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The interstellar medium (or ISM) is the name astronomers give to the tenuous gas and dust that pervade interstellar space. Whilst the ISM refers to the matter (interstellar matter, also abbreviated by ISM) that exists between the stars within a galaxy, the energy, in the form of electromagnetic radiation, that occupies the same volume is called the interstellar radiation field (or ISRF). The ISM consists of an extremely dilute (by terrestrial standards) plasma, gas and dust, consisting of a mixture of ions, atoms, molecules, larger dust grains, electromagnetic radiation, cosmic rays, and magnetic fields. The matter consists of about 99% gas and 1% dust by mass. It fills interstellar space. This mixture is usually extremely tenuous, with typical gas densities ranging from a few hundred to a few hundred million particles per cubic meter. As a result of primordial nucleosynthesis, the gas is roughly 90% hydrogen and 10% helium by number, with additional elements ("metals" in astronomical parlance) present in trace amounts. The ISM plays a crucial role in astrophysics precisely because of its intermediate role between stellar and galactic scales. Stars form within the densest regions of the ISM, molecular clouds, and replenish the ISM with matter and energy through planetary nebulae, stellar winds, and supernovae. In turn, this interplay between stars and the ISM helps determine the rate at which a galaxy depletes its gaseous content, and therefore its lifespan of active star formation.
The history of interstellar space The nature of the interstellar medium has received the attention of astronomers and scientists over the centuries. However, they first had to acknowledge the basic concept of "interstellar" space. The term appears to have been first used in print by Francis Bacon in 1626 where he wrote: "The Interstellar Skie.. hath .. so much Affinity with the Starre, that there is a Rotation of that, as well as of the Starre." (Sylva §354–5). Later, natural philosopher Robert Boyle summised: "The inter-stellar part of heaven, which several of the modern Epicureans would have to be empty." (1674 Excell. Theol. ii. iv. 178) Before modern electromagnetic theory early physicists postulated that an invisible luminiferous aether existed as a medium to carry lightwaves. It was assumed that this aether extended into interstellar space, as R. H. Patterson wrote in 1862, "This efflux occasions a thrill, or vibratory motion, in the ether which fills the interstellar spaces" (Ess. Hist. & Art 10). The advent of deep photographic imaging allowed Barnard to produce the first images of dark nebulea silouetted against the background star field of the Galaxy. In 1904 Hartmann detected spectroscopic absorption lines towards a pair of binary stars that could not have come from the stars themselves. The growing evidence for interstellar material led William Henry Pickering to comment in 1912 that "While the interstellar absorbing medium may be simply the ether, yet the character of its selective absorption, as indicated by Kapteyn, is characteristic of a gas, and free gaseous molecules are certainly there, since they are probably constantly being expelled by the Sun and stars..." The same year Victor Hess's discovery of cosmic rays, highly energetic charged particles that rain down on the Earth from space, led others to speculate whether they also pervaded interstellar space. The following year the Norwegian explorer and physicist Kristian Birkeland 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 sic 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). In 1930, Samuel L. Thorndike notes that ".. it could scarcely have been believed that the enormous gaps between the stars are completely void. Terrestrial aurorae are not improbably excited by charged particles from the Sun emitted by the Sun. If the millions of other stars are also ejecting ions, as is undoubtedly true, no absolute vacuum can exist within the galaxy". The Three Phase Model In 1969 Field, Goldsmith, & Habing put forward the a static two phase equilibrum model to explain the observed properties of the ISM. Their ISM consisted of a cold dense phase (T<300K), comprised of clouds of neutral and molecular hydrogen, and a warm intercloud phase (T~1,000K), comprised of rarefied neutral and ionized gas. McKee and Ostriker later added a dynamic third phase that represented the very hot (T = 1,000,000K) gas which had been expelled from supernovae and HII regions and constituted most of the volume of the ISM. Their 1977 paper formed the basis for further study over the past quarter-century. However, the relative proportions of the phases and their subdivisions are still a matter of considerable contention in scientific circles. The following table shows a breakdown of the properties and origin of the components of the three phases. Structures Features prominent in the study of the interstellar medium include molecular clouds, interstellar clouds, supernova remnants, planetary nebulae, and similar diffuse structures. Interstellar Extinction The medium is also responsible for extinction and reddening, the decreasing light intensity and dominant observable wavelengths of a star as the light travels through the medium. These effects are caused by scattering and absorption of photons and allows the ISM to be observed with the naked eye in a dark sky. The rifts that can be seen in the band of the Milky Way are caused by absorption of background starlight from the uniform disk of stars by molecular clouds within a few thousand light years. Far ultraviolet light is absorbed effectively by the neutral components of the ISM. 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 farther than a few hundred light years from Earth, because most of it is absorbed during the trip to Earth by intervening neutral hydrogen. Interstellar Radiation Field The interstellar radiation field (ISRF) is the sum total of all electromagnetic radiation travelling through interstellar space. See also | ||||||||
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