Lady in red :Three Faces of Andromeda

NASA's Spitzer Space Telescope has captured stunning infrared views of the famous Andromeda galaxy to reveal insights that were only hinted at in visible light.

Spitzer's 24-micron mosaic (top panel) is the sharpest image ever taken of the dust in another spiral galaxy. This is possible because Andromeda is a close neighbor to the Milky Way at a mere 2.5 million light-years away.

The Spitzer multiband imaging photometer's 24-micron detector recorded 11,000 separate snapshots to create this new comprehensive picture. Asymmetrical features are seen in the prominent ring of star formation. The ring appears to be split into two pieces, forming the hole to the lower right. These features may have been caused by interactions with satellite galaxies around Andromeda as they plunge through its disk.

Spitzer also reveals delicate tracings of spiral arms within this ring that reach into the very center of the galaxy. One sees a scattering of stars within Andromeda, but only select stars that are wrapped in envelopes of dust light up at infrared wavelengths.

This is a dramatic contrast to the traditional view at visible wavelengths (lower left panel), which shows the starlight instead of the dust. The center of the galaxy in this view is dominated by a large bulge that overwhelms the inner spirals seen in dust. The dust lanes are faintly visible in places, but only where they can be seen in silhouette against background stars.

The multi-wavelength view of Andromeda (lower right panel) combines images taken at 24 microns (blue), 70 microns (green), and 160 microns (red). Using all three bands from the multiband imaging photometer allows astronomers to measure the temperature of the dust by its color. The warmest dust is brightest at 24 microns while the coolest is most evident at 160 microns. The blue/white areas have the hottest dust, as seen in the bulge and in the star-forming areas along the arms. The cooler dust floating further out in the ring and arms are in the redder regions.

The data were taken on August 25, 2004, the one-year anniversary of the launch of the space telescope. The observations have been transformed into this remarkable gift from Spitzer -- the most detailed infrared image of the spectacular galaxy to date.

Release: October 13, 2005

NASA's Spitzer Space Telescope has captured a stunning infrared view of Messier 31, the famous spiral galaxy also known as Andromeda.

Andromeda is the most-studied galaxy outside our own Milky Way, yet Spitzer's sensitive infrared eyes have detected captivating new features, including bright, aging stars and a spiral arc in the center of the galaxy. The infrared image also reveals an off-centered ring of star formation and a hole in the galaxy's spiral disk of arms. These asymmetrical features may have been caused by interactions with the several satellite galaxies that surround Andromeda.

"Occasionally small satellite galaxies run straight through bigger galaxies," said Dr. Karl Gordon of the Steward Observatory, University of Arizona, Tucson, lead investigator of the new observation. "It appears a little galaxy punched a hole through Andromeda's disk, much like a pebble breaks the surface of a pond."

Approximately 2.5 million light-years away, Andromeda is the closest spiral galaxy and is the only one visible to the naked eye. Unlike our Milky Way galaxy, which we view from the inside, Andromeda is studied from the outside. Astronomers believe that Andromeda and the Milky Way will eventually merge together.

Spitzer detects dust heated by stars in the galaxy. Its multiband imaging photometer's 24-micron detector recorded approximately 11,000 separate infrared snapshots over 18 hours to create the new comprehensive mosaic. This instrument's resolution and sensitivity is a vast improvement over previous infrared technologies, enabling scientists to trace the spiral structures within Andromeda to an unprecedented level of detail.

"In contrast to the smooth appearance of Andromeda at optical wavelengths, the Spitzer image reveals a well-defined nuclear bulge and a system of spiral arms," said Dr. Susan Stolovy, a co-investigator from the Spitzer Science Center at the California Institute of Technology, Pasadena.

The galaxy's central bulge glows in the light emitted by warm dust from old, giant stars. Just outside the bulge, a system of inner spiral arms can be seen, and outside this, a well-known prominent ring of star formation.