Milky Way loses two (major) arms
Pastor Dale Morgan
Milky Way loses two (major) arms*
By Jeanna Bryner
Science and space news
USATODAY
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).
NASA: Our Milky Way Gets a Makeover
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 debued 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.