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Active Galactic Nuclei (AGN) produce more energy than
the rest of the galaxy put together! They often look like
stars because some are so bright that they can be seen
from billions of light years away. They are bright in
X-rays, visible light and radio band. The X-rays in AGNs
change the most - we reckon that the quickest variability
is 50 seconds per million Suns of mass in the black hole,
so for a black hole as heavy as 100 million Suns, we’d
expect to see changes over 1-2 hours.
WHAT ELSE CAN WE X-RAY?
The picture to the right shows a mosaic of X-ray photos
of the centre of our galaxy taken by Chandra, one of
NASA’s X-ray observatories. The colours are made
up of red (low energy X-rays), green (medium energy
X-rays) and blue (high energy X-rays). The bright white
spot in the middle is the stuff falling into the giant
black hole (don’t worry we’re too far away
to be in any danger!); however, you can also see hundreds
of ordinary stars, lone neutron stars, white dwarfs
and hot gas.
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