A Farewell To Arms

[Ronn! Blankenship]

Whitney Clavin 818-354-4673/818-648-9734
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin at jpl.nasa.gov

NEWS RELEASE: 
2008-094 
June 3, 2008

Two of the Milky Way's Spiral Arms Go Missing

St. Louis, Mo. -- 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.

Now, 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, who presented the new 
results at a press conference today at the 212th 
meeting of the American Astronomical Society in 
St. Louis, Mo. "We will keep revising our picture 
in the same way that early explorers sailing 
around the globe had to keep revising their maps."

An artist's concept of the structure of our 
two-armed Milky Way is online at 
http://www.nasa.gov/mission_pages/spitzer/multimedia/20080603a.html .

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 blind-folded."

Large infrared sky surveys in the 1990s led to 
some major revisions of these models, including 
the discovery of a large bar of stars in the 
middle of the Milky Way. Infrared light can 
penetrate through dust, so telescopes designed to 
pick up infrared light get better views of our 
dusty and crowded galactic center. In 2005, 
Benjamin and his colleagues used Spitzer's 
infrared detectors to obtain detailed information 
about our galaxy's bar, and found that it extends 
farther out from the center of the galaxy than previously thought.

The team of scientists now has new infrared 
imagery from Spitzer of an expansive swath of the 
Milky Way, stretching 130 degrees across the sky 
and one degree above and below the galaxy's 
mid-plane. This extensive mosaic combines 800,000 
snapshots and includes over 110 million stars.

Benjamin developed software that counts the 
stars, measuring stellar densities. When he and 
his teammates counted stars in the direction of 
the Scutum-Centaurus Arm, they noticed an 
increase in their numbers, as would be expected 
for a spiral arm. But, when they looked in the 
direction where they expected to see the 
Sagittarius and Norma arms, there was no jump in 
the number of stars. The fourth arm, Perseus, 
wraps around the outer portion of our galaxy and 
cannot be seen in the new Spitzer images.

The findings make the case that the Milky Way has 
two major spiral arms, a common structure for 
galaxies with bars. These major arms, the 
Scutum-Centaurus and Perseus arms, have the 
greatest densities of both young, bright stars, 
and older, so-called red-giant stars. The two 
minor arms, Sagittarius and Norma, are filled 
with gas and pockets of young stars. Benjamin 
said the two major arms seem to connect up nicely 
with the near and far ends of the galaxy's central bar.

"Now, we can fit the arms together with the bar, 
like pieces of a puzzle," said Benjamin, "and, we 
can map the structure, position and width of 
these arms for the first time." Previous infrared 
observations found hints of a two-armed Milky 
Way, but those results were unclear because the 
position and width of the arms were unknown.

Though galaxy arms appear to be intact features, 
stars are actually constantly moving in and out 
of them as they orbit the center of the Milky 
Way, like London commuters in a busy traffic 
circle. Our own sun might have once resided in a 
different arm. Since it was formed more than 4 
billion years ago, it has traveled around the galaxy 16 times.

Co-investigators of this research include Ed 
Churchwell, Marilyn Meade and Brian Babler of the 
University of Wisconsin, Madison; Barbara Whitney 
of the Space Science Institute, Madison, Wis.; 
Rémy Indebetouw of the University of Virginia, 
Charlottesville; and Christer Watson of 
Manchester College, Ind. NASA's Jet Propulsion 
Laboratory, Pasadena, Calif., manages the Spitzer 
mission for NASA's Science Mission Directorate, 
Washington. Science operations occur at the 
Spitzer Science Center at the California 
Institute of Technology, also in Pasadena. For 
more information about Spitzer, visit 
http://www.spitzer.caltech.edu/spitzer and http://www.nasa.gov/spitzer .



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