Space Between the Stars: Properties of the Interstellar Medium Steven R. Spangler University of Iowa
Dec 27, 2015
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
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
Properties of HI Gas in ISM
• n: 1-100 cc (also higher)
• T: 50-200 K (also higher)
• nT roughly constant at ~3000
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
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
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”