Explanation:
A major discovery was lurking in the data.
By accident, while preparing a talk on the
Galaxy's spiral arms for
a meeting of the
American Astronomical Society,
Tom Dame
(Harvard-Smithsonian CfA) found it -
a new
spiral arm in the Milky Way.
The arm is labeled in
this illustration as the Far 3kpc
Arm, located at a distance of 3 kpc
(kiloparsecs)
or about 10,000
light-years from
the galactic center, on the opposite side from the Sun.
Along with the Near 3kpc Arm
whose presence was known since the mid 1950s, the counterpart inner
arms now establish that the galaxy has a simple symmetry.
The arms are defined by shocked
interstellar gas
flowing along both sides of the Milky Way's
central bar.
Dame and his collaborator Patrick Thaddeus recorded the
presence of both inner spiral arms in their radio data
tracking emission
from carbon monoxide
molecules along the galactic plane.
How much star formation goes on in the counterpart arms?
Despite this depiction of stars and star forming regions
along the arms, the
last attempt
to search for star formation in the Near 3kpc Arm was in 1980 and
didn't turn up any.
The discovery of the Far 3kpc Arm has renewed
interest
in this and other
questions about
the center of
the Milky Way.
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