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. 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 - PowerPoint PPT PresentationTRANSCRIPT
Space Between the Stars: Properties of the Interstellar Medium
Steven R. Spangler
University of Iowa
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
How empty is interstellar space? A comparison with the Earth’s atmosphere
Point of talk: how we discovered the Interstellar Medium, and its properties
Emphasis on results from radio astronomy
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
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
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
21 cm HI spectra… University of Iowa SRT
21 cm HI spectra
The galaxy has been mapped in the 21cm
line
Properties of HI Gas in ISM
• n: 1-100 cc (also higher)
• T: 50-200 K (also higher)
• nT roughly constant at ~3000
Neutral hydrogen has also been mapped out in many other galaxies
visible
HI
Topic 2: where do stars come from?
Mass of star = 330,000 mass of Earth, mean density = 1.5 gm/cc
Hints from a century ago: star formation and Dark Clouds
Where is the gas from which the stars form?
Answer: gas is molecular and very cold
• Discovery was a contribution of radio astronomy
• Utilized observations of rotational transitions of molecules
Rotational transitions of (diatomic) molecules
Classical rotational Kinetic energy
Quantum mechanical Square of angular momentum
Energy levels of rotator
Energy levels of a quantum rotator
J=0
J=1
J=2
J=3
The sky in the glow of the carbon monoxide molecule
Physical properties of molecular cloud gas
• T=10 -70 K (and lower)
• N= 1.0E+05 - 1.0E+06
• Numerous molecular species
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
A Star is born….
A Star is born (Part 2) …
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
Interstellar chemistry as plasma chemistry
• Molecular clouds are weakly-ionized plasmas
• Ion-molecule reactions do not have formation barriers
The future of molecular cloud studies…ALMA
• 64 antenna interferometer• 2010 August, “first science”• 2012 December, “full science operations”
ALMA