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
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
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.; R?my 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 .
Media contact: Whitney Clavin 818-354-4673/818-648-9734
Jet Propulsion Laboratory, Pasadena, Calif.