A set of haikus:
Interstellar Medium
The Stuff Between Stars

outside of our world,
all around our galaxy
in between the stars

vast clouds of gas swirl
pushed and lit up by the stars
steered by magnetism

in places it’s cool
quiet diffuse hydrogen
with sparse specks of dust

some places it’s warm
and ionised by starlight
fluorescent but thin

the coldest places
are the molecular clouds
giant nebulae

in their frozen cores
compacting and compacting
new stars find their birth

nuclear fusion
sets old dust burning anew
the cycle repeats

hot, hotter it gets
turns old dust into new light

round old stars and new
hot hydrogen, ionised
beautiful halos

blown by stellar winds
the clouds form new cavities
gases pushed away

hot star-driven waves
form loops, arcs and shells of stars
daughters of giants.

The interstellar medium (hereafter ISM) was first discovered in 1904, with the observation of stationary calcium absorption lines superimposed on the Doppler shifting spectrum of a spectroscopic binary. Since the calcium lines were not changing in wavelength, they could not originate in the stellar atmospheres of the binary star, and so had to be between the telescope and the star. Since no terrestrial source was identified, the calcium had to be interstellar. Several more discoveries have been made since then: the discoveries of interstellar extinction (the dimming of starlight) in the 1930's, of polarization by interstellar dust grains in 1949, of 21-centimeter radio emission from atomic hydrogen in 1952, of the soft X-Ray background in the 1960's, and many others.

The ISM is now known to consist of several independent phases, including:

  • hot, ionized medium (HIM)
  • warm, ionized medium (WIM), also called diffuse ionized gas (DIG)
  • warm, neutral medium (WNM)
  • cold, neutral medium (CNM), and
  • dust
In addition, several other phases are observed and classified separately, including HII regions (the "II" is a Roman numeral two, indicating singly-ionized hydrogen), supernova remnants, neutral hydrogen clouds, and molecular clouds. Here's a brief description of each.

Hot, ionized medium

The HIM consists of very hot gas at temperatures higher than half a million kelvins. This gas is heated by supernovae explosions which blast large bubbles of hot gas into the surrounding interstellar medium. The gas is very thin -- less than 10-4 atoms per cubic centimeter -- but it fills nearly half the volume of a typical spiral galaxy. It is detectable in X-rays, and by the absorption lines of highly-ionized atoms (oxygen VI for example).

Warm, ionized medium

The WIM is cooler, with temperatures of a few tens of thousands of kelvins. Gas densities are higher than in the HIM, though it takes up only 20 percent of the volume. The WIM is interesting because a lot of energy is required to keep it ionized in a steady state. Part of this energy is supplied by massive, luminous stars (type O and B). It is easily detectable by observing galaxies at the wavelengths of ionized hydrogen, and takes up 20 to 40 percent of the total hydrogen Balmer line emission of a given galaxy. The WIM can also be detected by measuring the phase shift of pulsars with frequency caused by the effect of ionized plasmas on radio waves.

Warm, neutral medium

The WNM is cooler and denser still, with temperatures less than 10,000 kelvins. It takes up about 20 to 25 percent of the volume in a galaxy. Since the hydrogen gas is neutral (the ionization temperature is over 11,000 kelvins), we can detect it in the neutral hydrogen radio emission line at 21 centimeters (about 1.4 gigaHertz), and also in the absorption lines of weakly ionized metals (for example, the calcium ions mentioned in the first paragraph).

Cold, neutral medium

The CNM is made up of cooler hydrogen clouds at temperatures less than a few hundred kelvins, and molecular clouds containing a wide variety of molecules (H2, CO, water, and organic molecules) at very cold temperatures -- between 3 and 50 kelvins. The temperature can never drop below that of the cosmic microwave background at about 2.7 kelvins. This gas takes up only a small fraction of the volume in a spiral galaxy, but contains most of the mass of the gaseous ISM. This gas can be detected in radio and microwave emission. It is important to note that neutral hydrogen (both in the WNM and CNM) is an excellent absorber of of soft X-Rays with wavelengths longer than one angstrom. However, molecular hydrogen (H2) does not emit radiation (since the molecule is symmetric, it has no dipole to generate the radiation), so carbon monoxide (CO) is often used as a tracer to measure the amount of H2 in a given cloud.

Dust grains

Dust grains can be found scattered throughout the galaxy, as long as the temperature is not high enough (several thousand kelvins) to evaporate the grains. The dust acts to diminish and redden starlight, and is detectable primarily in this way. It can also emit infrared light of its own, and can be seen with microwave and long-wavelength infrared telescopes (for example the IRAS satellite). Dust is responsible for the pronounced lack of background galaxies seen along the plane of the Milky Way -- their light is absorbed by dust grains in the plane of our Galaxy, known as the zone of avoidance. Dust also makes it difficult to see into the core of our own Galaxy, and is responsible for such sights as the Horsehead Nebula and the Coal Sack. It is also responsible for the blue haze surrounding the Pleiades, which arises because dust grains preferentially scatter blue light from stars in the cluster.

Other stuff

Supernova remnants like the Crab Nebula and HII regions like the Orion Nebula are also considered a part of the ISM, and make up most of the more spectacular visible examples of it. Cosmic rays are also considered a part of the ISM and are responsible for ionizing some of the colder gas. Finally, magnetic fields also permeate the Galaxy, and provide some pressure support via the magnetic pressure, B2/(8*pi).

The ISM is different depending upon the type and environment of the galaxy you are observing. The phases mentioned here can be seen in most gas-rich spiral galaxies like our own Milky Way. Gas-poor elliptical galaxies, or spirals in galaxy clusters have already used up most of their gas to form stars, or have had it stripped away by ram pressure from the intracluster medium.

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