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Remarkable Hubble Space Telescope images of a star-forming ring in the disk of a distant galaxy are being presented by astronomers Ron Buta and Gene Byrd of the University of Alabama and Tarsh Freeman of Bevill State College at the 198th meeting of the American Astronomical Society in Pasadena, California. The images are of special interest because the distribution of new stars in the disk directly shows the shape of the orbit traced out over time by gas and newly formed stars.

``These images help us understand how stars form and then grow older in galaxies,'' said Buta. ``Giant clouds of interstellar gas and dust are moving along an elliptical orbit centered on the nucleus of the galaxy. Just like cars on a flat oval race track, when these clouds reach the more sharply curved ends of the ellipse, they slow down and may bump into each other. This leads to formation of young star clusters that are observed to be concentrated around the ends of the ellipse. As these star clusters age, they move away from the ends around the orbit, leading to a beautiful ring-shaped elliptical pattern.''

The ring is part of a galaxy known as NGC 3081. It is in the constellation Hydra at a distance of 100 million light years. The galaxy probably has a supermassive black hole at its center. However, this object has no impact on the star formation occurring in the ring, which lies more than 10,000 light years from the center.

The elongated shape of the orbit and the varying speed with which objects move along it are tied to the presence of a bar in the galaxy's disk. A bar is an elongated mass of old stars that extends in opposite directions in the disk from the galaxy's center. The slowly rotating bar pattern forces the gas clouds in a galaxy to move in non-circular orbits. More than 70 spiral galaxies have a bar in some form. ``We have calculated gas cloud orbital positions at different times in NGC 3081 based on the bar's gravitational field.'' said Byrd. ``The ring is caused by the way orbiting clouds of hydrogen gas in the plane of the galaxy are affected by the presence of the bar. This material is forced to move in an elliptical orbit around the ends of the bar. We can see how the star clusters age as they move away from their birth places at the ends of the elongated ring''

``The elliptical shape of the ring orbit is similar to the highly elliptical orbits of some objects in our solar system. However, there are fundamental differences between orbits in the solar system and orbits in the galaxy NGC 3081," said Freeman. ``In NGC 3081, the elliptical orbit is tied to the slowly rotating bar. Since the galaxy's bar is spread out symmetrically on both sides from the center, the elliptical orbits of the colliding clouds also extend symmetrically in the same directions on each side of the nucleus. We thought that the simple and beautiful ring pattern is created in NGC 3081 because the bar is not too strong and the disk gas is not too thick."

``We saw a surprising confirmation of our expection that the disk gas and dust was not too thick in our first views of the Hubble Space Telescope images," said Buta. ``Besides remarkably clear views of the star clusters of different ages around the ring, the Hubble Telescope also showed a group of more distant galaxies which could actually be seen looking through the disk of NGC 3081!"

For more information, contact:

Dr. Ron Buta (205-348-3792,
Dr. Gene Byrd (205-348-3793,
Mr. Tarsh Freeman (800-648-3271 ex. 5167, Color Image of NGC 3081

Color image of NGC 3081

R. Buta, G. Byrd, and T. Freeman
Color composite image of the spiral galaxy NGC 3081, showing the bright ring of star formation around the weak bar. Note the background galaxies to the lower left of center that are being seen through NGC 3081. The small, bluish color-objects in the ring are clusters of massive and very young stars. Notice how these young stars and clusters congregate near the ends of the ellipse. Because of the great distance (106 million light years), the young star clusters are almost star-like in appearance.
Deprojected Image of NGC 3081

Deprojected image of NGC 3081

R. Buta, G. Byrd, and T. Freeman
This image shows only the young clusters and massive stars that lined the ring, as well as background galaxies and foreground stars in our galaxy. The image has been deprojected to a face-on view and has been rotated so that the bar axis is horizontal. The bar is not seen in this image since it is made mostly of old stars, so we have marked its location with a red horizontal line. The red dotted ellipse indicates the orbit defined by the star-forming ring.

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