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The Milky Way Galaxy
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This image is from observations made by the COBE satellite.
COBE was
launched by NASA in November, 1989. It was designed to study the
radiation believed to be a remnant of the explosion that started the
expansion of the Universe. In scientific terms, it measured the diffuse
infrared and microwave background radiation.
The image is a near-infrared image of the Milky Way. It shows the Milky
Way from an edge-on perspective with the north pole of our Galaxy at the
top and the south pole at the bottom. At near-infrared wavelengths, the
dominant source of light is stars within our Galaxy. Even though our
solar system is part of the Milky Way, the view looks distant because
most of the light comes from the population of stars that are closer to
the galactic center (the big bulge in the middle of the disk) rather
than our own Sun.
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We live in 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 to pull us through 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 (http://adc.gsfc.nasa.gov/mw/milkyway.html) 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.
References:
- 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.
INFRARED
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.
Reference:
- Wheelock, S. L., et al. 1994, IRAS Sky Survey Atlas Explanatory Supplement, JPL Publication 94-11 (Pasadena: JPL) Order: CASI HC A08/MF A02
Optical
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.
Reference:
- Laustsen, S., Madsen, C., West, R. 1987, Exploring the Southern Sky, (
Berlin: Springer-Verlag)
X-Ray
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.
Reference:
- Snowden, S. L., et al. 1995 Astrophys. J., 454, 643
Online data access:
ROSAT All-Sky Survey at MPE (http://wave.xray.mpe.mpg.de/rosat/survey/)
ROSAT data archives at the HEASARC (http://heasarc.gsfc.nasa.gov/docs/rosat/rhp_archive.html)
Gamma Ray
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.
References:
- Hunter, S. D., et al. 1997, Astrophys. J., 481, 205
- Thompson, D. J., et al. 1996, Astrophys. J. Suppl., 107, 227
Online data access:
EGRET instrument team's Home Page (http://lheawww.gsfc.nasa.gov/docs/gamcosray/EGRET/egret.html)
EGRET data from the Compton Observatory SSC (http://cossc.gsfc.nasa.gov/egret/)
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