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The space between the stars is referred to as the interstellar medium, abbreviated ISM. By volume, the ISM is the largest "object" in the Milky Way Galaxy. Although the mass of the interstellar medium is not known with certainty, the ISM has at least enough matter to make billions of stars like our Sun.

The ISM is 99% gas and 1% dust. Hydrogen is by far the most abundant element in the ISM, accounting for about 90% of the gas. The remaining 10% of the gas is composed of other elements, with helium having by far the greatest abundance. Carbon- and silicate-grains make up the dust component of the ISM, with particle sizes ranging from 0.1 - 1 micrometers (4 - 40 millionths of an inch), although some grains are considerably smaller. Over a hundred different molecules have been found to date in interstellar space, both in clouds and on interstellar grains.
Photo of the Rose Nebula Although the interstellar medium has an average density of about 1 atom for every cubic centimeter, the distribution of atoms is far from uniform. Most conspicuous are the clouds of gas and dust that populate the ISM. The clouds are called nebulae (singular = nebula), and three types are shown on this page. The first type is emission nebulae, among the most famous, most photographed, and most beautiful objects in the heavens. The Rosette Nebula (at left) is a good example, its characteristic red color being due to light released by hydrogen atoms around 10,000 K (17,500 F). The Orion Nebula, a favorite of backyard astronomers, is another example of an emission nebula. These objects are often found near young stars. Few molecules are hardy enough to exist at the high temperatures of emission nebulae, and so consequently scientists find that they contain mostly atoms and atomic ions.
Absorption nebulae, a different type of gas-dust cloud, are also found in the interstellar medium. They appear dark, indicative of their low temperatures, sometimes as low as 10 K (- 441 F). At such temperatures, atoms can bond together to form molecules like water (H2O), carbon monoxide (CO), and methane (CH4). In turn, these molecules can stick to the cold dust grains in an absorption nebula and be altered by high-energy light to form still other, different molecules. The adjacent photo shows an irregular dark patch where stars seem to be missing. The dark patch is actually Bernard 86, an absorption nebula. Photo of a dark nebula
Photo of the Pleides representing a reflection nebula Reflection nebulae are a third type of gas-dust cloud found in the interstellar medium. These nebulae normally display a blue color due to light scattered by their dust grains, the light originating in nearby stars. A famous example is the nebulosity around the stars of the Pleiades cluster in Taurus.
All three types of nebulae can sometimes be found in a single photograph. Here is an image of the Horsehead Nebula. Can you find emission, absorption, and reflection regions? Photo of the Horsehead Nebula
In addition to these three nebular types, the space between the stars contains gaseous hydrogen atoms and dust particles. Cosmic rays also traverse the ISM, and interstellar magnetic fields have been detected by radio astronomers. And aside from being the home of a fascinating variety of astronomical objects, the interstellar medium serves as a type of "chemical recycling center" for stars. Old stars die either in supernova explosions or by casting off material more gently and forming planetary nebulae. In either case the material released from a star's demise goes into the ISM. This material in turn, over millions of years, finds its way into clouds of gas and dust from which other new stars subsequently form, stars which at the time of their own death will once again return to the interstellar medium.

- Reggie L. Hudson (Code 691)

Laboratory Branches participating in this area of research:
Code 691

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Page last updated June 26, 2001