Press Release

Seeing Double: NASA Spitzer Captures Our Galaxy’s Twin

By SpaceRef Editor
June 28, 2004
Filed under , ,
Seeing Double: NASA 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.

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 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

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,

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

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, Cambridge, Mass.

Additional information about the Spitzer Space Telescope is available
at .

SpaceRef staff editor.