Space Between the Stars: Properties of the Interstellar Medium
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Transcript of 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 = 9.46E+17 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 Earth’s 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 n=2, need T=20,000K
• 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
N=1,S=0 F=1
F=0
5.9E-06 eV
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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
J=0
J=1
J=2
J=3
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The sky in the glow of the carbon monoxide molecule
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Physical properties of molecular cloud gas
• T=10 -70 K (and lower)
• N= 1.0E+05 - 1.0E+06
• Numerous molecular species
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The physics of star formation: the Jean’s Mass
If a mass larger than the Jean’s mass is Compressed, gravity dominates gas pressureAnd 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 studies…ALMA
• 64 antenna interferometer• 2010 August, “first science”• 2012 December, “full science operations”
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ALMA