Milky Way Has Two Main Arms, Not Four

June 4, 2008 -- Living inside the Milky Way as we do, it's hard to come up with a realistic picture of what the galaxy looks like, but that hasn't stopped astronomers from trying.

For years, they envisioned a lovely spiral galaxy with a quartet of long arms wrapping around a central core like a mother's hug. The arms even have names: Norma, Scutum-Centaurus, Sagittarius and Perseus.

They are the birthplace of stars.

Pretty picture, but not quite right.

"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.

Sky surveys of infrared radiation began painting a new picture of reality in the 1990s. Astronomers discovered a large pouch stars in the galaxy's mid-section, bulging like an expanded waistline. Later images taken with the infrared Spitzer Space Telescope showed the bar extending out farther from the center of the galaxy than previously thought.

Now it appears the Milky Way has only two main arms, not four, as is common with galaxies containing a central bar.