6 The Interstellar Medium ISM

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6 - The Interstellar Medium (ISM)
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density of matter 1 cm-3 in spiral arms By
number 90 H 10 He lt1 everything else
(mostly O, C, N, Ne)
Face-on? (M 51)
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Emission-Line Nebulae shine via emission lines
from the gas
Planetary Nebulae
Supernova Remnants
H II Regions (i.e. H)
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Reflection Nebulae shine by reflected starlight
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• Dark Clouds
• Giant Molecular Clouds
• Bok Globules
• Diffuse Clouds

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• DUST
• Solid Grains ? C, Si, O, ? T 1200 K
• Absorb Scatter starlight
• Polarization ? Transmission Scattering
• Thermal Emission

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Interstellar Extinction Reddening
B,I,K
B,V,I
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Dust Composition
Spectral Features at 0.2 ?m (UV) - Carbon
materials 10 20 ?m (IR) - Silicates
Grains in the laboratory More to come - STARDUST
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Schematic Representation
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Giant Molecular Clouds Mass 104-107 M? Size
20-80 pc Density 200-106 cm-3 Temperature 10
K Composition H2, CO, Dust, ..
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Taurus-Auriga Cloud
visible light (stars) CO
emission line (gas)
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H II Regions
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hot young stars ionize the gas
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recombination and de-excitation emits energy at
longer wavelengths
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Mass 102-104 M? Size 10 pc Density 102
cm-3 Temperature 8000 K Composition H, He0,
He1, O1, O2, etc.
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Intercloud Medium Warm Hot T 70 K 5x105
K n 0.3-20 cm-3 0.03 cm-3 21-cm H0 X-rays,
O5, etc.
? 21 cm
N
N
N
N
21-cm H0 emission line
X-ray sky (ROSAT)
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Planetary Nebulae
Mass 0.1-1 M? Size ? 0.1 pc Density 103-104
cm-3 Temperature 10,000-15,000 K Composition
H, He1, He2, O1, O2, etc.
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Reflection Nebulae
Pleiades
light echo of V838 Mon (more complicated)
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Molecules in the ISM
H C N O
H2 hydrogen CO carbon monoxide H2O water H2CO form
aldehyde C3H2 cyclopropenylidene
HC11N (forget it!)
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Polycyclic Aromatic Hydrocarbons (PAHs) (ISM
Auto Exhaust!)
C6H6 - benzene C24H12 -
coronene (H atoms not shown)
(no perfect fit to data, yet)
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C60, C70, etc. - Fullerenes
C60 - buckminsterfullerene buckyballs
Might be present in ISM, but evidence is not
solid