Interstellar Medium (ISM)

The Milky Way galaxy has a mass > 100 billion M(sun). Out of this total mass, the material in-between the stars (the Interstellar Medium) is about 5 billion M(sun) making the gas and dust contribution to the Milky Way galaxy about 5 % or so. This gas and dust is quite important as this is the material out of which stars are formed.

• The ISM material is primarily gas -- the ISM is roughly 90 % hydrogen and 10 % helium with just a touch of everything else. The dust particles are agglomerations of several billions of atoms thought to be composed of things like carbon (graphite) and silicates. Note that this means that the dust makes a very small contribution to the mass of the ISM (< 1 %) and that it makes an even smaller contribution to the number density of the ISM.
• On average, there is 1 gas particle per cubic centimeter in the ISM. There is fewer than 1 dust particle per cubic football field in the ISM!!! Dust is very rare.
• However, despite its low abundance, dust is the primary agent which causes the extinction of the light from distant stars. It is the reason that we cannot see more than a thousasnd or so parsec of the Milky Way galaxy in visible light. This is a result of the fact that dust is able to absorb a broad range of radiation (i.e., radiation over a broad range of wavelengths) while gas is only able to absorb radiation of very specific wavelengths!

The gas and dust is not spread uniformly throughout the Milky Way galaxy. It is confined mainly to the disk of the Galaxy, and furthermore is spread uniformly throughout the disk.

• There is a very dilute gas at a temperature of around 1,000,000 Kelvins which fills roughly 90 % of the volume of the disk. The gas has a very low density however -- 0.0001 to 0.001 particles per cubic centimeter -- and so does not make a significant contribution to the gas and dust mass (~ 0.1 %).

• There are large clouds of neutral hydrogen (H I regions). They have sizes of 5 parsecs, masses of ~ 100 M(sun), temperatures of 50 - 100 Kelvins, and densities of tens of particles per cubic centimeter. They comprise roughly 40 % of the mass of the gas and dust.

• There are large clouds known as Giant Molecular Clouds (GMC) (e.g., the Orion complex). It is now well-established that these objects are the sites of star formation.
  • mainly hydrogen molecules (H2)
  • temperatures of 50 Kelvins and densities of 100 to 1,000 particles per cubic centimeter
  • sizes of 10's of parsecs
  • Masses of 10,000 to 10,000,000 M(sun) -- 100,000 M(sun) is typical
  These are actually cloud complexes being composed of many smaller denser clumps of material:

• Ionized Hydrogen Clouds (HII regions). Sites of young massive star formation. Associated with spiral arms and GMC.'s. These are red, glowing pictures that you see in the text. They glow red because they radiate large amounts of hydrogen balmer beta lines (which falls in the red portion of the spectrum).
• Reflection Nebulae. One sees the reflection of blue light by the dust particles in the cloud, i.e., the light from a hot star is reflected by the dust in the cloud.
• Dark clouds are regions of high dust concentration. The dust very effectively blocks out the stars which sit behind them making what appears to be a hole in the sky.