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Seeing the Milky Way take shape.

WHEN ASTRONOMERS try to determine the shape of our galaxy, they suffer from a lack of perspective. Given our location within the galactic disk, the only way to derive the galaxy's shape is to reconstruct the Milky Way from the inside out.

Astronomers have previously noted that the distribution of stars near the galactic center appears asymmetric, leading them to conclude that our galaxy has been stirred by a central bar. Following a recent examination of our galaxy's interior, it seems that the Milky Way's backbone is longer than they had thought.

A team led by Robert A. Benjamin (University of Wisconsin, Whitewater) reviewed the Spitzer Space Telescope's infrared star catalog, the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (Glimpse), to study how certain types of stars are grouped together. Glimpse lists nearly 30 million stars in the galactic plane. "What we noticed is one half of the galactic center [contains] 25 percent more stars than the other half," says Edward Churchwell (University of Wisconsin, Madison), coauthor of the team's paper in the September 10th Astrophysical Journal Letters.

Spitzer's camera sorted out carbon stars (which shine brightly in the infrared) from the opaque glow of countless stars near the galaxy's core. "Once you know what kind of stars populate the bar," says Churchwell, "the shape just jumps out at you."

By knowing the shape and orientation of the long stellar bar and by using other barred spiral galaxies as examples, scientists can envision what our galaxy should look like. The new results imply that the bar is a thin strip of stars spanning roughly 29,000 light-years and angled 45[degrees] to the imaginary line that connects the Sun and the galactic center (about 28,000 light-years away). The bar is more than one-fourth the diameter of the galactic disk, which is approximately 100,000 light-years across.

Astronomers previously thought the bar was much shorter, perhaps only 15,000 light-years long (S&T: September 2004, page 50). "I agree that this is further evidence of a bar," says William H. Waller (Tufts University), "but the parameters should be considered preliminary."

Churchwell says that the longer bar may help funnel material into the galactic center, where it can induce star formation.

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Title Annotation:astronomical research
Author:Johnston, Lisa R.
Publication:Sky & Telescope
Geographic Code:1USA
Date:Jan 1, 2006
Previous Article:Lasering the galactic center.
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