Credit: NASA/JPL-Caltech/M. Regan (STScI), and the SINGS Team

Morphology of Our Galaxy's 'Twin'

NASA's Spitzer Space Telescope has captured these infrared images of a nearby spiral galaxy that resembles our own Milky Way. The targeted 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. This inclined galaxy was discovered in 1784 by William Herschel, who also discovered infrared light.

The evolution of this galaxy is a story that depends significantly on the amount and distribution of gas and dust, the locations and rates of star formation, and on how the energy from star formation is recycled by the local environment. The new Spitzer images are allowing astronomers to "read" this story by dissecting the galaxy into its separate components.

The main image, measuring 12.6 by 8.2 arcminutes, was obtained by Spitzer's infrared array camera. It is a four-color composite of invisible light, showing emissions from wavelengths of 3.6 microns (blue), 4.5 microns (green), 5.8 microns (yellow) and 8.0 microns (red). These wavelengths are roughly 10 times longer than those seen by the human eye.

The infrared light seen in this image originates from two very different sources. At shorter wavelengths (3.6 to 4.5 microns), the light comes mainly from stars, particularly ones that are older and cooler than our Sun. This starlight fades at longer wavelengths (5.8 to 8.0 microns), where instead we see the glow from clouds of interstellar dust. This dust consists mainly of a variety of carbon-based organic molecules known collectively as polycyclic aromatic hydrocarbons. Wherever these compounds are found, there will also be dust granules and gas, which provide a reservoir of raw materials for future star formation.

These shorter- and longer-wavelength views are shown separately as insets. Perhaps the most intriguing feature of the longer-wavelength image is a ring of dust girdling the galaxy center. This ring, with a radius of nearly 20,000 light-years, is invisible at shorter wavelengths, yet has been detected at sub-millimeter and radio wavelengths. It is made up in large part of polycyclic aromatic hydrocarbons. Spitzer measurements suggest that the ring contains enough gas to produce four billion stars like the Sun. Starlight was systematically subtracted from the longer-wavelength picture to enhance dust features.

Three other galaxies are seen below NGC 7331, all about 10 times farther away. From left to right are NGC 7336, NGC 7335 and NGC 7337. The blue dots scattered throughout the images are foreground stars in the Milky Way; the red ones are galaxies that are even more distant.

The Spitzer observations of NGC 7311 are part of a large 500-hour science project, known as the Spitzer Infrared Nearby Galaxy Survey, which will comprehensively study 75 nearby galaxies with infrared imaging and spectroscopy.

About the Object	Object Name: NGC 7331 Object Type: Spiral Galaxy Position (J2000): RA: 22h37m04s Dec: +34d24m56s Distance: 49,000,000 light-years or 15 Mpc Magnitude: 10.35 Constellation: Pegasus
About the Data	Image Credit: NASA/JPL-Caltech/J.D. Smith (Univ. of Arizona), Michael Regan (STScI), and the SINGS Team (PI: Robert Kennicutt (Univ. of Arizona)) Instrument: IRAC Wavelength: 3.6 (blue), 4.5 (blue-green), 5.6 (yellow), 8.0 (red) microns Exposure Dates: December 2-3, 2003 Exposure Time: 240 seconds per pixel Image Scale: 12.6 x 8.2 arcmin Orientation: North is 101 degrees CCW from up Release Date: June 28, 2004
Observers	Robert Kennicutt, Principal Investigator (Steward Observatory, University of Arizona) Lee Armus (Spitzer Science Center, Caltech) George Bendo (Steward Observatory, University of Arizona) Daniela Calzetti (Space Telescope Science Institute) Daniel Dale (University of Wyoming) Bruce Draine (Princeton University) Charles Engelbracht (Steward Observatory, University of Arizona) Karl Gordon (Steward Observatory, University of Arizona) George Helou (Caltech) David Hollenbach (NASA Ames Research Center) Thomas Jarrett (Caltech) Lisa Kewley (Harvard Center for Astrophysics) Claus Leitherer (Space Telescope Science Institute) Aigen Li (Steward Observatory, University of Arizona) Sangeeta Malhotra (Space Telescope Science Institute) Martin Meyer (Space Telescope Science Institute) Eric Murphy (Yale University) Michael Regan (Space Telescope Science Institute) George Rieke (Steward Observatory, University of Arizona) Marcia Rieke (Steward Observatory, University of Arizona) Helene Roussel (Caltech) John-David Smith (Steward Observatory, University of Arizona) Michele D. Thornley (Bucknell University, Space Telescope Science Institute) Fabian Walter (National Radio Astronomy Observatory)

Individual Images

	Composite view of NGC 7331. Screen-Resolution (450x292): JPEG High-Resolution (1239x805): JPEG | Mac TIFF | PC TIFF Credit: NASA/JPL-Caltech/M. Regan (STScI), and the SINGS Team
	Shorter-wavelength view of NGC 7331. Screen-Resolution (450x292): JPEG High-Resolution (1239x805): JPEG | Mac TIFF | PC TIFF Credit: NASA/JPL-Caltech/M. Regan (STScI), and the SINGS Team
	Longer-wavelength view of NGC 7331. Screen-Resolution (450x292): JPEG High-Resolution (1239x805): JPEG | Mac TIFF | PC TIFF Credit: NASA/JPL-Caltech/M. Regan (STScI), and the SINGS Team
	Visible-light view of NGC 7331. Note different image orientation (North is up) and crop. Screen-Resolution (441x450): JPEG High-Resolution (1177x1200): JPEG | Mac TIFF | PC TIFF Credit: Daniel Bramich (ING) and Nik Szymanek

The Spitzer Space Telescope is a NASA mission managed by the Jet Propulsion Laboratory. This website is maintained by the Spitzer Science Center, located on the campus of the California Institute of Technology and part of NASA's Infrared Processing and Analysis Center.