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Andromeda's twin peaks.

With dust obscuring the center of our own galaxy, astronomers sometimes turn for answers to Andromeda, the nearest galaxy similar to the Milky Way. The core of this spiral neighbor, researchers reason, should contain features resembling those at the heart of our galaxy.

But Andromeda may not fully merit its reputation as a Milky Way look-alike. Newly released images, taken in 1991 by the Hubble Space Telescope, suggest that Andromeda has two distinct clusters of stars at its core. The Milky Way has just one.

The brighter cluster, visible from Earth, was thought to lie at Andromeda's exact center. But Hubble has uncovered a second, dimmer nucleus of stars at the true center of the galaxy. The brighter cluster is offset from the dimmer center by about 5 light-years, says Tod R. Lauer of the National Optical Astronomy Observatories in Tucson, Ariz.

When lauer and his colleagues first examined the Hubble images, they were hoping to find an intense spike of light -- evidence supporting the notion that a black hole lurks at Andromeda's heart. Instead, they found two less intense peaks.

Other spiral galaxies have double centers, but Andromeda's twin peaks are more difficult to explain, Lauer says.

The brighter cluster could represent the remnant of a smaller galaxy that collided with Andromeda and was swallowed up by it a billion or so years ago, he says. However, the black hole thought to lie at Andromeda's center would have torn apart the remanant core in just a few hundred thousands years. The remnant might have survived longer if it had its own black hole, but its gravity would likely have distorted the true center of Andromeda, and the images don't show this, he adds.

Alternatively, the two peaks might actually be part of the same star cluster. A thick ring of dust cutting across the galaxy's center could create the illusion of two separate peaks, Lauer notes. But this explanation also poses problems, he says. Dust grams of average size would scatter visible light, causing the galaxy to appear redder--brighter at longer wavelenghts. Alas, Hubble detected no such effect.

A ring of larger dust grains could create the twin-peak illusion without producing reddening. But the dominance of larger grains at Andromeda's core would pose another puzzle: Why and when were the more typical, finer dust grains destroyed?

For now, admits Lauer, the Hubble images "raise more questions than they answer."
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Title Annotation:possible second star cluster observed with the Hubble Telescope
Publication:Science News
Article Type:Brief Article
Date:Aug 7, 1993
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