New Milky Way Map Created; Shows Fewer Main Arms

John Roach
for National Geographic News
June 3, 2008
Astronomers unveiled today what they are calling the best map ever produced of the Milky Way galaxy.

The new view shows our spiral galaxy as it would look face-on to a very distant observer.

The map is based on findings about the structural evolution of the Milky Way presented this week at the 212th meeting of the American Astronomical Society in St. Louis, Missouri.

Robert Benjamin of the University of Wisconsin-Whitewater was among the scientists who presented results at a briefing today with reporters.

The researchers determined that the Milky Way actually has two fewer major arms than previously believed.

In barred spiral galaxies like our own, major arms have a high density of stars, produce lots of new stars, and are clearly connected to the long bar of stars at the galactic center.

(See an image of the bar of stars at the center of the Milky Way.)

By contrast, minor arms have high gas density and presumably less star formation.

Scientists had long thought that the Milky Way has four major arms. But the new images show that the spirals are actually made of two major arms and two minor ones.

"These major arms plus the bar could be the things that really stand out if you were looking at the Milky Way galaxy from, say, [our nearest galactic neighbor] Andromeda," Benjamin said.

Sharper Resolution

Mapping the Milky Way is an unusual challenge, one that some experts have likened to a fish attempting to map the Pacific Ocean.

In particular, dust and gases tend to obscure much of our view of the galaxy's structure.

The new map is based primarily on data collected from infrared cameras on the Spitzer Space Telescope.

"Using these wavelengths of light, you can see through the dust and begin to actually glimpse at the true structure of our galaxy," John Gallagher, an expert on galaxy evolution at the University of Wisconsin-Madison, said at the briefing.

The sharp resolution of these cameras also allows astronomers to see how massive stars evolve and how cosmic structures take shape.

"You can really start to see individual objects, a few structures … and really start to understand what's going on in our galaxy," said Sean Carey, who works with one of the cameras at the Spitzer Science Center in Pasadena, California.

Radio Telescope Survey

The map also includes data on the density and location of the arms from the Very Long Baseline Array, a network of ten radio telescopes.

"We are beginning now to trace out the spiral structure of the Milky Way with solid distances," said Mark Reid of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts.

Thomas Dame, also of the CfA, presented the discovery of an unusual spiral arm called the Far 3-kiloparsec that hugs the galaxy's central bar.

The short arm appears to be a mirror image of the Near 3-kiloparsec arm on the opposite side of the galactic center, ending a 50-year search for the twin.

"Just knowing that we have these two lovely symmetric spiral arms deep in the inner galaxy," he said, "gives us some faint hope that perhaps we live in one of the rare beautiful bispiral galaxies."

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