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The source of a great attraction.

The source of a great attraction

The Milky Way and its galactic neighbors appear caught up in the irresistible gravitational pull of a vast agglomeration of matter whose center lies about 150 million light-years away. Known as the Great Attractor, the concentration of mass significantly alters the rate of which these galaxies spread apart as the universe expands. Alan M. Dressler of the Pasadena, Calif.-based Observatories of the Carnegie Institution of Washington and Sandra M. Faber of the University of California, Santa Cruz, have now amassed sufficient data to confirm that Great Attractor's existence and to define its extent. Working independently, Robert A. Schommer of Rutgers University in New Brunswick, N.J., and his colleagues have obtained similar results.

Dressler and Faber were members of the international team of astronomers that first identified a strong local perturbation in the smooth outward flow of matter associated with the universe's expansion (SN: 3/22/86, p. 182). Galaxies in a large region of space seemed to be streaming in the general direction of a collection of galaxies known as the Hydra-Centaurus supercluster. There entire region was expanding less rapidly than the universe as a whole, suggesting the presence of an unidentified center of attraction somewhere beyond the Hydra-Centaurus galaxies.

But the original measurements of galactic distances and velocities did not extend as far as the postulated attractor's center, where gravity would pull equally in all directions and any galaxies present would have no motion beyond that associated with the expansion of space. "We had walked on the side of a [cloud-enshrouded] mountain and inferred its presence without seeing its peak," Dressler says. The new measurements by the two research groups include galaxies that appear to lie near the attractor's center. A few of the galaxies even seem to be falling toward Earth, indicating they sit on the attractor's far side. These data enable astronomers to establish the attractor's sphere of influence.

By almost any measure, the Great Attractor represents the dominant structure in our part of the universe. Roughly spherical, it stretches 300 million light-year across the sky and contains a mass equivalent to tens of thousands of galaxies. But its enormous gravitational influence results not so much from its large mass density as from its vast extent. Compared with some galactic superclusters, "it's not a particularly dense structure," Dressler says. "It's just very big." The Milky Way itself is actually inside but on the edge of the Great Attractor.

The new data also seem to rule out the presence of an even stronger attractor just beyond the Great Attractor. Astronomers have detected concentrations of galaxies that lie farther out (SN: 4/15/89, p. 230), but these concentrations don't appear to influence galactic motion in the vicinity of the Great Attractor. Nevertheless, astronomers suspect that features on the scale of the Great Attractor are common in the universe. "We found one . . . after sampling only a small fraction of the universe," Dressler notes.
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Title Annotation:expansion of the universe
Author:Peterson, Ivars
Publication:Science News
Date:Jan 27, 1990
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