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.
Benjamin and colleagues used Spitzer to make a mosaic of 800,000 images spanning 130 degrees across the sky and one degree above and below the galaxy's mid-plane. It encompassed more than 110 million stars.
Using newly developed software to measure star densities, the researchers found the Sagittarius and Norma arms were severely lacking. Scutum-Centaurus was rich enough and Perseus, which wraps around the outer portion of the galaxy, was hidden from view.
At a press conference, Benjamin said the two major arms seem to connect up nicely with the near and far ends of the galaxy's central bar.
"We can fit the arms together with the bar, like pieces of a puzzle," he said.
The two minor arms, Sagittarius and Norma, are believed to be filled with gas and pockets of young stars, while the big arms are dense with both young hot stars and older cooler red giant stars.
Our solar system is located near a small spur between Sagittarius and Perseus called the Orion arm, but it may not have started off there.
After traveling around the galaxy for the past 4 billion years or so since its formation, the sun may have once resided in a different arm, Benjamin noted.
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