Press Release

Source: Chaisson & McMillan; Chapter 18

The Milky Way galaxy has a total mass > 100 billion M_sun. The material in-between the stars (the Interstellar Medium) contains about 5 billion M_sun ---> the gas and dust are about 5 % of mass of our Galaxy. Gas and dust are, however, quite important as they are the material out of which stars form.
MAKE-UP OF THR ISM
• The ISM material is primarily gas:
  • it is roughly 90 % hydrogen and 10 % helium, with just a touch of everything else.

• Dust is very rare in the ISM.
  • Dust particles are complexes of several billion atoms; they are usually composed of carbon (graphite) or silicates. This further compounds the fact that dust is only a small contribution to the mass of the ISM (< 1 %) in that it makes the dust an even smaller contribution to the number of particles in the ISM.

  • Dust particles are roughly 10^-5 cm or a few thousand Angstroms in size--that is, dust particles are a little smaller than, but on the order of the wavelength of visible light. Dust easily blocks light whose wavelengths are shorter than their size (optical and UV) but is relatively transparent to light with longer wavelengths (IR and microwave).

• On average, there are 10⁶ hydrogens per cubic meter (1 hydrogen per cubic centimeter) in the ISM. (The air in this room has roughly 10¹⁸-10¹⁹ molecules per cubic centimeter -- a sugar cube is roughly a cubic centimeter.) There is fewer than a single dust particle per cubic football field in the ISM!!! Dust is exceedingly rare.

• Despite its scarcity, dust plays a crucial role in star formation (it cools collapsing clouds as they forms--more later on this point) and dust is the primary agent that attenuates visible light as we peer in our Galaxy. Dust is the reason we cannot see into the interiors of the ISM clouds (in visible light) and the reason for the dark horsehead we saw in the Horsehead Nebula, and the dark lanes in the Milky Way galaxy. Because of this, we must study the star formation process in other parts of the EM spectrum. The most fruitful observations have been made in the infrared (IR) and radio.

Why is dust such an efficient blocker of starlight (compared to the vastly more abundant gas-- recall)?

The gas and dust are not spread uniformly throughout the Milky Way. They are mostly confined to the disk of the Galaxy. Furthermore, they are not spread uniformly throughout the disk. The ISM is lumpy. We have:

• Coronal gas -- dilute gas at temperatures of around 10⁶ K, which fills roughly 90 % of the volume of the disk. The coronal gas has very low density -- around 0.1 % to 1 % of the average ISM density -- and so does not make a significant contribution to the gas and dust mass of our Galaxy (~ 0.1 %).

• Neutral hydrogen Clouds (H I regions). The H I clouds have sizes of ~15 light years, masses of ~ 100 M_sun, T ~ 50 - 100 K, and densities of tens of millions of particles per cubic meter. They make up roughly 40 % of the mass in the gas and dust.

• Giant Molecular Clouds (GMC) -- It is well-established that GMCs are the clouds in which star formation occurs. (Giant Molecular Clouds: Orion, Orion nebula, Orion mosaic, Video 1, Video 2.)
  GMC's are actually complexes of smaller clumps of material:
  • small dense clumps of material composed mainly of hydrogen molecules (H₂) with:
    • T ~ 50 K
    • densities of 100 million to biilions of particles per cubic meter
    • sizes of 10's of light years
    • masses of 10⁴ to 10⁷ M_sun -- 10⁵M_Sun is typical
  • Ionized Hydrogen Clouds (HII regions). (Eagle nebula, Eagle nebula HST). Sites of 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 alpha lines (which fall in the red portion of the spectrum).

  • Reflection Nebulae (Pleiades, NGC1999). Reflection form around hot stars, e.g., see the Trifid Nebula. 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. The explanation for the blue color of reflection nebulae is similar to the reason for why the sky is blue.

  • 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.