Interstellar matter

The interstellar medium—the space between the stars—consists of nearly empty space. It is the vacuum of the universe. It would be totally empty if not for a smattering of gas atoms and tiny solid particles—interstellar matter.

On average, the interstellar matter in our region of the galaxy consists of about one atom of gas per cubic centimeter and 25 to 50 microscopic solid particles per cubic kilometer. In contrast, the air at sea level on Earth contains about 1,019 molecules of gas per cubic centimeter.

In some regions of space, however, the concentration of interstellar matter is thousands of times greater than average. Where there is a large enough concentration of gas and particles (also called cosmic dust), clouds form. Most of the time these clouds are so thin they are invisible. At other times they are dense enough to be seen and are called nebulae (plural for nebula).

Cosmic dust

Cosmic dust accounts for only 1 percent of the total mass in the interstellar medium; the other 99 percent is gas. Scientists believe the dust is primarily composed of carbon and silicate material (silicon, oxygen, and metallic ions), possibly with solid carbon dioxide and frozen water and ammonia. A dark nebula is a relatively dense cloud of cosmic dust. The nebula is dark because much of the starlight in its path is either absorbed or reflected by dust particles. When starlight is reflected, it shines off in every direction, meaning only a small percentage is sent in the direction of Earth. This process effectively blocks most of the starlight from Earth's view.

Even individual particles of cosmic dust affect the quality of starlight. Random dust particles absorb or reflect some light from various stars, causing them to appear far dimmer than they actually are. Scientists have theorized that without the presence of cosmic dust, the Milky Way would shine so brightly that it would be light enough on Earth to read at night.

Most dark nebulae resemble slightly shimmering, dark curtains. However, in cases where a dense cloud of dust is situated near a particularly bright star, the scattering of light may be more pronounced, forming a reflection nebula. This is a region where the dust itself is illuminated by the reflected light.

Words to Know

Cosmic dust: Solid, microscopic particles found in the interstellar medium.

Interstellar medium: The space between the stars, consisting mainly of empty space with a very small concentration of gas atoms and tiny solid particles.

Light-year: Distance light travels in one year, about 5.9 trillion miles (9.5 trillion kilometers).

Nebula: An interstellar cloud of gas and dust.

Red giant: Stage in which an average-sized star (like our sun) spends the final 10 percent of its lifetime. Its surface temperature drops and its diameter expands to 10 to 1,000 times that of the Sun.

Interstellar gas

In contrast to solid particles, interstellar gas is transparent. Hydrogen accounts for about three-quarters of the gas. The remainder is helium plus trace amounts of nitrogen, oxygen, carbon, sulfur, and possibly other elements.

While interstellar gas is generally cold, the gas near very hot stars is heated and ionized (electrically charged) by ultraviolet radiation given off by those stars. The glowing areas of ionized gas are called emission nebulae. Two well-known examples of emission nebulae are the Orion nebula, visible through binoculars just south of the hunter's belt in the constellation of the same name, and the Lagoon nebula in the constellation Sagittarius. The Orion nebula is punctuated by dark patches of cosmic dust.

Interstellar space also contains over 60 types of polyatomic (containing more than one atom) molecules. The most common substance is molecular hydrogen (H₂); others include water, carbon monoxide, and ammonia. Since these molecules are broken down by starlight, they are found primarily in dense, dark nebulae where they are protected from the light by cosmic dust. These nebulae—known as molecular clouds—are enormous. They stretch across several light-years and are 1,000 to 1,000,000 times as massive as the Sun.

Origin of interstellar matter

Scientists have proposed various theories as to the origins of interstellar matter. Some matter has been ejected into space by stars, particularly from stars in the final stages of their lives. As a star depletes the supply of fuel on its surface, the chemical composition of the surrounding interstellar medium is altered. Massive red giant stars have been observed ejecting matter, probably composed of heavy elements such as aluminum, calcium, and titanium. This material may then condense into solid particles, which combine with hydrogen, oxygen, carbon, and nitrogen when they enter interstellar clouds.

It is also possible that interstellar matter represents material that did not condense into stars when the galaxy formed billions of years ago. Evidence supporting this theory can be found in the fact that new stars are born within clouds of interstellar gas and dust.

[See also Galaxy; Star ]

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 / ˈizəm/ • n. inf., chiefly derog. a distinctive practice, system, or philosophy, typically a political ideology or an artistic movement: of all the isms, fascism is the most repressive. DERIVATIVES: ist n.

ISM Abbr. for interstellar matter or interstellar medium.

ISM. Incorporated Society of Musicians (Britain).

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ISM Iesus Salvator Mundi (Latin: Jesus, Saviour of the World)
• Imperial Service Medal
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• Astronomy interstellar medium