[Spitzer News] Spitzer Captures Stellar Coming of Age in Our Galaxy

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Tue Jun 3 12:38:05 PDT 2008

In this issue:

1) Spitzer Captures Stellar Coming of Age in Our Galaxy
2) Two of the Milky Way's Spiral Arms Go Missing



More than 800,000 snapshots from NASA's Spitzer Space Telescope have  
been stitched together to create a new "coming of age" portrait of  
stars in our inner Milky Way galaxy.

The image depicts an area of sky 120 degrees wide by two degrees  
tall. It was unveiled today at the 212th meeting of the American  
Astronomical Society in St. Louis, Mo.

"This is the highest-resolution, largest, most sensitive infrared  
picture ever taken of our Milky Way," said Sean Carey of NASA's  
Spitzer Science Center at the California Institute of Technology,  
Pasadena, Calif. Carey is lead investigator for one of two teams  
responsible for the new picture. "Where previous surveys saw a single  
source of light, we now see a cluster of stars. With this data, we  
can learn how massive stars form, map galactic spiral arms and make a  
better estimate of our galaxy's star-formation rate," Carey explained.

"I suspect that Spitzer's view of the galaxy is the best that we'll  
have for the foreseeable future. There is currently no mission  
planned that has both a wide field of view and the sensitivity needed  
to probe the Milky Way at these infrared wavelengths," said Barbara  
Whitney of the Space Science Institute, Madison, Wis. Whitney is a  
member of the second astronomy team.

Because Earth sits inside our dusty, flat, disk-shaped Milky Way, we  
have an edge-on view of our galactic home. We see the Milky Way as a  
blurry, narrow band of light that stretches almost completely across  
the sky. With Spitzer's dust-piercing infrared eyes, astronomers  
peered 60,000 light-years away into this fuzzy band, called the  
galactic plane, and saw all the way to the other side of the galaxy.

The result is a cosmic tapestry depicting an epic coming-of-age tale  
for stars. Areas hosting stellar embryos are identified by swaths of  
green, which are organic molecules, called polycyclic aromatic  
hydrocarbons, illuminated by light from nearby newborn stars. On  
Earth, these molecules are found in automobile exhaust and charred  
barbeque grills, essentially anywhere carbon molecules are burned  

The regions where young stars reside are revealed as "bubbles," or  
curved ridges in the green clouds. These bubbles are carved by the  
winds from young starlets blowing away their natal dust. The starlets  
appear as yellow and red dots, and the wisps of red that fill most  
bubbles are composed of graphite dust particles, similar to very  
small pieces of pencil lead.

Blue specks sprinkled throughout the photograph are individual older  
Milky Way stars. The bluish-white haze that hovers heavily in the  
middle two panels is starlight from the galaxy's older stellar  
population. A deep, careful examination of the image also shows the  
dusty remnants of dying and dead stars as translucent orange spheres.

"With these Spitzer data, we've been able to catalogue more than 100  
million stars," said Edward Churchwell of the University of  
Wisconsin, at Madison. Churchwell is principal investigator of one of  
the teams.

"This picture shows us that our Milky Way galaxy is a crowded and  
dynamic place. We have a lot to learn. I've definitely found a lot of  
things in this map that I didn't expect to see," said Carey.

This infrared composite incorporates observations from two Spitzer  
instruments. Data from the infrared array camera were collected and  
processed by The Galactic Legacy Infrared Mid-Plane Survey  
Extraordinaire team, led by Churchwell. The Multiband Imaging  
Photometer for Spitzer Galactic Plane Survey Legacy team, led by  
Carey, processed observations from Spitzer's multiband imaging  
photometer. Blue represents 3.6-micron light, green shows light of 8  
microns and red is 24-micron light.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the  
Spitzer Space Telescope mission for NASA's Science Mission  
Directorate, Washington. Science operations are conducted at the  
Spitzer Science Center at the California Institute of Technology,  
also in Pasadena. Caltech manages JPL for NASA.




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."

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- 

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  

"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  

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.



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