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Milky Way loses two arms

NASA says new infrared images indicate that our galaxy is trimmer than scientists have long thought.

Milky Way galaxy old and new
On the left is an artist's rendition of the revised structure of the Milky Way galaxy; on the right is an earlier sketch, showing the four-armed structure. Since the 1950s, scientists have continued to revise their models of the Milky Way. NASA/JPL-Caltech

Call it the Milky Way Light, a trimmed-down version of the galaxy we know and love.

Using new infrared imaging from the Spitzer Space Telescope, NASA scientists say our spiraled Milky Way galaxy is actually made up of just two main arms. For years, astronomers have mapped out the galaxy with four primary arms. The two arms on the chopping block--Norma and Sagittarius--haven't disappeared entirely; they've just been demoted to the lowly status of minor arm, according to NASA.

Scientists have studied parts of the galaxy for many years, but say telescopes tuned to detect infrared light give them the best picture of its layout because they can penetrate through dust. Infrared images taken in the 1990s led them to discover the large bar of creamy nougat stars in the center of the galaxy. Spitzer's new infrared shots, coupled with software that measures stellar density, indicate that Norma and Sagittarius aren't as thick as astronomers had thought.

"We will keep revising our picture in the same way that early explorers sailing around the globe had to keep revising their maps," said Robert Benjamin of the University of Wisconsin at Whitewater, who presented the results at a press conference Tuesday.