Our Galaxy

The Milky Way is a spiral galaxy containing over 100 billion stars. It is over 100,000 light years (or 600,000,000,000,000,000 miles) in diameter and has a disk containing spiral arms and a dense central sphere or bulge.

The center of our galaxy is not visible at optical wavelengths because it is hidden behind numerous clouds of gas and dust. However we can view the center of our galaxy in the infrared, since infrared rays can penetrate gas and dust. The image to the right is a combination of infrared data from the 2MASS and MSX projects. The Galactic plane runs horizontally along the image, and the Galactic center is the bright (yellow) object near the middle. In the blue regions (2MASS) many stars invisible to optical telescopes can be seen in the infrared. The red areas (MSX) show the distribution of dust near the center of our galaxy. The center of our galaxy is one of the brightest infrared sources in the sky. It is about one thousand times brighter in the infrared than at radio wavelengths. Infrared observations show that the center of our galaxy consists of a very dense crowding of stars and that stars and gases near the center are orbiting very rapidly (probably due to the existence of a black hole). Because infrared radiation can pass more easily through gas and dust, infrared pictures reveal the structure of our galaxy much better than visible pictures can. Our galaxy, the Milky Way, radiates about half of its luminosity in the infrared.

2MASS Project, Umass, IPAC/Caltech, NSF, NASA

Michael Hauser (STScI),
the COBE/DIRBE Science Team, and NASA
To the left is an infrared image of the entire sky from the COBE satellite. The bright band in the middle of the image is our Milky Way galaxy. This is how our galaxy appears from our vantage point in the sun's orbit about the center of our galaxy. Our solar system is located far out in the disk of our galaxy at a distance of about 30,000 light years from the center of our galaxy (that is about 1,760,000,000,000,000,000 miles). It is interesting to compare images of our galaxy at several different wavelengths .

The Spitzer Space Telescope will be too sensitive to observe the bright center of our galaxy, however, surveys of the galactic plane will be conducted to study the structure of the inner galaxy. A 240 degree survey extending from 10 to 70 degrees in longitude on either side of the galactic center, and from -1 to +1 degree in latitude will be carried out using the IRAC instrument onboard Spitzer. Spitzer will also map regions of the Milky Way to characterize cirrus and count infrared sources. In addition, Spitzer will study star clusters, and star forming regions within our Milky Way galaxy.