The Interstellar Medium

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The Interstellar Medium and Star Formation
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interior to suns orbit.
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Reminder.
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suns vicinity
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500 pc view of the local ISM
http/spacsun.rice.edu/twg
Tim Glover The Guide to the
Galaxy
Sun
Orion
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On a smaller scale, the sun and local fluff
are near the center of a bubble of relatively
low density.
red, yellow high density
sun
Locally the density is about 0.5 cm-3, but the
bubble has a density more like 0.01 cm-3 and a
temperature 105 106 K. The sun is moving to
the left in this diagram (towards Vega) at about
20 km s-1.
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The sun is in a local region of somewhat higher
density called the local fluff. The density
there is 0.1 atom cm-3. The sun is moving
towards Vega.
Blue OB Yellow AFG Red KM
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Mass 35 40 20 lt5
H I H2 H I H II
. basically hydrogen in various forms
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very rough a large range is observed
also studied in 21 cm
(sometimes up to 1000 cm-3)
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.01 0.1 cm-3
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Mass 35 40 20 lt5
H I H2 H I H II
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Coronal Gas

• Small fraction of the ISM mass
• Large fraction of the volume (50)
• Emission lines in the ultraviolet, e.g., O VI
• Found in vicinity of supernova remnants and
  above disk. Heated by supernova shocks? Cosmic
  rays?

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H II Regions

• About 700 in our Galaxy
• Ionized by intense uv-flux from stars within,
  especially O stars
• Detected in radio from high n transitions in
  hydrogen. Also optical emission in Balmer and
  Lyman series. Oxygen forbidden lines make them
  appear greenish.T 104 K
• Most abundant between 4 to 8 kpc and in spiral
  arms of the Milky Way. Trace regions of
  recent star formation.
• Brightest is less than a million years old.
• Often are embedded in have molecular clouds
• Colorful, but not a major part of the Galaxys
  mass or volume

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The Great Nebula in Orion. An illuminated portion
of a nearby (1300 ly) giant molecular cloud. The
field of view here is 32 arc min. Each arc min at
this distance is about 0.4 ly.
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The Great Nebula in Orion. A larger field of
view. 46 arc min. The inner regions are glowing
in the lines of excited hydrogen which together
with some green from oxygen emission give the
inner nebula a yellowish color. The entire nebula
is 29 x 26 light years.
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The Trapezium cluster of stars illuminates the
Orion Nebula. These are the brightest members of
a substantial cluster many of whose members
lie hidden behind gas and dust. 8 arc min wide
field of view. Distance 1300 light years.
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Lagoon Nebula in Sagitarius 5000 ly away
spans 90 x 40 arc min and 130 by 60 light years.
Another H II region on the boundary of a
molecular cloud (like Orion)
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Molecular Hydrogen

• Traced by the radio emission of CO which is found
  in the same (cold, dense) conditions with a
  near constant proportionality to molecular
  hydrogen. H2 itself is not observable in the
  radio.
• Mostly concentrated in a ring around the center
  of the Galaxy
• interior to the suns orbit at 4 to 6 kpc
• Mostly clumped into clouds ranging in mass from
  several solar masses to 106 solar masses and
  sizes from a few ly to 600 ly
• Molecular clouds contain dust that shields the
  molecules from destruction by starlight.
• Some evidence that about 90 of the molecular
  hydrogen is contained in 5000 giant cloud
  complexes with typical masses 105 that of the
  sun and sizes gt65 ly.

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123 different molecules have now been detected in
space. Most are organic. HC11N is the heaviest.
http//www.cv.nrao.edu/awootten/allmols.html
http//dsnra.jpl.nasa.gov/IMS/
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These infrared line features are
characteristic of ices and silicates.
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0.2 m
1 m 10-6 m 10-4cm
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Star Formation Scenario - varies
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