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.

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.

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.