close-up view of the extreme conditions
surrounding supermassive black holes.
 Schematic of
Supermassive Black Hole with Jet
NASA/CXC/SAO
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The energy radiated by a supermassive black hole can be
stupendous as it gobbles up stars. The power of an AGN depends
on the mass of the central black hole and the rate at which it
swallows or accretes matter. One explanation for the explosive
activity of Cen A's central black hole is that a collision with
one or more smaller galaxies has provided a generous supply of
gas for the supermassive black hole to accrete. The dust lanes
seen in optical images that stretch across the middle of the
galaxy may be a remnant of such a collision.
 X-ray/Optical composite of Cen A
NASA/CXC/SAO
/AURA/NOAO/NSF
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One of the most intriguing features of supermassive black holes
is that they do not suck up ALL the matter that falls within
their sphere of influence. Some of the matter falls inexorably
toward the black hole, and some explodes away from the black
hole in high-energy jets that move at near the speed of light.
The X-ray jets in Chandra's image of Cen A extend from the
center of the galaxy, out to a distance of more than 15,000
light years.
The presence of a bright X-ray jet means that magnetic fields
are continually accelerating electrons to extremely high
energies over enormous distances. Exactly how this happens is a
major puzzle that Chandra may help to solve.
Return to Centaurus A (25
Oct 99)
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