Interstellar matter

(astronomy) The gaseous and dust material between the stars.

The material between the stars, constituting several percent of the mass of stars in the Milky Way Galaxy. Being the reservoir from which new stars are born in the Galaxy, interstellar matter is of fundamental importance in understanding both the processes leading to the formation of stars, including the solar system, and ultimately the origin of life in the universe. Among the many ways in which interstellar matter is detected, perhaps the most familiar are attractive photographs of bright patches of emission-line or reflection nebulosity. However, these nebulae furnish an incomplete view of the large-scale distribution of material, because they depend on the proximity of one or more bright stars for their illumination. Radio observations of hydrogen, the dominant form of interstellar matter, reveal a widespread distribution throughout the thin disk of the Galaxy, with concentrations in the spiral arms. The disk is very thin (scale height 135 parsecs for the cold material, where 1 pc is equal to 3.26 light-years, 1.92 × 10¹³ mi, or 3.09 × 10¹³ km) compared to its radial extent (the distance from the Sun to the galactic center is about 8000 pc, for example). Mixed in with the gas are small solid particles, called dust grains, of characteristic radius 0.1 micrometer. Although by mass the grains constitute less than 1% of the material, they have a pronounced effect through the extinction of starlight. Striking examples of this obscuration are the dark rifts seen in the Milky Way. On average, the density of matter is only 15 hydrogen atoms per cubic inch (1 hydrogen atom per cubic centimeter; in total, 2 × 10⁻²⁴ g · cm⁻³), but because of the long path lengths over which the material is sampled, this tenuous medium is detectable. Radio and optical observations of other spiral galaxies show a similar distribution of interstellar matter in the galactic plane.

A hierarchy of interstellar clouds, concentrations of gas and dust, exists within the spiral arms. Many such clouds or cloud complexes are recorded photographically. However, the most dense, which contain interstellar molecules, are often totally obscured by the dust grains and so are detectable only through their infrared and radio emission. These molecular clouds, which account for about half of the interstellar mass, contain the birthplaces of stars. See also Galaxy, external; Milky Way Galaxy.

Molecules

The presence of molecules in interstellar space was first revealed by optical absorption lines. Unfortunately, most species produce no lines at optical wavelengths, and so the discovery of large numbers of molecules had to await advances in radio astronomy, for it is in this spectral band that molecular rotational transitions take place.

An intriguing phenomenon seen in some molecular emission lines is maser amplification to very high intensities. The relative populations of the energy levels of a particular molecule are determined by a combination of collisional and radiative excitation. The population distribution is often not in equilibrium with either the thermal gas or the radiation field, and if a higher energy sublevel comes to have a higher population than a lower one, the electromagnetic transition between the states is amplified through the process of stimulated emission. The best-understood masers are the hydroxyl (OH), water (H₂O), and silicon monoxide (SiO) masers in the circumstellar envelopes of cool mass-losing red supergiant stars. The other masers are found in dense molecular clouds, in particular near compact sources of infrared and radio continuum emission identified as massive stars just being formed. See also Maser.

At present, 119 interstellar molecular species are identified. The known molecules are largely organic. Only 21 are inorganic and stable. There are 32 stable organic species, and 66 unstable species, mostly organic, including carbon chains as complex as HC₁₁N. Most of the unstable species were unknown terrestrially before their interstellar identification.

Astrochemistry

Because of the very low temperatures and densities of the interstellar medium, molecules cannot form from atoms by normal terrestrial processes. The most important interstellar formation process involves gas-phase chemistry, particularly involving molecular ions. Ion-molecule reactions satisfy the requirements of minimal activation energy and of rapid two-body rates. The (positive) ions are initiated by the cosmic-ray ionization of H₂, producing H₃⁺, which then reacts with other abundant species such as carbon monoxide (CO) and N₂ to produce a large number of the observed species such as HCO⁺ and N₂H⁺. Ion fragments themselves react rapidly at low temperatures to form larger ions, which eventually recombine with electrons or neutralize by reaction with easily ionized metals. Slower reactions involving neutral atoms and molecules also produce several species.

Interstellar grains can act as passive repositories of frozen molecular material while a cloud is in a cold dense phase. The same frozen material can undergo chemical reactions within the icy mantle, modifying the chemical composition before subsequent evaporation. Finally, molecules may be catalyzed from interstellar atoms arriving on dust grains. The ubiquitous H₂ molecule is the best example of a species that cannot form in the gas phase at sufficient rate but that catalyzes on grains and desorbs efficiently because of its high volatility.

Strong shocks abound in the interstellar medium, resulting from supernovae and expanding H II regions. These shocks briefly heat and compress the gas, producing required conditions for many high-temperature chemical reactions.