EANA: Interstellar Medium

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Interstellar Medium

Outer space is not a complete vacuum. Although the interstellar regions are more devoid of matter than any vacuum artificially created on Earth, there is matter in space. Interstellar media have very low densities and consist mainly of gas (99%) and dust. In total, approximately 15% of the visible matter in our Galaxy is composed of interstellar gas and dust.

Besides abundant hydrogen and helium, 114 interstellar and circumstellar gaseous molecules are currently identified in the interstellar medium, see Figure 2. Ultraviolet irradiation of dust grains may result in the formation of complex organic molecules or even total carbonization of the sample, forming - according to the local environmental conditions - carbonaceous matter such as amorphous carbon, hydrogenated amorphous carbon or coal- and kerogen-like material.

Figure 1: Representation of the Interstellar Medium and its composition of objects. 

Listing of interstellar and circumstellar molecules as compiled per October 2000 (Al Wootten). Observations indicate the presence of molecules larger than 12 atoms, such as polycyclic aromatic hydrocarbons (PAHs), fullerenes, and other in the interstellar medium.

Figure 2: List of Molecules that have been observed in space.

Apart from ultraviolet irradiation, cosmic rays and thermal processing are the most important processes which modify interstellar dust are grain-grain collisions, sputtering and grain growth. The incorporation of interstellar matter in meteorites and comets in the pre-solar nebula (as evidenced by isotopic measurements) provides the basis of the cosmic dust connection. Polycyclic aromatic hydrocarbons (PAHs), fullerenes and complex aromatic networks have a strong common link and their evolutionary cycle is dominated by ultraviolet irradiation, as shown by simulation experiments in laboratory studies.

The Infrared Space Observatory ISO, launched in November 1995, has provided extraordinary results concerning the nature of cosmic dust particles. ISO allowed a new definition of the composition of interstellar ices, thermal processing in the protostellar vicinity and gas-grain chemistry. A comparison of interstellar and cometary ices using recent ISO data have revealed important similarities between interstellar ices and volatiles measured in the coma of some comets.

The striking identification of fosterite around some young stars and in comet Hale-Bopp, indicates that the disk around such stars could contain comets. The link between processes in dark embedded clouds and comets seems to be evident and studies on the connection between interstellar, cometary and meteoritic dust provide important constraints on the formation of the Solar System and early evolution on Earth. 

Figure 3: Photo: ESA. Launched on 17 November 1995 by an Ariane 4 launcher, ISO provided astronomers with a facility of unprecedented sensivity for detailed exploration of the infrared Universe, ranging from objects in the Solar System right out to the most distant extragalactic sources.

Read more about: Interstellar Clouds...........