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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 an Astrophysicist

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