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Interstellar Matter

Interstellar matter

The interstellar mediumthe space between the starsconsists 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 particlesinterstellar 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 nebulaeknown as molecular cloudsare 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 ]

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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.

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ism

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.

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ISM

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ism

ismchasm, spasm •enthusiasm • orgasm • sarcasm •ectoplasm • cytoplasm • iconoclasm •cataplasm • pleonasm • phantasm •besom • dirigisme •abysm, arrivisme, chrism, chrisom, ism, prism, schism •Shiism, theism •Maoism, Taoism •egoism • truism • Babism • cubism •sadism • nudism • Sufism • ageism •holism • cataclysm • monism • papism •verism • aneurysm • purism • Nazism •sexism • racism • paroxysm • autism •macrocosm • microcosm • bosom

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ISM

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

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Interstellar Matter

Interstellar Matter

Components of the interstellar medium

Significance of the interstellar medium

Resources

The constellation of Orion, the hunter, is most easily visible on a clear winter night. A row of three stars makes up his belt. Hanging from his belt is his sword, a smaller row of three fainter stars. Looking at the center star in the sword with a pair of binoculars or a small telescope, small fuzzy patch of interstellar gas and dust is visible, called the Orion Nebula. Space is not empty. The matter in the space between the stars is called interstellar matter or the interstellar medium. The interstellar medium consists of atoms, ions, and molecules of gas and dust grains. It is both concentrated into clouds and spread out between stars and the clouds. The interstellar medium is tenuous enough to qualify as a vacuum on Earth, but it plays a crucial role in the evolution of the galaxy. Stars are born out of the interstellar medium, and when stars die they recycle some of their material back into the interstellar medium.

Components of the interstellar medium

The interstellar medium can be broadly classified into gas and dust components. The average density of the interstellar gas is roughly one hydrogen atom per cubic centimeter. This density can however vary considerably for different components of the interstellar gas. The components of the interstellar gas include: cold atomic gas clouds, warm atomic gas, the coronal gas, HII regions (a volume of space where hydrogen [H] is in an ionized state rather than in its usual neutral state), and molecular clouds. Most of the atoms in space are hydrogen, about 90%. The remaining atoms are helium, at 9%, and all the other elements making up the other 1%.

Gas

The cold atomic gas clouds consist primarily of neutral hydrogen atoms. Astronomers refer to neutral hydrogen atoms as HI, so these clouds are also called HI regions. These gas clouds have densities from 10 to 50 atoms per cubic centimeter and temperatures about 50 to 100K (-369.4 to -279.4°F [-223 to -173°C]). They can be as large as 30 light-years in diameter (one light-year is the distance in vacuum that light travels in one year) and contain roughly 1,000 times the mass of the sun.

The warm atomic gas is much more diffuse than the cold atomic gas. Its density only averages one atom per ten or more cubic centimeters. The temperature is much warmer and can range from 3,000 to 6,000K (4,940.6 to 10,340.6°F [2,727 to 5,727°C]). Like the cold atomic clouds, the warm atomic gas is primarily neutral hydrogen. For both the warm and cold atomic gas, 90% of the atoms are hydrogen, but other types of atoms are mixed in at their normal cosmic abundances. The atomic gas accounts for roughly half the mass and volume of the interstellar medium. The warm diffuse gas is spread out between the clumps of the cold gas clouds.

The coronal gas is named for its similarity to the suns corona, which is the outermost layer of the sun. The coronal gas like the suns corona is both very hot and diffuse. The average temperature and density of the coronal gas are roughly 1,799,541°F (99,727°C) and one atom per 1,000 cubic centimeter, respectively. The coronal gas is most likely heated by supernova explosions in the galaxy. Because the temperature is so high, the hydrogen atoms are ionized, meaning that the electrons have escaped from the nuclei.

Astronomers often call ionized hydrogen HII, so HII regions are clouds of ionized hydrogen. HII regions have temperatures of roughly 17,541°F (9,727°C) and densities of a few thousand atoms per cubic centimeter. These HII regions are generally associated with regions of star formation. Newly formed stars are still surrounded by the clouds of gas and dust out of which they were formed. The hottest and most massive stars emit significant amounts of ultraviolet light that has enough energy to eject the electrons from the hydrogen atoms. An ionized HII region forms around these stars. Like the other atomic clouds, 90% of the atoms in HII regions are hydrogen, but other types of atoms are also present. These other types of atoms also become ionized to varying degrees.

The ionized atoms emit visible light so many HII regions can be seen in small telescopes and are quite beautiful. The Orion Nebula, for example, is the closest example of a glowing HII region that is heated by newly formed stars. These HII regions are also called emission nebulae. Molecular clouds are also associated with star formation. Giant molecular clouds have temperatures below -369.4°F (-223°C), but can contain several thousand molecules per cubic centimeter. They can also be quite large. They range up to 100 light-years in size and typically contain 100,000 times the mass of the sun. These clouds appear dark because they block the light from stars behind them. The most massive contain as much as 10 million times the mass of the sun. Roughly half the mass of the interstellar medium is found in molecular clouds. Like the atomic gas, most of the molecules are hydrogen molecules, but hydrogen molecules are difficult to detect. Molecular clouds are therefore most commonly mapped out as carbon monoxide (CO) clouds because the CO molecule is easy to detect using a radio telescope.

So far more than 80 different types of molecules have been found in molecular clouds, including some moderately complex organic molecules. The most common molecules are the simplest ones, containing only two atoms. These include molecular hydrogen (H2), some carbon monoxide (CO), the hydroxyl radical (OH), and carbon sulfide (CS), followed by the most common three-atom molecule, water (H2O). More complex species are relatively rare. However, molecules having as many as 13 atoms have been identified, and even larger species are suspected.

How can all these molecules form in interstellar space? For molecules to form atoms have to get close together. In even the densest interstellar clouds the atoms are too spread out. How can they get close? The details are poorly understood, but astronomers think that dust grains play a crucial role in interstellar chemistry, particularly for such important species as molecular hydrogen. The atoms on the surface of the dust grains can get close enough to form molecules. Once the molecules form, they do not stick to the dust grains as well as atoms so they escape the surface of the dust grain.

Dust

In addition to gas, the other major component of interstellar matter is dust. Dust grains permeate the entire interstellar medium, in clouds and between them. Interstellar dust grains are usually less than a millionth of a meter in radius. Their compositions are not well known, but likely compositions include silicates, ices, carbon, and iron. The silicates are similar in composition to the silicate rocks found on the moon and in Earths mantle. The ices can include carbon dioxide, methane, and ammonia ice as well as water ice. Astronomers think that a typical grain composition is a silicate core with an icy mantle, but pure carbon grains may be present as well.

Dust exists in diffuse form throughout the interstellar medium. In this diffuse form, each dust grain typically occupies the volume of a cube the length of a football field on each side (one million cubic meters). Astronomers detect this diffuse interstellar dust by the extinction and reddening of starlight. The dust grains block starlight, creating extinction, and they, also, preferentially block blue light over red light, causing reddening. Stars therefore appear redder in color than they otherwise would appear. This extinction and reddening is similar to the effect that makes sunsets red, especially over a smoggy city.

Astronomers can see dust grains more directly in dense regions, that is, in interstellar clouds. Two types of clouds showing the effects of dust are dark clouds and reflection nebulae. Astronomers see dark clouds by their effect on background stars. They block the light from stars behind the cloud, so a region of the sky is seen with very few stars. Reflection nebulae are dust clouds located near a star or stars. They shine with reflected light from the nearby stars, and are blue in color because the grains selectively reflect blue light.

Significance of the interstellar medium

Neutral hydrogen atoms in the interstellar medium emit radio waves at a wavelength of 8 in (21 cm). Studies of this 8-in (21-cm) emission are not just important for studying the interstellar medium. Mapping the distribution of this interstellar hydrogen has revealed to scientists the spiral structure of the Milky Way galaxy. It has also revealed that about 3% of the Milky Way galaxy is interstellar gas and 1% is interstellar dust.

The interstellar medium is intimately intertwined with the stars. Stars are formed from the collapse of gas and dust in molecular clouds. The leftover gas around newly formed massive stars forms the HII regions. At various times stars return material to the interstellar medium. This recycling can be gentle in the form of stellar winds, or it can be as violent as a supernova explosion. The supernovas are a particularly important form of recycling in the interstellar medium. The material recycled by supernovas is enriched in heavy elements produced by nuclear fusion in the star and in the supernova itself. With time the amount of heavy elements in the composition of the interstellar medium and of stars formed from the interstellar medium slowly increases. The interstellar medium therefore plays an important role in the chemical evolution of the galaxy.

Dark matter is interstellar material in the universe that is non-luminousthat is, material that does not emit or reflect light and that is therefore invisible. Everything seen when looking through a telescope is visible because it is either emitting or reflecting light; stars, nebulae, and galaxies are examples of luminous objects. However, luminous matter appears to make up only a small fraction of all the matter in the universe, perhaps only a few percent. The rest of the matter is cold, dark, and hidden from direct view.

The identity of the universes dark matter remains a subject of dispute among physicists. Dark matter is known to exist thanks to observations with NASAs Chandra X-ray Observatory, along with the Hubble Space Telescope, the European Southern Observatorys Very Large Telescope and the Magellan optical telescopes. However, astronomers do not know what makes up dark matter. Most proposals include dark matter as part of interstellar matter. For example,

KEY TERMS

Dark cloud A cloud of dust that block light from stars behind it.

HI region A cloud of neutral hydrogen.

HII region A cloud of ionized hydrogen.

Interstellar medium The matter between the stars.

Ion An atom that has lost or gained one or more electrons. In astronomy it will virtually always have lost electrons.

Molecular cloud An interstellar cloud of molecules.

Nebula An interstellar cloud of gas and/or dust.

Reflection nebula A cloud of dust that glows from reflected starlight.

subatomic particles known as neutrinos pervade the universe in very great numbers. In 1998, they were proven by astronomers to have a small mass, ending a decades-long dispute among physicists about whether they are without mass (massless). It is now thought that each neutrinos mass is so small that neutrinos can account for at most a fifth of the dark matter in the universe. However, other research shows that particles of some unknown kind, generically termed wimps (weakly-interacting massive particles), may permeate the space around galaxies. They may be held together in clouds by gravity. Whatever ever makes up dark matter is still up for debate.

Dark matter has long been thought to play a crucial role in determining the fate of the universe. The most widely accepted theory regarding the origin and evolution of the universe is the big bang theory, which provides an elegant explanation for the well-documented expansion of the universe. One question is whether the universe will expand forever, propelled by the force of the big bang, or eventually stop expanding and begin to contract under its own gravity, much as a ball thrown up into the air eventually turns around and descends. The deciding factor is the amount of mass in the universe: the more mass, then the more overall gravity. From about 1995 to the present, strong scientific evidence shows that the expansion of the universe, far from slowing down, is accelerating. If this result is confirmed, which it has not been as of October 2006, then the fate of the universe is at last definitely known: it will expand forever, becoming darker, colder, and more diffuse.

To account for the observed acceleration, physicists have postulated a dark energy, still mysterious in origin, that pervades the universe and actually helps to push things apart rather than keep them together. Since energy (even dark energy) and matter are interchangeable, some of the universes dark matter may thus turn out to be not matter at all, but energy.

See also Stellar evolution.

Resources

BOOKS

Bacon, Dennis Henry, and Percy Seymour. A Mechanical History of the Universe. London: Philip Wilson Publishing, Ltd., 2003.

Dopita, Michael A. Astrophysics of the Diffuse Universe. Berlin, Germany, and New York: Springer, 2003.

Freeman, Kenneth C. In Search of Dark Matter. Berlin, Germany, and New York: Springer, 2006.

Krishna, K.S. Krishna. Dust in the Universe: Similarities and Differences. Singapore and London, UK: World Scientific, 2005.

Lequeux, James. The Interstellar Medium. Berlin, Germany, and New York: Springer, 2005.

Paul A. Heckert

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Interstellar Matter

Interstellar matter

On a clear winter night go outside to a dark location and look for the constellation Orion, the hunter. A row of three stars makes up his belt. Hanging from his belt is his sword, a smaller row of three fainter stars. If you look at the center star in the sword with a pair of binoculars or a small telescope , you will see a small fuzzy patch of interstellar gas and dust, called the Orion Nebula. Space is not empty. The matter in the space between the stars is called interstellar matter or the interstellar medium. The interstellar medium consists of atoms , ions, molecules, and dust grains. It is both concentrated into clouds and spread out between stars and the clouds. The interstellar medium is tenuous enough to qualify as a vacuum on the earth , but it plays a crucial role in the evolution of the galaxy . Stars are born out of the interstellar medium, and when stars die they recycle some of their material back into the interstellar medium.


Components of the interstellar medium

The interstellar medium can be broadly classified into gas and dust components. The average density of the interstellar gas is roughly one hydrogen atom per cubic centimeter. This density can however vary considerably for different components of the interstellar gas. The components of the interstellar gas include: cold atomic gas clouds, warm atomic gas, the coronal gas, HII regions, and molecular clouds.


Gas

The cold atomic gas clouds consist primarily of neutral hydrogen atoms. Astronomers refer to neutral hydrogen atoms as HI, so these clouds are also called HI regions. These gas clouds have densities from 10–50 atoms per cubic centimeter and temperatures about 50–100K (-369.4–-279.4°F [-223–-173°C]). They can be as large as 30 light years and contain roughly 1,000 times the mass of the sun .

The warm atomic gas is much more diffuse than the cold atomic gas. Its density only averages one atom per ten or more cubic centimeters. The temperature is much warmer and can range from 3,000–6,000K (4,940.6–10,340.6°F [2,727–5,727°C]). Like the cold atomic clouds, the warm atomic gas is primarily neutral hydrogen. For both the warm and cold atomic gas 90% of the atoms are hydrogen, but other types of atoms are mixed in at their normal cosmic abundances. The atomic gas accounts for roughly half the mass and volume of the interstellar medium. The warm diffuse gas is spread out between the clumps of the cold gas clouds.

The coronal gas is named for its similarity to the sun's corona, which is the outermost layer of the sun. The coronal gas like the sun's corona is both very hot and very diffuse. The average temperature and density of the coronal gas are roughly 1,799,541°F (99,727°C) and one atom per 1,000 cubic centimeter. The coronal gas is most likely heated by supernova explosions in the galaxy. Because the temperature is so high, the hydrogen atoms are ionized, meaning that the electrons have escaped from the nuclei.

Astronomers often call ionized hydrogen HII, so HII regions are clouds of ionized hydrogen. HII regions have temperatures of roughly 17,541°F (9,727°C) and densities of a few thousand atoms per cubic centimeter. What causes these HII regions? They are generally associated with regions of star formation . Newly formed stars are still surrounded by the clouds of gas and dust out of which they were formed. The hottest and most massive stars emit significant amounts of ultraviolet light that has enough energy to eject the electrons from the hydrogen atoms. An ionized HII region forms around these stars. Like the other atomic clouds, 90% of the atoms in HII regions are hydrogen, but other types of atoms are also present. These other types of atoms also become ionized to varying degrees.

The ionized atoms emit visible light so many HII regions can be seen in small telescopes and are quite beautiful. The Orion Nebula mentioned in the opening paragraph of this article is the closest example of a glowing HII region that is heated by newly formed stars. These HII regions are also called emission nebulae. Molecular clouds are also associated with star formation. Giant molecular clouds have temperatures below -369.4°F (-223°C), but can contain several thousand molecules per cubic centimeter. They can also be quite large. They range up to 100 light years in size and typically contain 100,000 times the mass of the sun. These clouds appear dark because they block the light from stars behind them. The most massive contain as much as 10 million times the mass of the sun. Roughly half the mass of the interstellar medium is found in molecular clouds. Like the atomic gas, most of the molecules are hydrogen molecules, but hydrogen molecules are difficult to detect. Molecular clouds are therefore most commonly mapped out as carbon monoxide (CO) clouds because the CO molecule is easy to detect using a radio telescope.

So far more than 80 different types of molecules have been found in molecular clouds, including some moderately complex organic molecules. The most common molecules are the simplest ones, containing only two atoms. These include molecular hydrogen (H2), some carbon monoxide (CO), the hydroxyl radical (OH), and carbon sulfide (CS), followed by the most common three-atom molecule, water (H2O). More complex species are relatively rare. However, molecules having as many as 13 atoms have been identified, and even larger species are suspected.

How can all these molecules form in interstellar space? For molecules to form atoms have to get close together. In even the densest interstellar clouds the atoms are too spread out. How can they get close? The details are poorly understood, but astronomers think that dust grains play a crucial role in interstellar chemistry , particularly for such important species as molecular hydrogen. The atoms on the surface of the dust grains can get close enough to form molecules. Once the molecules form, they do not stick to the dust grains as well as atoms so they escape the surface of the dust grain.


Dust

In addition to gas, the other major component of interstellar matter is dust. Dust grains permeate the entire interstellar medium, in clouds and between them. Interstellar dust grains are usually less than a millionth of a meter in radius. Their compositions are not well known, but likely compositions include silicates, ices, carbon, and iron . The silicates are similar in composition to the silicate rocks found on the Moon and in the earth's mantle. The ices can include carbon dioxide , methane, and ammonia ice as well as water ice. Astronomers think that a typical grain composition is a silicate core with an icy mantle, but pure carbon grains may be present as well.

Dust exists in diffuse form throughout the interstellar medium. In this diffuse form each dust grain typically occupies the volume of a cube the length of a football field on each side (one million cubic meters). We detect this diffuse interstellar dust by the extinction and reddening of starlight. The dust grains block starlight, creating extinction, and they also preferentially block blue light over red light, causing reddening. Stars therefore appear redder in color than they otherwise would. This extinction and reddening is similar to the effect that makes sunsets red, especially over a smoggy city.

We can see dust grains more directly in dense regions, that is, in interstellar clouds. Two types of clouds showing the effects of dust are dark clouds and reflection nebulae. We see dark clouds by their effect on background stars. They block the light from stars behind the cloud, so we see a region of the sky with very few stars. Reflection nebulae are dust clouds located near a star or stars. They shine with reflected light from the nearby stars, and are blue in color because the grains selectively reflect blue light.


Significance of the interstellar medium

Neutral hydrogen atoms in the interstellar medium emit radio waves at a wavelength of 8 in (21 cm). Studies of this 8 in (21 cm) emission are not just important for studying the interstellar medium. Mapping the distribution of this interstellar hydrogen has revealed to us the spiral structure of the Milky Way galaxy.

The interstellar medium is intimately intertwined with the stars. Stars are formed from the collapse of gas and dust in molecular clouds. The leftover gas around newly formed massive stars forms the HII regions. At various times stars return material to the interstellar medium. This recycling can be gentle in the form of stellar winds, or it can be as violent as a supernova explosion. The supernovas are a particularly important form of recycling in the interstellar medium. The material recycled by supernovas is enriched in heavy elements produced by nuclear fusion in the star and in the supernova itself. With time the amount of heavy elements in the composition of the interstellar medium and of stars formed from the interstellar medium slowly increases. The interstellar medium therefore plays an important role in the chemical evolution of the galaxy.

See also Stellar evolution.


Resources

books

Bacon, Dennis Henry, and Percy Seymour. A Mechanical History of the Universe. London: Philip Wilson Publishing, Ltd., 2003.

Morrison, David, Sidney Wolff, and Andrew Fraknoi. Abell'sExploration of the Universe. 7th ed. Philadelphia: Saunders College Publishing, 1995.

Verschuur, Gerrit L. Interstellar Matters. New York: Springer-Verlag, 1989.

periodicals

Knapp, Gillian. "The Stuff Between The Stars." Sky & Telescope 89 (May 1995): 20-26.


Paul A. Heckert

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dark cloud

—A cloud of dust that block light from stars behind it.

HI region

—A cloud of neutral hydrogen.

HII region

—A cloud of ionized hydrogen.

Interstellar medium

—The matter between the stars.

Ion

—An atom that has lost or gained one or more electrons. In astronomy it will virtually always have lost electrons.

Molecular cloud

—An interstellar cloud of molecules.

Nebula

—An interstellar cloud of gas and/or dust.

Reflection nebula

—A cloud of dust that glows from reflected starlight.

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