INTERSTELLAR MEDIUM

The interstellar medium is the material which fills the space between the stars. Many people imagine outer space to be a complete vacuum, devoid of any material. Although the interstellar regions are more devoid of matter than any vacuum artificially created on earth, there is matter in space. These regions have very low densities and consist mainly of gas (99%) and dust. In total, approximately 15% of the visible matter in the Milky Way is composed of interstellar gas and dust.

Approximately 99% of the interstellar medium is composed of interstellar gas, and of its mass, about 75% is in the form of hydrogen (either molecular or atomic), with the remaining 25% as helium. The interstellar gas consists partly of neutral atoms and molecules, as well as charged particles, such as ions and electrons. This gas is extremely dilute, with an average density of about 1 atom per cubic centimeter. In comparison, the air we breathe has a density of approximately 30,000,000,000,000,000,000 molecules per cubic centimeter. Even though the interstellar gas is very dilute, the amount of matter adds up over the vast distances between the stars. The interstellar gas is typically found in two forms 1) cold clouds of neutral atomic or molecular hydrogen; and 2) hot ionized hydrogen near hot young stars.

The cold clouds of neutral or molecular hydrogen are the birthplace of new stars if they become gravitationally unstable and collapse. The neutral and molecular forms emit radiation in the radio band of the electromagnet.

The ionized hydrogen is produced when large amounts of ultraviolet radiation are released by hot newly-formed stars. This radiation ionizes the surrounding clouds of gas. Visible light is emitted when electrons recombine with the ionized hydrogen, which is seen as beautiful red colors of emission nebulae like the Trifid Nebula or the Orion Nebula.

Interstellar dust is not like the dust that you might find around your house; it is made of very different substances. These dust particles are extremely small, just a fraction of a micron across, which happens to be approximately the wavelength of blue light waves. The particles are irregularly shaped, and are composed of silicates, carbon, ice, and/or iron compounds.

When light from other stars passes through the dust, a few things can happen. If the dust is thick enough, the light will be completely blocked, leading to dark areas. These dark clouds are known as dark nebulae. The Horsehead Nebula is an example.

Light passing through a dust cloud may not be completely blocked, although all wavelengths of light passing through will be dimmed somewhat. This phenomenon is known as extinction. The extinction is caused by the light being scattered off of the dust particles out of our line of sight, preventing the light from reaching us. The amount that the light is dimmed depends upon the thickness and density of the dust cloud, as well as the wavelength of the light.

Because of the size of the dust particles, scattering of blue light is favored. Therefore, less of the blue light reaches us, which means that the light that reaches us is more red than it would have been without the interstellar dust. This effect is known as interstellar reddening. This process is similar to those that make the sun red at sunset. In turn, a dust cloud that is illuminated by star light, when viewed from the side, appears blue, as in the close-up of the "Egg Nebula". This is similar to the blue sky we see, which is produced by sunlight scattered by the Earth's atmosphere.

Aside from passing through, or being blocked from passing through, interstellar dust, light may also be reflected from the clouds of dust. This is seen as a reflection nebula, as is seen in the Horsehead Nebula image as a bright spot. A reflection nebulae is a region of dusty gas surrounding a star where the dust reflects the starlight, making it visible to us.

Some studies of interstellar gas and dust can be performed here on Earth. For example, we can determine the size and location of cold interstellar gas by performing electromagnetic observations: clouds of interstellar gas are capable of producing radiation, which is detected in the form of a 21-cm wavelength radio wave. However, this radiation is extremely faint, and equipment sensitive enough to detect it was not invented until 1951. Since then, researchers have been able to do extensive research about interstellar gas, but these indirect measurements do not allow for much study of individual particles` charges and abundances.

For observers trapped within the magnetic field that surrounds the Earth, learning about the interstellar medium through direct observation of particles was quite difficult until the advent of satellites. Part of the reason that life was able to evolve on Earth is that the magnetic acts as a shield against high-energy charged particles, like certain cosmic rays. However, the shielding action that protects us also makes it nearly impossible for scientists to sample particles from the sun, interstellar space, and the galaxy as a whole.

In 1958, the Explorer 1, was launched. It sent back data about the radiation belts that surround the Earth, and this information sparked much debate as to the source of such high-energy radiation. Eventually, several sources including solar and interstellar radiation, were identified and there began the quest to study the sources as close to first-hand as possible. Now, many satellites have studied or are now studying interstellar matter. Indeed, the AMPTE mission was the first to discover the existence of pick-up ions. Many of these missions use a "time of flight" to determine the charge and size of particles that enter the detector. Knowing these properties, researchers can tell where a particle came from, how old it is, or even a little about its history.

Our sun and solar system are currently moving through a cloud of interstellar gas. This cloud is approximately 60 light years across, with our sun being only appoximately 4 light years from the edge. Our local cloud, which features a density of 0.1 particles per cubic centimeter, and a temperature of about 6000-7000 is immersed in the "Local Bubble," which has extremely low densities (approximately 0.001 particles per cubic centimeter) and very high temperatures (approx. 1,000,000 K). The local bubble is about 300 light years in diameter, and may have been created by a supernova explosion. We know this because the material from our local cloud can be sampled within the solar system.

Our sun is moving through the local interstellar gas cloud approximately into the direction of Scorpio with a speed of about 25 km/sec. As a result of this motion, an interstellar wind with that speed is blowing through our planetary system. We measure this interstellar wind as helium. This radiation would turn the neutral gas atoms into ions and electrons that would then be swept out of the solar system by the gale-force solar winds, with speeds of 300 - 1000 km/sec. Until the late 1960's, astonomers thought that in this manner, the Sun would sweep the space around itself clean because of its fierce UV radiation. However, the Sun rushes through the gas, so that there is not enough time for the atoms to lose their electrons and become charged until the atoms are actually very close to the sun.

The newer view is that there is not enough time for the atoms to lose their electons and become charged as the sun rushes through the gas. Typically, there is 1 atom in each 10 cm3 of interstellar gas and 10 ions in each cm3 of solar wind (near Earth). The particles are so far apart that the solar wind and interstellar gas flow through each other without being disturbed by collisions. Therefore, we can even take samples of the interstellar gas near the Earth.

The interstellar atoms change their behaviour as soon as they are ionized. This happens more frequently closer to the sun, where sunlight gets more intense. The Sun's ultraviolet (UV) radiation is the most important player in this game.

The newly-created ion, with its positive electric charge, can no longer cross magnetic field lines. Ions are forced to gyrate around a field line, like a gymnast swings around the horizontal bar in the Giant Swing. They are an excellent tool for determining the density of the local insterstellar cloud, and to help us find out of which elements the interstellar medium consists.

The magnetic field is taken along with the solar wind and the motion of an interstellar ion that starts at rest after the ionization. Now the motion of the ion looks like that of the valve on a bicycle wheel (when on street level, the valve does not move, but on the top of the wheel, the valve moves twice as fast as the bicycle itself.)

The interstellar medium contains particles that have origins in many different events. The majority of the interstellar gas and dust that we see was produced by star death. We do not ordinarily think of stars as transient objects, but even stars are born, and must also eventually die.

Stars produce energy by the process of fusion. This releases a fantastic amount of energy, which then allows the reaction to continue elsewhere in the star.

When a star has fused all of its hydrogen into helium, it begins fusing the helium into lithium, and so on up the periodic. If a star is massive enough, it will produce elements all the way up to iron. Each time the star begins fusing a new element, the delicate balance between its gravitational pull and the pressure inside it also undergoes a change, and so the star may grow or shrink. In many of these changes, the star throws off a layer of matter at very high velocities, as in the example at right. These expanding shells of matter surround some of the stars we see from Earth. The most dramatic star deaths ie. novae and supernovae, are created by super-massive stars, when the force of gravity is no longer enough to keep the star from expanding.

These stars explode in a tremendous release of energy, and matter is thrown off at incredible speeds and very high temperatures. When this matter encounters patches of interstellar gas, it excites and ionizes them, producing an emission.

Interstellar material also comes from stars which are still undergoing stable fusion reactions. Our own sun sends out streams of particles and radiation ie. the solar winds, that interact with particles flowing into our solar system. Given that all stable stars also produce a similar "wind", a substantial portion of the interstellar medium can be accounted for in this way.

Clouds of interstellar dust have accreted over millions of years. This dust is a varied mix of compounds and elements; some interstellar clouds even contain organic molecules like acetylene and acetaldehyde, known precursors of amino acids. The atoms needed to create these clouds of molecules have generally come from past supernovae.

Many of the denser clouds become stellar nurseries. Our own solar system was probably born in this way; and we are all made of stardust. Eventually, these stars will also die, some more spectacularly than others, and begin the cycle again. When matter in the expanding universe becomes too sparse for gravity to pull it together, this process will end.