interstellar matter

interstellar matter matter in a galaxy between the stars, known also as the interstellar medium.

Distribution of Interstellar Matter

Compared to the size of an entire galaxy, stars are virtually points, so that the region occupied by the interstellar matter constitutes nearly all the physical volume of a galaxy. Although the density of interstellar matter is far lower than in the best laboratory vacuum, the total mass contained between stars is about 5% of the mass of the universe. Interstellar matter is mostly gaseous, but about 1% is interstellar grains or dust. The grains are not distributed uniformly in space but are found in clumpy clouds.

Some of the interstellar material is visible, sometimes through small telescopes, as nebulae . In the vicinity of bright stars the grains appear as glowing regions because of the intensity of the light they scatter; these regions are called reflection nebulae. Regions where the clouds are so thick that they obscure all starlight are called dark nebulae. Highly ionized matter, densely clustered around a hot star, is visible by the light emitted by the ions and electrons when they recombine; this is called an emission nebula.

Composition and Properties of Interstellar Matter

The interstellar gas, which constitutes about 99% of the interstellar matter, consists mostly of hydrogen and helium. In addition to the spectra (see spectrum ) of those elements, some spectral lines not formed under ordinary laboratory conditions ( "forbidden lines" ) are seen. The prominent green color of certain emission nebulae is due to a forbidden line of doubly ionized oxygen. In H I regions (regions of unionized hydrogen), neutral hydrogen atoms absorb and emit radio waves with a wavelength of 8 in. (21 cm), due to a reorientation of the proton spin in the magnetic field produced by the electron spin (see magnetic resonance ). Besides atomic hydrogen and helium, many molecules, including formaldehyde and water vapor, have been detected in the interstellar medium by the techniques of radio astronomy .

The interstellar gas is electrically neutral at points far removed from any star (H I regions) but is highly ionized (the electrons are detached from their atoms) in the immediate vicinity of the most massive and hottest stars (H II regions). The gas is virtually transparent to visible light; there is weak optical absorption by certain trace atoms (sodium, calcium) and molecules (cyanogen, carbon hydride). However, within a short distance from a hot star nearly all its ultraviolet light is absorbed; the energy from this light maintains the state of ionization in the circumstellar H II region, which is called the Strömgren sphere (for Bengt Strömgren, the Danish astrophysicist who postulated its existence in the 1930s) and is the source of emission nebulae.

The nongaseous interstellar matter exists in the form of tiny solid particles called interstellar grains or dust. The grains are believed to be elongated in shape, and aligned with the magnetic field; they are believed to contain graphite or silicate material as well as polycyclic aromatic hydrocarbons. The clouds obscure the view of the galaxy in certain directions, particularly in the direction of the galactic center. They polarize and selectively scatter the starlight passing through them; blue light is scattered more than red light so that stars partially obscured by interstellar matter appear redder than their true color. Since the distances and intrinsic luminosities of many stars are estimated from analysis of their spectra, this effect, called interstellar reddening, has been responsible for errors in calculating the distances and luminosities of these stars.

interstellar matter (ISM) The material between the stars, also called the interstellar medium. The ISM in our Galaxy consists of 99 % gas and 1 % fine dust particles by mass. There are also a few very energetic cosmic-ray particles. The ISM is spread very thinly, with an average density of only 1 particle/cm³. However, there are enormous variations about this average. The densest parts, which include maser regions containing 10¹⁰ hydrogen molecules/cm³, are also the coldest, with temperatures close to absolute zero. The hottest material has temperatures of 10⁸ K, and densities of 1 particle/cm³ or less. The ISM is the reservoir of matter from which new stars form.

ISM Abbr. for interstellar matter or interstellar medium.