Space Between the Stars: Properties of the Interstellar Medium

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Space Between the Stars Properties of the
Interstellar Medium

• Steven R. Spangler
• University of Iowa

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An intuitive argument for the emptiness of space
the clarity of distant stars

• Yardstick for interstellar distances the light
  year
• 1 light year 9.46E17 cm 63,235 au
• Vega 26 ly
• Altair 17 ly
• Antares 390 ly

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How empty is interstellar space? A comparison
with the Earths atmosphere
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Point of talk how we discovered the Interstellar
Medium, and its properties
Emphasis on results from radio astronomy
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A physical argument for how interstellar space
could be invisible

• If H is in the ground state, no Balmer
  transitions in absorption or emission
• For 1 occupancy of n2, need T20,000K
• Demo

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How to detect cold neutral hydrogen in
interstellar space?

• A rare example of a theoretical search followed
  by observational confirmation
• Arose from need for a spectral line in radio
  astronomy
• The 21 cm line of hydrogen

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Structure of the ground state of hydrogen
Classical physics
Quantum mechanical Angular momentum numbers
Two states of H ground state
N1,S0
F1
5.9E-06 eV
F0
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21 cm HI spectra University of Iowa SRT
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21 cm HI spectra
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The galaxy has been mapped in the 21cm line
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Properties of HI Gas in ISM

• n 1-100 cc (also higher)
• T 50-200 K (also higher)
• nT roughly constant at 3000

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Neutral hydrogen has also been mapped out in many
other galaxies
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visible
HI
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Topic 2 where do stars come from?
Mass of star 330,000 mass of Earth, mean
density 1.5 gm/cc
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Hints from a century ago star formation and
Dark Clouds
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Where is the gas from which the stars form?
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Answer gas is molecular and very cold

• Discovery was a contribution of radio astronomy
• Utilized observations of rotational transitions
  of molecules

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Rotational transitions of (diatomic) molecules
Classical rotational Kinetic energy
Quantum mechanical Square of angular momentum
Energy levels of rotator
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Energy levels of a quantum rotator
J3
J2
J1
J0
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The sky in the glow of the carbon monoxide
molecule
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Physical properties of molecular cloud gas

• T10 -70 K (and lower)
• N 1.0E05 - 1.0E06
• Numerous molecular species

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The physics of star formation the Jeans Mass
If a mass larger than the Jeans mass is
Compressed, gravity dominates gas pressure And
it continues to contract
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A Star is born.
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A Star is born (Part 2)
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Molecular Clouds as Chemistry Sets in the Sky

• Number of molecules discovered in molecular
  clouds 152
• 8 species with 10 or more atoms
• Deuterated species overrepresented

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Interstellar chemistry as plasma chemistry

• Molecular clouds are weakly-ionized plasmas
• Ion-molecule reactions do not have formation
  barriers

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The future of molecular cloud studiesALMA

• 64 antenna interferometer
• 2010 August, first science
• 2012 December, full science operations

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ALMA