What We See
Photography changes what we can see in the universe
Giovanni G. Fazio, senior physicist at the Harvard-Smithsonian Center for Astrophysics, explains how infrared photography enables us to see aspects of the past, present, and future of the universe.
This dazzling infrared image of the nearby spiral galaxy Messier 81 (M81), taken on November 6, 2003, was one of the first pictures made by the Infrared Array Camera (IRAC) on NASA’s Spitzer Space Telescope. Spitzer is the fourth and last in the series of what NASA calls its “Great Observatories.” NASA’s Great Observatories Program is a series of four orbiting observatories, each observing the Universe in a different kind of light. The Hubble Space Telescope (HST) views the Universe in ultraviolet and visible light, while the Compton Gamma-Ray Observatory (CGRO) and the Chandra X-Ray Observatory (CXRO) view it in gamma-rays and x-rays, respectively.
The Infrared Array Camera (IRAC) is one of three instruments on Spitzer and this image, released to the public at a press conference at NASA Headquarters on December 18, 2003, demonstrated that Spitzer was “ready for business” and opened a new window to the Universe. In the years since then, Spitzer has been operating very successfully, producing numerous and spectacular images that help us make new discoveries about our Universe.
M81—located in the northern constellation of Ursa Major (which includes the Big Dipper) at a distance of twelve million light-years from Earth—is easily visible using binoculars or a small telescope. Because it is relatively close by to our Milky Way galaxy, M81 provides astronomers with a unique opportunity to study the anatomy of another galaxy. The challenge for astronomers has been to see and photograph this galaxy as clearly as possible, which was difficult to do in the past because M81 contains clouds of gas and dust particles that conventional human vision and photographic technology cannot “see” through. The Spitzer, fitted out with cameras that record infrared radiation, made it possible, for the first time, to get an unobstructed view of this galaxy’s constituent parts.
Since our eyes cannot see infrared radiation, images made with the IRAC need to be altered by applying what we call “false color” to them. Specific colors are assigned to the various infrared wavelengths captured in IRAC images to make the light emitted from M81 visible to the human eye. In the image reproduced here, 3.6 microns wavelength is colored blue, 4.5 microns is colored green, 5.8 microns is yellow, and 8.0 microns is colored red. Because different wavelengths originate from different key constituent parts of the galaxy, we can use these colors to “dissect” the galaxy. For example, old stars in the galaxy emit radiation primarily at 3.6 and 4.5 microns, and therefore we see the distribution of stars and particularly, the central stellar bulge of the galaxy, in bluish-white color. At longer wavelengths, 5.8 and 8.0 microns, we see interstellar dust that has been heated by visible and ultraviolet light from nearby stars, and that appears as grand spiral arms. This dust actually consists of very small particles of silicates (like sand) and polycyclic aromatic hydrocarbons, which trace the molecular gas distribution in the galaxy. The gas/dust mixture, in the form of giant clouds within the spiral arms, is the reservoir of raw materials for future star formation. The clumpy red knots (8.0 microns) within the spiral arms indicate regions where massive stars are being born. Note that the white stars scattered throughout the image are foreground stars within our own Milky Way galaxy.
The high spatial resolution and sensitivity of the Spitzer Space Telescope/IRAC have allowed us, for the first time, to distinguish between several key constituents of the galaxy: the old stars, “hot” interstellar dust, and the embedded sites of massive star formation. These images have also permitted the determination of the mass of the galaxy in stars, quantitative measurements of the galaxy’s overall dust content and distribution in the spiral arms, as well as the rate at which new stars are being born in these arms. Through photography, and for the first time, we clearly see the anatomy of a galaxy, and indicators of its past, present, and future.
While I serve as the principal investigator of IRAC, Dr. Karl D. Gordon, University of Arizona, and Dr. Steven P. Willner, Harvard Smithsonian Center for Astrophysics, were the principal observers for this amazing image which, since it was made, has taken on a life of its own. It has appeared on the cover of a popular astronomy text book, and as the large backdrop for a NASA exhibition booth at an American Astronomical Society meeting. More images and information about the Spitzer Space Telescope are available at http://www.spitzer.caltech.edu.
- Spiral Galaxy Messier 81 (M81), 2003
- Spitzer Space Telescope / IRAC