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The Great Attractor

The Milky Way and Andromeda galaxies are the dominant structures in a galaxy cluster called the Local Group which is, in turn, an outlying member of the Virgo supercluster. Andromeda--about 2.2 million light-years from the Milky Way--is speeding toward our galaxy at 200,000 miles per hour.

Local Group in Virgo

This motion can only be accounted for by gravitational attraction, even though the mass that we can observe is not nearly great enough to exert that kind of pull. The only thing that could explain the movement of Andromeda is the gravitational pull of a lot of unseen mass--perhaps the equivalent of 10 Milky Way-size galaxies--lying between the two galaxies.
JPEG Image (9.9 KB)

Furthermore, our entire Local Group is hurtling toward the center of the Virgo cluster at one million miles per hour.

Virgo Cluster
The Virgo cluster lies some 50 million light years from Earth. Only the central region is shown above, containing two giant elliptical galaxies, M84 and M86. The visible part of the cluster is but a small portion of what seems to be out there. Nevertheless, the Virgo Cluster, along with several other large clusters, are in turn speeding towards a gigantic unseen mass named The Great Attractor.
JPEG Image (27.2 KB); Caption; Credits and Copyright

Jeremiah Ostriker, Princeton University, on-camera
Movie/Sound Byte
QuickTime Movie (4.1 MB); Sound File (1.7 MB); Text

Based on the velocities at these scales, the unseen mass inhabiting the voids between the galaxies and clusters of galaxies amounts to perhaps 10 times more than the visible matter.

Even so, adding this invisible material to luminous matter brings the average mass density of the universe still to within only 10-30 percent of the critical density needed to "close" the universe.

Might the universe be "open" after all? Cosmologists continue to debate this question, just as they are also trying to figure out the nature of the missing mass, or "dark matter."

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Copyright, (c) 1995: Board of Trustees, University of Illinois

NCSA. Last modified 11/2/95.