Milky Way Loses Two of Four Arms

For decades, astronomers have pictured our galaxy as sporting four major, spiral arms.

But new images effectively sever two appendages, revealing the Milky Way has just two major arms.

"We're not proposing that they change the positions of the arms," said Robert Benjamin of the University of Wisconsin, Whitewater. "What we're proposing is a change in the emphasis of the arms."

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Benjamin will present his team's results Wednesday here at a meeting of the American Astronomical Society (AAS).

The results are among a handful of presentations at the meeting to paint an evolving picture of our galactic home base.

For instance, other results presented here this week suggest a completely new arm of stars wraps around one side of the galactic bulge. And another group has identified with more accuracy the location and relative distance of the spiral arms.

Spotlight on a galaxy

The Milky Way debuted as a spiral celebrity in 1951 when astronomical morphologist William Morgan of the Yerkes Observatory presented his results showing the galaxy's three arms of hot stars, which he were then named Perseus, Orion and Sagittarius.

"Those were the first three arms of the spiral galaxy," Benjamin told "Actually, he got a standing ovation at the AAS meeting, which is something I've never seen."

Beginning in the 1960s and through the 1980s, several groups of scientists used radio astronomy to map out the Milky Way's structure, coming up with various results on how the spiral arms looked and the number of arms.

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

The galactic image that stuck, Benjamin said, was one with the four spiral arms, now called Norma, Scutum-Centaurus, Sagittarius and Perseus.

Our sun lies near a small, partial arm called the Orion Arm, or Orion Spur, located between the Sagittarius and Perseus arms.

Spiral structure

The new survey of an extensive swath of the Milky Way was done with NASA's Spitzer Space Telescope, which detects infrared light.

All objects that emit any heat can be seen in infrared, and this wavelength penetrates dust, so the new mosaic includes 800,000 snapshots and more than 110 million stars.

Using a star-counting method, Benjamin and his colleagues noticed an increase in the number of stars in the direction of the Scutum-Centaurus Arm, but not in the direction of the Sagittarius and Norma arms.

(The fourth arm, Perseus, wraps around the outer portion of our galaxy and cannot be seen in the new Spitzer images.)

The two major arms, according to these findings, are the Scutum-Centaurus and Perseus arms.

The findings confirm an earlier observation by a team of astronomers, making a strong case that the Milky Way has two major spiral arms, a common structure for galaxies with bars.

These major arms have the greatest densities of both young, bright stars and older, so-called red-giant stars.

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