ST. LOUIS —
For decades, astronomers have pictured our galaxy as sporting four major,
spiral arms, however 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." Benjamin will present his team's results today 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 by Thomas
Dame of the Harvard-Smithsonian Center for
Astrophysics (CfA) this week suggest a completely new arm of stars wraps around one side of the galactic center.
This new arm is a virtual twin of a known arm on the near side of the galactic center. And another group led by
Mark Reid of CfA has identified with more accuracy the location and relative distances of the stars
within 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 SPACE.com.
"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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