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The Question
(Submitted September 24, 1999)
The great attractor has been described as an agglomeration of matter.
What would it look like, a galaxy? What are its proportions and what
are its effects on the Milky Way and other local galaxies.
The Answer
The Great Attractor is far bigger than a galaxy. In the terminology
of astronomers, there are clusters of galaxies containing maybe hundreds
of galaxies, and superclusters containing many clusters. The Great Attractor
is a supercluster, or something even bigger (the terminology becomes a
bit fuzzy when it comes to the largest scale structures in the universe!).
The gravity of the Great Attractor has been pulling the Milky Way
in its direction --- the motion of local galaxies indicated there was
something massive out there that are pulling the Milky Way, the Andromeda
Galaxy, and other nearby galaxies towards it. For a while, nobody could
see what it was, because it lies behind the plane of our Galaxy --- that
means the gas and dust in our Galaxy obscures the light from the Great
Attractor, and it is outshone by the stars and other objects in our Galaxy.
X-ray observations with the ROSAT satellite then revealed that Abell 3627,
a previously known cluster of galaxies, was much more massive than
originally suspected, containing many more galaxies. Optical astronomers
had missed a great number of galaxies, because of the obscuration, but
with hindsight (and with better observations), could spot many more
galaxies. It is now thought that the Great Attractor is probably a
supercluster, with Abell 3627 near its center.
There is an optical image of Abell 3627 at:
http://antwrp.gsfc.nasa.gov/apod/ap960218.html
Hope this helps.
Koji Mukai, Rich Mushotzky & Maggie Masetti
for Ask a High Energy Astronomer
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