The Physics
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Density of Outer Space

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Bibliographic Entry Result
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Chaisson, Eric, & Steve McMillan. Astronomy Today. New York: Prentice Hall, 1993: 418. "It averages roughly 1 atom per cubic centimeter, but density as great as 1000 atoms/cm3 and as small as 0.1 atom/cm3 have been found." 0.1–1000 atoms/cm3
MacMillan Encyclopedia of Physics. New York: MacMillan, 1996: 779. "On average, the density of matter in the space between the 1011 stars of the Milky Way is 0.1 neutral hydrogen atoms (H) per cubic centimeter." 0.1 atom/cm3
Mammana, Deniss L. Interstellar Space. New York: Popular Science, 2000: 220. "On the average, this haze contains about one atom per cubic centimeter." 1 atom/cm3
Pananides, Nicholas A. & Thomas Arny. Introductory to Astronomy Second Edition. 1979: 293. "The density of the gas cloud is incredibly small by terrestrial standards -- the cloud contains roughly one hydrogen atom per cubic centimeter." 1 atom/cm3
Mitten, Simon & Jacqueline. The Young Oxford Book of Astronomy.1995: 94. "If you went out in space to a spiral arm of the galaxy, you would find one to two atoms of gas per cubic inch!" 0.06–0.12 atom/cm3

Space, the final frontier. These are the words that might come to mind when you gaze up at the stars on a clear night. But wait a second, what is that dark stuff that separates all the stars? If you said it is space then you are partially correct. Outer space is divided into many levels and the one that separates the stars is called interstellar space.

It is often a misconception that space is a vacuum or simply empty. Space is a nearly perfect vacuum, even better than the best ones made in labs on earth, but it is not devoid of everything. The fact is that space is filled with tiny particles called cosmic dust and elements like hydrogen and helium. This applies for interstellar space also and all the previously mention particles make up what is known as the interstellar medium.

The interstellar medium is mainly made of lone hydrogen atoms. They do not even exist as pairs as they do on earth. I mentioned before that space is filled with hydrogen atoms. The actual density of hydrogen as it exist in interstellar space is on the average of about 1 atom per cubic centimeter. In the extremes, as low as 0.1 atom per cubic centimeter has been found in the space between the spiral arms and as high as 1000 atoms per cubic centimeter are known to exist near the galactic core.

The interstellar medium also contains cosmic dust. These particles are much bigger than hydrogen atoms. However, there are far fewer particles of cosmic dust than there are hydrogen atoms in the same volume of space. It is estimated that cosmic dust is 1000 times less common than hydrogen atoms in the interstellar medium.

Da Wei Cai -- 2000

Bibliographic Entry Result
(w/surrounding text)
Cutnell, John D. & Johnson, Kenneth W. Physics, 3rd Edition. New York: Wiley, 1995: 441. "In certain regions of outer space the temperature is about 3 K, and there are approximately 5 × 106 molecules per cubic meter." 5 atoms/cm3

Editor's Supplement -- 2001

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