Interstellar Matter

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Interstellar Matter

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 sun’s corona, which is the outermost layer of the sun. The coronal gas like the sun’s 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 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). 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-luminous—that 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 universe’s dark matter remains a subject of dispute among physicists. Dark matter is known to exist thanks to observations with NASA’s Chandra X-ray Observatory, along with the Hubble Space Telescope, the European Southern Observatory’s 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 neutrino’s 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 universe’s dark matter may thus turn out to be not matter at all, but energy.

See also Stellar evolution.