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The galaxy next door

NASA / JPL-Caltech / CfA
Red waves of dust swirl around a blue sea of stars in this color-coded infrared image of the Andromeda Galaxy from NASA's Spitzer Space Telescope.


The Andromeda Galaxy — the nearest spiral to our own — is all dressed up in reddish, dusty swirls in a new infrared portrait from NASA's Spitzer Space Telescope. The picture, which has plenty of scientific as well as aesthetic value, is just one of the visual delights coming out of this week's meeting of the American Astronomical Society.

Infrared light serves as a thermal signature for the dust being heated up by the galaxy's young stars — a signature that doesn't come through nearly so well in visible light. As detailed in today's image advisory from Spitzer's science team, the infrared readings were used to produce new estimates of the number of stars in the galaxy.

The readings confirm that Andromeda, 2.5 million light-years away in the constellation of the same name, puts our own Milky Way galaxy to shame in the star department: Andromeda has roughly 1 trillion stars, compared with the Milky Way's 400 billion stars.

"This is the first time the stellar population of Andromeda has been determined using the galaxy's infrared brightness," Pauline Barmby of the Harvard-Smithsonian Center for Astrophysics said in today's advisory. "It's reassuring to know our numbers are in agreement with previous estimates of the mass of the stars based on the stars' motion."

Spitzer's view is actually built up from about 3,000 individual picture frames, stitched together in a submarine-shaped mosaic that also takes in a companion galaxy above Andromeda's disk (NGC 205) and another below (M32). In the color-coded image, blue represents the infrared light from older stars, and red represents the glow from dust made up of molecules called polycyclic aromatic hydrocarbons, or PAHs. In space, the substance is often associated with dense clouds of new stars; on Earth, PAHs are associated with barbecue pits and car exhaust.

George Helou, deputy director of the Spitzer Space Science Center at the California Institute of technology, marveled at the detailed tracings of star-forming material. "The challenge is to understand what shapes the distribution of this gas and dust, and what modulates the star formation at different locations," he said.

The data behind the image were gathered in January and August 2005, and the results were released today at the American Astronomical Society's spring meeting in Calgary, Alberta. The twice-yearly AAS gathering is a chance for astronomers from around the world to share their results — some of which have been held back for the occasion, and some of which may not be quite ready for prime time (or, more accurately, publication in a peer-reviewed journal).

In addition to the heavyweight studies on extrasolar planets and supernovae, you can always find some eye-pleasers among the presentations (always with a serious scientific point, of course). Here are a couple of links to other stunning images on the Web:

  • Massive galaxy clusters were used as "cosmic telescopes" to spot gravitationally lensed images of infant galaxies born in the first billion years after the beginning of the universe.
  • The Gemini South Telescope captured two stunning pictures of nebulae created by stars blowing themselves apart at the end of their lives.
  • Meanwhile, the Gemini North Telescope looked deep into the heart of the Andromeda galaxy to map a crowded star field as well as the dusty nucleus.

Stay tuned for more from the AAS meeting in the next day or two. And if you're curious about how the Spitzer Space Telescope and infrared astronomy fit into the grander scheme of things, check out our backgrounders on the Spitzer mission and the electromagnetic spectrum.