Interstellar Medium
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Wikipedia-Article "Interstellar Medium"
In astronomy, the interstellar medium (or ISM) is the matter and energy content that exists between the stars (or their immediate circumstellar environment) within a galaxy. The ISM plays a crucial role in astrophysics precisely because of its intermediate role between stellar and galactic scales. Stars themselves form within cold regions of the ISM, and replenish the ISM with matter and energy through stellar winds and supernovae. In turn, this interplay between stars and the ISM sets the rate at which a galaxy depletes its gaseous content and therefore determines its lifespan of active star formation.
The ISM consists of an extremely dilute (by terrestrial standards) plasma, consisting of a mixture of atoms, molecules, dust, electromagnetic radiation, cosmic rays, and the magnetic field. The matter normally consists of about 99% gas particles and usually 1% dust. It fills interstellar space. This mixture is usually extremely tenuous, with typical gas densities ranging from a few single to a few hundred particles per cubic centimeter. As a result of primordial nucleosynthesis, the gas is roughly 90% hydrogen and 10% helium, with additional elements ("metals" in astronomical parlance) present in trace amounts.
| Component | Proportion | Temperature (K) |
Density (atoms/cm³) |
State |
|---|---|---|---|---|
| HI clouds | 50 - 100 | 1 - 103 | neutral hydrogen atoms | |
| Molecular clouds | 20 - 50 | 103 - 105 | neutral hydrogen molecules | |
| Warm Ionized Medium (WIM) |
50% | 103 - 104 | 0.01 | partially ionized plasma |
| H II regions | 104 | 102 - 104 | mostly ionized plasma | |
| Coronal gas | 105 - 106 | 10-4 - 10-3 | fully ionized plasma | |
| Source | ||||
The medium is also responsible for extinction, namely the decreasing light intensity of a star as the light travels through the medium. This extinction is caused by refraction and absorption of photons in certain wavelengths.
For example, a typical absorption wavelength of atomic hydrogen lies at about 121.5 nanometers, the Lyman-alpha transition. Therefore, it is nearly impossible to see light emitted at that wavelength from a star, because most of it is absorbed during the trip to Earth by Lyman-alpha absorption.
The interstellar medium is usually divided into three phases, depending on the temperature of the gas: hot (millions of kelvins), warm (thousands of kelvins), and cold (tens of kelvins). This "three-phase" model of the ISM was initially developed by McKee and Ostriker in a 1977 paper, which has formed the basis for further study over the past quarter-century. The relative proportions of the phases are still a matter of considerable contention in scientific circles.
Features prominent in the study of the interstellar medium include molecular clouds, interstellar clouds, supernova remnants, planetary nebulae, and similar diffuse structures.
History
Originally, astronomers thought that space was an empty vacuum. In 1913, Norwegian explorer and physicist Kristian Birkeland may have been the first to predict that space is not only a plasma, but also contains "dark matter". He wrote: "It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It does not seem unreasonable therefore to think that the greater part of the material masses in the universe is found, not in the solar systems or nebulae, but in "empty" space. (See "Polar Magnetic Phenomena and Terrella Experiments", in The Norwegian Aurora Polaris Expedition 1902-1903 (publ. 1913, p.720).