The Milky Way Galaxy
We livein the Milky Way Galaxy. If you were looking down on the Milky Way, it would look like a large pinwheel rotating in space. Our Galaxy is a spiral galaxy that formed approximately 14 billion years ago. Contained in the Milky Way are stars, clouds of dust and gas called nebulae, planets, and asteroids. Stars, dust, and gas fan out from the center of the Galaxy in long spiraling arms. The Milky Way is approximately 100,000 light-years in diameter. Our solar system is 26,000 light-years from the center of the Galaxy. All objects in the Galaxy revolve around the Galaxy's center. It takes 250 million years for our Sun (and the Earth with it) to make one revolution around the center of the Milky Way.
When you look up at the night sky, most of the stars you see are in one of the Milky Way arms. Before we had telescopes, people could not see many of the stars very clearly. They blurred together in a white streak across the sky. A myth by the ancient Greeks said this white streak was a "river of milk". The ancient Romans called it the Via Galactica, or "road made of milk". This is how our Galaxy became known as the Milky Way.
It is interesting to note that astronomers capitalize the "G" in galaxy when talking about our Milky Way!
Today, astronomers have been able to observe the Milky Way in all regions of the electromagnetic spectrum. They have had to be clever in making the observations since they are having to look through the disk of the Galaxy from our location in one of the arms! Below, you can see that the Milky way looks very different in different wavelengths of light. You can read more about this at the Multiwavelength Milky Way Web site.
Radio (Atomic Hydrogen)
Column density of atomic hydrogen derived from radio surveys of the 21-cm spectral line of hydrogen. On a large scale the 21-cm emission traces the "warm" interstellar medium, which is organized into diffuse clouds of gas and dust that have sizes of up to hundreds of light years. Most of the image is based on the Leiden-Dwingeloo Survey of Galactic Neutral Hydrogen. This survey was conducted over a period of 4 years using the Dwingeloo 25-m radio telescope.
- Burton, W. B. 1985, Astron. Astrophys. Suppl. Ser., 62, 365
- Hartmann, Dap, & Burton, W. B., "Atlas of Galactic Neutral Hydrogen," Cambridge Univ. Press, (1997, book and CD-ROM)
- Kerr, F. J., et al. 1986, Astron. Astrophys. Suppl. Ser.
Composite mid and far-infrared intensity observed by the Infrared Astronomical Satellite (IRAS). Most of the emission is thermal, from interstellar dust warmed by absorbed starlight, including that in star-forming regions embedded in interstellar clouds. Emission from interplanetary dust in the solar system, the "zodiacal emission," has been modeled and subtracted; the black, wedge-shaped areas indicate gaps in the IRAS survey.
- Wheelock, S. L., et al. 1994, IRAS Sky Survey Atlas Explanatory Supplement, JPL Publication 94-11 (Pasadena: JPL) Order: CASI HC A08/MF A02
Intensity of visible light from a mosaic of wide-field photographs by Laustsen, Madsen, & West (1987). Owing to the strong obscuration by interstellar dust the light is primarily from stars within a few thousand light-years of the Sun, nearby on the scale of the Milky Way, which has a diameter on the order of 100,000 light years. Nebulosity from hot, low-density gas is widespread in the image. Dark patches are due to absorbing dust clouds.
- Laustsen, S., Madsen, C., West, R. 1987, Exploring the Southern Sky, (Berlin: Springer-Verlag)
Composite X-ray intensity observed by the Position-Sensitive Proportional Counter (PSPC) instrument on the Roentgen Satellite (ROSAT). In the Milky Way, extended soft X-ray emission is detected from hot, shocked gas. At the lower X-ray energies especially, the interstellar medium strongly absorbs the X-rays, and cold clouds of interstellar gas are seen as shadows against background X-ray emission. Color variations indicate variations of absorption or of the temperatures of emitting regions. The black regions indicate gaps in the ROSAT survey.
- Snowden, S. L., et al. 1995 Astrophys. J., 454, 643
Online data access:
Intensity of high-energy gamma-ray emission observed by the Energetic Gamma -Ray Experiment Telescope (EGRET) instrument on the Compton Gamma-Ray Observatory (CGRO). The image includes all photons with energies greater than 100 MeV. At these extreme energies, most of the celestial gamma-rays originate in collisions of cosmic rays with hydrogen nuclei in interstellar clouds. The bright, compact sources indicate high-energy phenomena associated with pulsars.
- Hunter, S. D., et al. 1997, Astrophys. J., 481, 205
- Thompson, D. J., et al. 1996, Astrophys. J. Suppl., 107, 227
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