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Matrix Software > Learng Astrology > Astrophysical Directions > The Galaxy > Interstellar Dust

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Back to The Galaxy   |   Back to Astrophysical Direction 

Interstellar Dust

Interstellar space is not empty, but filled with fine particles of dust (grains, smoke) and gas often mixed in clouds. These small grains of matter -- clouds of smoke or dust -- have absorbing efficiency and like cigarette smoke, diffuse or scatter starlight. Few single clouds absorb more than 3 magnitudes (see Stellar Magnitudes), but the accumulation in depth of many individual clouds in the vast cloud complexes of the "Great Rift" in the Milky Way can produce, in places, almost total obscuration. The Coal Sack dark nebula in the south Milky Way results from a dark cloud some 40 light years across absorbing somewhat more than one magnitude. It is located at a distance of some 500 light years from our Sun. Almost all of the gas and dust is concentrated in the equatorial plane of the galaxy and our observing situation in the Milky Way (in optical wavelengths) is similar to that of an edge-on external galaxy.

Distribution of Neutral Hydrogen in our Galaxy

The galactic center is totally obscured in the visual part of the spectrum by the dense dust clouds. Except for a few 'windows' (see below), this dust prevents us from seeing more than a few thousand parsecs in any direction in the galactic plane. The dust tends to clump in clouds associated with the spiral arms (see section on Spiral Arms). The most famous window through which we can look to greater distances than average is in the direction of the globular cluster NGC 6522. The most transparent or homogeneous window in the galactic equator is toward Puppis, between galactic longitudes 240°-250° (245° optimum view).

Visual windows in the plane of our Galaxy

Dust particles are not the only form of obscuring matter in interstellar space. Many kinds of gas pervade the space between the stars. The most abundant gas, Hydrogen, was discovered in the 1950s, when its emission at a radio wavelength of 21 centimeters was detected. Using radio astronomy, the first detailed map of the spiral-arm structure of our galaxy was produced. (see figure A) The gas is concentrated in the spiral-arms

Copyright (c) 1997-99 Michael Erlewine


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