Interstellar Matter (Encyclopedia of Science)
The interstellar mediumhe space between the starsonsists of nearly empty space. It is the vacuum of the universe. It would be totally empty if not for a smattering of gas atoms and tiny solid particlesnterstellar matter.
On average, the interstellar matter in our region of the galaxy consists of about one atom of gas per cubic centimeter and 25 to 50 microscopic solid particles per cubic kilometer. In contrast, the air at sea level on Earth contains about 1,019 molecules of gas per cubic centimeter.
In some regions of space, however, the concentration of interstellar matter is thousands of times greater than average. Where there is a large enough concentration of gas and particles (also called cosmic dust), clouds form. Most of the time these clouds are so thin they are invisible. At other times they are dense enough to be seen and are called nebulae (plural for nebula).
Cosmic dust accounts for only 1 percent of the total mass in the interstellar medium; the other 99 percent is gas. Scientists believe the dust is primarily composed of carbon and silicate material (silicon, oxygen, and metallic ions), possibly with solid carbon dioxide and frozen water and ammonia. A dark nebula is a relatively dense cloud of cosmic dust. The nebula is dark because much of the starlight in its path is either absorbed or...
(The entire section is 846 words.)
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