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Mercury Summer 2008 Table of Contents

New View of the Milky Way

JPL / CalTech

For decades, astronomers have been blind to what our galaxy, the Milky Way, really looks like. After all, we sit in the midst of it and can't step outside for a bird's-eye view.

New images from NASA's Spitzer Space Telescope are shedding light on the true structure of the Milky Way, revealing that it has just two major arms of stars instead of the four it was previously thought to possess.

"Spitzer has provided us with a starting point for rethinking the structure of the Milky Way," said Robert Benjamin of the University of Wisconsin, Whitewater. "We will keep revising our picture in the same way that early explorers sailing around the globe had to keep revising their maps."

Since the 1950s, astronomers have produced maps of the Milky Way. The early models were based on radio observations of gas in the galaxy, and suggested a spiral structure with four major star-forming arms, called Norma, Scutum-Centaurus, Sagittarius, and Perseus. In addition to arms, there are bands of gas and dust in the central part of the galaxy. Our Sun lies near a small, partial arm called the Orion Arm, or Orion Spur, located between the Sagittarius and Perseus arms.

"For years, people created maps of the whole galaxy based on studying just one section of it, or using only one method," said Benjamin. "Unfortunately, when the models from various groups were compared, they didn't always agree. It's a bit like studying an elephant blindfolded."

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