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) --the gas and dust--have total mass about 10-15 % of the visible mass of the Milky Way Galaxy. Despite this, the gas and dust are quite important as they are the material out of which stars form.

Make-Up of the ISM

• The ISM material is primarily gas:
  • it is roughly 90 % hydrogen and 10 % helium, with just a touch of everything else.

• The dust is very rare. Dust particles are complexes of several billion atoms; they are usually composed of carbon (graphite) and silicates. This means that dust makes a very small contribution to the mass of the ISM (< 1 %) and that it makes an even smaller contribution to the number of particles in the ISM.
  • 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.) There is fewer than a single dust particle per cubic football field in the ISM!!! Dust is exceedingly rare.

  • Despite its rarity, dust is the primary agent which attenuates visible light. Dust is the reason we cannot see into the interiors of the ISM clouds (in visible light). 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. (Further, dust is also the reason we cannot see more than a few thousand or so light years of the Milky Way galaxy in visible light [the visible disk of the Milky Way galaxy has a diameter of around 100,000 light years] and why there are dark patchs in glowing gas clouds ( horsehead nebula, N81 in LMC.)

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

Structure of the ISM

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.)
  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). 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.