When one looks up into the night sky we only see stars and the occasional planet. Most of outer space is empty, meaning that the density of atoms is much lower than even the best vacuums in our labs. Deep imaging of the skies showed that there are numerous regions where interstellar matter, in the form of gas and dust, collects to form clouds and nebula. Since these clouds are diffuse, they are difficult to see with the naked eye.
The first indication that there was interstellar gas and dust was dark lanes in the Milky Way. Since we live in a disk galaxy, then looking outward we see a band of light in the sky which, if magnified, breaks down into the many stars in our Galaxy. A deep photo of the Milky Way shows that there are dark regions or lanes.
We understand now that there is gas and dust blocking the starlight which produces these dark lanes, such as the CoalSack Nebula. In fact, most of our Galaxy is blocked from our view by patches of gas and dust.
Astrophotograph in the 19th century showed that the dark lanes or holes in the Milky Way did not have sharp edges. That, in fact, detail studies of star clusters at various distances from us showed that the intensity of light from remote stars is reduced as it passes through the sparse material of the interstellar medium. Herschel tried to use star counts to measure the size of the Galaxy and where our position is within it. His result was the diagram below, but what he really discovered was that interstellar extinction limits our line of sight.
Not only is the intensity of the light decreased, called interstellar extinction, it is also reddened, called interstellar reddening. The blue component of light is more easily scattered than the red component (which is why the sky is blue during the day, scattered sunlight). Thus, light from remote stars has part of its blue component scattered before it reaches the Earth.
Maps of interstellar reddening demonstrated that the interstellar medium composed mostly of hydrogen and helium gas (99%) and traces of dust. Dust, in an interstellar sense, is very small (few microns in size) particles of carbon and silicon. Dust is fragile because it can be broken down by UV photons, but is very important in dark nebula as sites for the formation of molecules.
Most of the interstellar medium is in the form of neutral hydrogen gas (HI). The typical densities of neutral hydrogen in the Galaxy is one atom per cubic centimeter. This gas is cold and the electron is usually in its ground state. However, protons and electrons can have spin. This spin produces a magnetic field such that when the spins are aligned the ground state has a slightly higher energy then when the spins are opposed. This is called hyperfine splitting of the ground state for hydrogen and results in the emission of 21 cm radio waves from HI clouds.
The spins of the electron can be changed by collisions with other hydrogen atoms, although this is very rare because the densities are so low, or the transition can happen naturally after a few million years. On the other hand, there are billions and billions of hydrogen atoms in the typical cloud of gas. So the result is that 21 cm is a strong measure of the amount of HI gas in the Galaxy.
A scan of our Milky Way galaxy at 21 cm shows that the distribution of neutral hydrogen is concentrated in the spiral arms. The Sun is marked as the yellow arrow, Galactic center is a blue dot. Notice how there is a cone of avoidance behind the Galactic center due to confusion in the HI signal.
Atoms can bond together to form molecules. The most common atoms in the Universe are H, He, C, N, O, thus we have the expectation that the most common molecules are made from these atoms. However, the bonds between molecules is very weak (look how easy it is to breakdown water) and interstellar space is full of UV and x-ray photons which can break these bonds. So interstellar molecules are only found in the dark centers of dense nebula of gas and dust.
Just as electrons orbit around the nucleus in quantized energy states, atoms can rotate around each other, also in quantized speeds. Changes in the rotation, by collisions or interactions, will emit photons.
Radiation from interstellar molecules is a good tracer of the dense, thick regions of the interstellar medium. These dense regions are usually the sites of protostar formation and collapse of large clouds of gas to form into star clusters.
Interstellar dust is produced in the envelopes around red supergiant stars. Stellar winds and the planetary nebula phase eject this dust into the interstellar medium. Dust particles are mostly carbon and silicate grains that are a few microns in size. Although small, they completely absorb any light that strikes them and are heated by collisions with gas molecules. They will be warmed to the ambient temperature of the surrounding region and re-radiate that energy in the far infrared.
Although solid objects, dust grains are not immune to destruction. Collisions with high speed gas particles, UV photons and other grains will breakdown dust grains. For this reason, dust is only found in the cores of dark nebula where they are shielded from destructive effects.
Dust grains serve as sites for the formation of molecules and organic compounds. Their cold surfaces act as catalysts by allowing atoms to stick to them so there there is time for a second atom to land, interact, and form a molecule. Collections of dust and molecular gases are called molecular clouds.
We divide the interstellar medium into three types dependent on their temperature (called the phase of the interstellar medium): cold (10's K), warm (100 to 1000's K), hot (millions K). The colder a cloud of gas, the more of its output emission is in the long wavelengths; radio and microwave. Hot regions of interstellar gas are bright in their own optical emission.
Note also that the various temperatures determine the type of matter that will exist. Cold temperatures are suitable to the formation of molecules. Warmer temperatures will find only atoms, such as neutral hydrogen. Under higher temperatures, atoms become ionized (HII regions). The coldest regions of space are the dark nebula.
Some dark nebula are found through star counts, such as the Coalsack. Others are visible because an emission nebula is behind them, so they are illuminated from behind, such as the Horse Head Nebula shown below. This happens often because the same gas that is associated with the dark nebula can also be heated by nearby stars to glow as an emission nebula.
The Horse Head Nebula is several tens of parsecs across and would envelop the local neighborhood of stars around our solar system. Stars seen in the image are foreground stars since the nebula is opaque.
Some dark nebula are surrounded by emission regions, giving the impression that the dark cores are being dissolved by the hotter gas as in the Eagle Nebula shown below.
A great deal of information is contained in the above image, the irregular shape of the dark regions, the hot vapor off the edges heated by nearby stars, the straight rays of dissolved gas. This nebula indicates that the interaction between dark nebula and bright, hot gas is dynamic and ongoing.
Often in the centers of dark nebula are dense concentrations of gas and dust called molecular clouds. They are called molecular because the temperatures are so low (only a few 10's K) that H2 and CO can form on the surface of dust grains. Dense gas makes for instability to gravity, and large pieces of the cloud can collapse to form protostars. Thus, molecular clouds are important as "nurseries" for young star clusters.
Molecular clouds are probed with far infrared and sub-millimeter telescopes to study the conditions that lead to star formation and the details of protostar evolution. As the young stars evolve and heat the leftover gas, the molecular cloud will be destroyed and turned into a HII region such as the Orion Nebula.
A perspective plot of the location of molecular clouds towards the center of the Galaxy is shown below. The location of the Sagittarius and Scutum spiral arms are clearly outlined.
There are many processes that go on inside a molecular cloud that can enhance or inhibit the formation of stars. For example, turbulence in the core of a cloud would compress some regions and enhance star formation. Magnetic fields would retard the collapse of a cloud and slow down the production of new stars.
Often there is found a blue haze around bright, hot stars. This haze is called a reflection nebula and is caused by scattering of blue light off a background cloud of dust. The most famous is the Pleiades, a cluster of young stars in front of a dust cloud.
Remember that only the blue component of a star's light is scattered, so the reflection will look bluer than the original star.
Near regions of star formation are often found powerful sources of microwave radiation, called OH or H2 masers. The radiation from newborn stars causes collisions with the molecules and `pumps' them into excited energy states. Some of these energy states are meta-stable, meaning that they will only transition downward when stimulated by a passing photon. The electromagnetic field of the passing photon causes the excited electron to transition downward emitting another photon of the same energy and wavelength. This is the process behind the laser.
If many molecules in the cloud are pumped to their excited states, then one photon can cause a downward transition of billions of molecules. The released radiation would have a particular wavelength signature instead of being spread out in a thermal spectrum.
Once stars are formed in the center of a molecular cloud, the stars will emit large amounts of UV radiation and heat the cloud. The gas will ionize and become an HII region, a region of ionized hydrogen (HII) surrounded by cooler, neutral hydrogen (HI).
The largest example of an HII region in the sky is the Orion Nebula.
The size of the HII region is directly proportional to the number of UV photons emitted to ionize the gas. You can increase the number of UV photons by either have large mass stars or simply more stars. The glow from an HII region is the recombination of ionized hydrogen, so the size of an HII region is simply the point in space where the rate of recombination equals the rate of photoionization from the central stars. This region is called a Stromgren sphere and is a useful method of calculating the number of hot O and B stars that are produced in the Galaxy by measuring the sizes of HII regions (i.e. the current star formation rate).
Also found in bright HII regions are new stars, and with them supernova. Supernova explosions over thousands of years will send shock waves back into the molecular cloud where the they were born from. This will cause compression in the cloud and enhance the formation of even more new stars as shown below.
The style of star formation will lead to the spiral patterns we see in galaxies, where star formation travels along the dense clouds found in the spiral arms.
Hotter and more compact than HII regions are planetary nebula, which got their names cause they look like distant fuzzy planets in old telescopes. Since planetary nebula are the ejected envelopes of AGB stars, the regions around them are empty, unlike the dense clouds around HII regions. The planetary nebula are heated solely from the interior AGB core. Thus, planetary nebula offer a unique chance to watch the interaction of radiation and low density gas.
Most planetary nebula are spherically symmetric shells with some icicle shapes on the inner edge of the outrushing shell, such as IC 3568. The green color is from emission lines of oxygen. If the original star was rotating, then a disk can form with holes along the long axis for radiation to escape, such as NGC 6826, NGC 7009 and NGC 3918. `Fliers' develop where there is a leak of UV light.
Often a reminant disk shell or old solar system will form a disk near the star. In this case, the expanding gas moves faster out the poles of the solar system to develop a dumbbell shape as in Hubble 5. The strangest case is where the core star has a hotspot not aligned with the rotation axis. This hotspot could be ejected charged particles which are falling back onto the magnetic poles of the stars. The result is a beam or jet of UV radiation which will paint helix shaped wobbles on the planetary nebula, as in NGC 5307.
The most violent, and therefore hottest, ejection of gas into the interstellar medium is from supernova explosions. Many thin, arc-like nebula are found through out the Galaxy that are remnants of expanding shells of gas moving away from dead supernova at supersonic velocities. One such supernova remnant is the Cygnus Loop shown below.
Supersonic motion is always accompanied by a shock wave that compresses the medium in front of it. This compression causes the gas to heat and glow. The most recent supernova near the Earth was in 1024 B.C. and its remnant is seen as the Crab Nebula shown below. In the center of the Crab Nebula is a fast rotating pulsar.
The shock wave from a supernova sweeps up matter in front of it and continues to heat this gas. Because supernova remnants are so hot, they emit a great deal of their energy in the x-ray region of the spectrum, shown in the x-ray picture of Cas A below.
Notice that although the shock wave starts out as a symmetric explosion, the supernova remnant develops structure and asymmetry. This is due to the fact that the density distribution of the interstellar medium is lumpy. The supernova remnant expands fastest in directions where the density is low. When pockets of dense gas are swept up, they radiate strongly and are visible as bright spots. So as a supernova remnant ages, it appears less round and regular.
As millions of years pass, the supernova remnant slows down and merges with the interstellar medium. All the heavy elements produced in the original supernova explosion are mixed into newly forming molecular clouds enhancing the number of heavy elements in future stars and solar systems.
A supernova remnant is a major source of energy for the interstellar medium. The region behind the shock wave is low in density, but very hot. Since its density is low, it cools at a very slow pace. The lumpy, foam-like nature to the interstellar medium is tracing the past history of supernovae. If the interstellar medium is dense, or the past supernova rate is small, then the interstellar medium has isolated bubbles of hot gas. If the interstellar medium is thin, or the supernova rate is high, then the interstellar medium becomes filled with connect bubbles or tunnels of hot gas.
During both the day and night there is a continual shower of high speed particles into the Earth's atmosphere called cosmic rays. Most of these cosmic rays are protons and helium nuclei that have been accelerated to relativistic velocities (close to the speed of light). When cosmic rays collide with atoms in the Earth's atmosphere all their energy is converted back into matter (E=mc2) as a shower of particles that rains down to the Earth's surface.
Cosmic ray showers are usually discovered with a cosmic ray detector, a large array of simply mirrors and phototubes to catch the Cerenkov radiation from the incoming particles. These array of detectors have shown that the cosmic rays arrive to the Earth from isotropic directions, meaning that there is no particular source to be deduced from their collisions with the Earth's atmosphere. A lack of a preferred direction implies that cosmic rays are deflected by the Galaxy's magnetic field and, thus, lose their orientation after many deflections over time.
We can make some deductions about cosmic rays based on their frequency and energies. Since their energies and speeds are so high, they easily escape from the Galaxy's gravitational field. Since there are large numbers of cosmic rays seen everyday, this implies that they must be replenished at a steady rate. The only source in the Galaxy to produce particles at these energies are supernova explosions.