Seeing Double: Spitzer Captures Our Galaxy's 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.
Image above: NASA's Spitzer Space Telescope captured these infrared images of a nearby spiral galaxy that resembles our own Milky Way. The galaxy, known as NGC 7331 and sometimes referred to as our galaxy's twin, is found in the constellation Pegasus at a distance of 50 million light-years. + Click for full image
Image coutesy: NASA/JPL-Caltech/STScI.
The picture, which can be viewed at
, 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
"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
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
The new 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.
These findings will appear in two papers in the September
issue of a special supplement to the Astrophysical Journal.
Dr. Michael W. Regan of the Space Telescope Institute,
Baltimore, Md., is lead author of a paper detailing
observations from the infrared array camera, and Smith is
lead author of a paper on the infrared spectrograph results.
The Spitzer Infrared Nearby Galaxies Survey project is
conducted by a team of about 25 scientists from 12
institutions, and is led by principal investigator Dr.
Robert C. Kennicutt of the University of Arizona, Tucson.
Launched August 25, 2003, the Spitzer Space Telescope is the
fourth of NASA’s Great Observatories, a program that also
includes the Hubble Space Telescope, Chandra X-ray
Observatory and Compton Gamma Ray Observatory.
JPL manages the Spitzer Space Telescope mission for NASA's
Office of Space Science, Washington, D.C. Science operations
are conducted at the Spitzer Science Center at the
California Institute of Technology in Pasadena. JPL is a
division of Caltech. Spitzer's infrared spectrograph was
built by Cornell University, Ithaca, N.Y., and Ball
Aerospace Corporation, Boulder, Colo. The instrument’s
development was led by Dr. Jim Houck of Cornell. Spitzer's
infrared array camera was built by NASA Goddard Space Flight
Center, Greenbelt, Md. The camera's development was led by
Dr. Giovanni Fazio of Smithsonian Astrophysical Observatory,
Additional information about the Spitzer Space Telescope is available at http://www.spitzer.caltech.edu
NASA's Jet Propulsion Laboratory