Credit: NASA/JPL-Caltech/K. Gordon (University of Arizona) & S. Willner (Harvard-Smithsonian Center for Astrophysics), N.A. Sharp (NOAO/AURA/NSF)

Messier 81

The magnificent spiral arms of the nearby galaxy Messier 81 are highlighted in this image from NASA's Spitzer Space Telescope. Located in the northern constellation of Ursa Major (which also includes the Big Dipper), this galaxy is easily visible through binoculars or a small telescope. M81 is located at a distance of 12 million light-years.

The main image is a composite mosaic obtained with the multiband imaging photometer and the infrared array camera. Thermal infrared emission at 24 microns detected by the photometer (red, bottom left inset) is combined with camera data at 8.0 microns (green, bottom center inset) and 3.6 microns (blue, bottom right inset).

A visible-light image of Messier 81, obtained with a ground-based telescope at Kitt Peak National Observatory, is shown in the upper right inset. Both the visible-light picture and the 3.6-micron near-infrared image trace the distribution of stars, although the Spitzer image is virtually unaffected by obscuring dust. Both images reveal a very smooth stellar mass distribution, with the spiral arms relatively subdued.

As one moves to longer wavelengths, the spiral arms become the dominant feature of the galaxy. The 8-micron emission is dominated by infrared light radiated by hot dust that has been heated by nearby luminous stars. Dust in the galaxy is bathed by ultraviolet and visible light from nearby stars. Upon absorbing an ultraviolet or visible-light photon, a dust grain is heated and re-emits the energy at longer infrared wavelengths. The dust particles are composed of silicates (chemically similar to beach sand), carbonaceous grains and polycyclic aromatic hydrocarbons and trace the gas distribution in the galaxy. The well-mixed gas (which is best detected at radio wavelengths) and dust provide a reservoir of raw materials for future star formation.

The 24-micron multiband imaging photometer image shows emission from warm dust heated by the most luminous young stars. The infrared-bright clumpy knots within the spiral arms show where massive stars are being born in giant H II (ionized hydrogen) regions. Studying the locations of these star forming regions with respect to the overall mass distribution and other constituents of the galaxy (e.g., gas) will help identify the conditions and processes needed for star formation.

About the Object	Object Name: Messier 81 (NGC 3031; UGC 5318) Object Type: Spiral galaxy Position (J2000): RA: 09h55m33.2s DEC +69d03m55s Distance: 12,000,000 light-years (3.6 Mpc) Magnitude: 7.9 Constellation: Ursa Major (the Big Bear)
About the Data	Image Credit: NASA/JPL-Caltech/K. Gordon (University of Arizona) & S. Willner (Harvard-Smithsonian Center for Astrophysics), N.A. Sharp (NOAO/AURA/NSF) Instruments used: IRAC, MIPS Wavelengths: 3.6-4.5 (blue), 5.8-8.0 (green), 24 (red) microns Exposure dates:     November 6, 2003 (IRAC)     November 24, 2003 (MIPS) Exposure time:    80 seconds per position (MIPS)    50 seconds per position (IRAC) Image Size: 23.3 x 17.8 arcmin [1.2 arcsec per pixel (IRAC)] Orientation: North is rotated 91 degrees clockwise from the vertical Release Date: December 18, 2003
Observers	Karl D. Gordon, Principal Investigator (University of Arizona) Steven P. Willner, Principal Investigator (Harvard-Smithsonian Center for Astrophysics) Almudena Alonso-Herroro (University of Arizona) Philip Appleton (SSC/Caltech) Matthew L. N. Ashby (Harvard-Smithsonian Center for Astrophysics) Pauline Barmby (Harvard-Smithsonian Center for Astrophysics) Chad Engelbracht (University of Arizona) Giovanni G. Fazio (Harvard-Smithsonian Center for Astrophysics) David Frayer (SSC/Caltech) George Helou (SSC/Caltech) Joannah Hinz (University of Arizona) Robert Kennicutt (University of Arizona) Karl Misselt (University of Arizona) Michael A. Pahre (Harvard-Smithsonian Center for Astrophysics) Pablo Perez-Gonzalez (University of Arizona) George Rieke (University of Arizona) Marcia Rieke (University of Arizona) Susan Stolovy (SSC/Caltech) Lisa Storrie-Lombardi (SSC/Caltech)

Individual Images

	Multiwavelength composite image of M81. Screen-Resolution (450x341): JPEG High-Resolution (1158x878): JPEG | Mac TIFF | PC TIFF Credit: NASA/JPL-Caltech/K. Gordon (University of Arizona) & S. Willner (Harvard-Smithsonian Center for Astrophysics)
	Visible-light comparison of M81. Screen-Resolution (450x341): JPEG High-Resolution (1158x878): JPEG | Mac TIFF | PC TIFF Credit: N.A. Sharp (NOAO/AURA/NSF)
	24-micron image of M81. Screen-Resolution (450x341): JPEG High-Resolution (1158x878): JPEG | Mac TIFF | PC TIFF Credit: NASA/JPL-Caltech/K. Gordon (University of Arizona) & S. Willner (Harvard-Smithsonian Center for Astrophysics)
	8-micron image of M81. Screen-Resolution (450x341): JPEG High-Resolution (1158x878): JPEG | Mac TIFF | PC TIFF Credit: NASA/JPL-Caltech/K. Gordon (University of Arizona) & S. Willner (Harvard-Smithsonian Center for Astrophysics)
	3.6-micron image of M81. Screen-Resolution (450x341): JPEG High-Resolution (1158x878): JPEG | Mac TIFF | PC TIFF Credit: NASA/JPL-Caltech/K. Gordon (University of Arizona) & S. Willner (Harvard-Smithsonian Center for Astrophysics)

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