A Better Portrait of Our Milky Way

By Eliza Strickland | June 4, 2008 3:15 pm

Milky Way galaxy starsIt’s like an out-of-body experience, but on a galactic scale.

Astronomers have put together the best image yet of the Milky Way, the spiral galaxy that is home to the Earth, and more than 200 billion other stars. To map our galaxy from within, they used infrared cameras on NASA‘s Spitzer Space Telescope, which allowed them to see through cosmic clouds of dust and gas.

The resulting data painted a radically different picture of our galaxy, and revealed that only two great arms of stars spiral out from the center of the Milky Way, not four as previously thought.

“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 [National Geographic News].

Earlier attempts at understanding what the Milky Way might look like from the outside were continually frustrated, because astronomers simply didn’t have a wide enough lens to take a comprehensive look. “For years, people created maps of the whole galaxy based on studying just one section of it, or using only one method,” said Robert Benjamin of the University of Wisconsin, Whitewater, who this week presented a new view of the Milky Way at a meeting of the American Astronomical Society in St. Louis, Mo. “Unfortunately, when the models were compared, they didn’t always agree. It’s a bit like studying an elephant blind-folded,” he said [Discovery News].

To make the current image, the research team first took over 800,000 images of the Milky Way, and patched them together into one mosaic image that contains over 110 million stars. Then they looked at the picture that emerged and found some surprises.

In the biggest change to our self-image, astronomers announced that the Milky Way only has two major arms, the long, dense clusters of stars that spiral outwards. Astronomers had previously believed that the galaxy had four, called Scutum-Centaurus, Perseus, Sagittarius and Norma. The new data demotes the latter two to minor arms.

Benjamin developed software that counts the stars, measuring stellar densities. When he and his teammates counted stars in the direction of the Scutum-Centaurus Arm, they noticed an increase in their numbers, as would be expected for a spiral arm. But, when they looked in the direction where they expected to see the Sagittarius and Norma arms, there was no jump in the number of stars. The fourth arm, Perseus, wraps around the outer portion of our galaxy and cannot be seen in the new Spitzer images [NASA]. The major arms contain a vibrant mix of bright young stars and older red giants, while the minor arms are thought to consist mostly of gas, with some pockets of young stars tucked inside.

Image: NASA/JPL-Caltech


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