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Galactic Twin
What would our Milky Way galaxy look like if we could travel outside
it and snap a picture? It might look a lot like a new image by NASA's
Spitzer Space Telescope of a spiral galaxy called NGC 7331 - a virtual
twin of our Milky Way.
The picture shows our twin as
never before. Its swirling arms spin outward from a central bulge of
light, which is outlined by a ring of actively forming stars.
"Being inside our galaxy makes it difficult to see what's going on in
the center," said Dr. J.D. Smith, a member of the team that observed
NGC 7331, and an astronomer at the University of Arizona, Tucson. "By
looking at a very similar galaxy, we gain a bird's eye-view of what
the entire Milky Way might look like."
Such an outside perspective will teach astronomers how our own galaxy,
as well as others like it, might have formed and evolved.
The latest observations are the first in a large-scale effort to
observe 75 nearby galaxies with Spitzer's highly sensitive infrared
eyes. Called Spitzer Infrared Nearby Galaxies Survey, the program will
combine Spitzer data with that from other ground- and space-based
telescopes operating at wavelengths ranging from ultraviolet to radio
to create a comprehensive map of the selected galaxies.
The program's first target, NGC 7331, was chosen in part for its
striking similarities to the Milky Way. While these so-called twin
galaxies do not share the same parents, they have many features in
common, including number of stars, mass, spiral arm pattern and
star-formation rate of a few stars per year. Whether the Milky Way has
an inner star-forming ring like that of NGC 7331 is not known. NGC
7331 is located about 50 million light-years away in the constellation
Pegasus.
The Spitzer image demonstrates the power of the telescope's
infrared eyes to dissect galaxies into their various parts. Taken by
the telescope's infrared array camera, the false-colored picture
readily distinguishes NGC 7331's arms (brownish red), central bulge
(blue) and star-forming ring (yellow). The composition of materials
making up these regions was also revealed by the Spitzer observations:
the central bulge consists primarily of older stars; the ring
possesses a large amount of gas and dusty organic molecules called
polycyclic aromatic hydrocarbons, which typically glow when
illuminated by newborn stars; and the arms contain these same dust
grains to a lesser degree. Polycyclic aromatic hydrocarbons are also
found on Earth, on burnt toast and in car exhaust among other places.
Data from Spitzer's infrared spectrograph instrument were also used to
show that the center of NGC 7331 harbors either an unusually high
concentration of massive stars, or a moderately active black hole
about the same size as the one lurking at the core of our galaxy.
Image Credit: NASA/JPL-Caltech/STScI
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