Download Class 12 : Star formation I : The Interstellar Medium (ISM)

Class 12 :
Star formation
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Interstellar gas
Gravitational collapse; the Jeans Mass
Protostars and T-Tauri stars
I : The Interstellar Medium (ISM)
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Galactic disk has significant dust and gas
between stars
But the gas not uniform…
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Hot ISM (T=106-107K, n=10-4-10-2cm-3; ionized)
Warm ISM (T=6000-10000K; n=0.2-0.5cm-3; ionized)
Cold ISM (50-100K; n=20-50cm-3; neutral/dusty)
HII regions (8000K; n=102-104cm-3; ionized)
Molecular clouds (10-20K; n=102-106cm-3; molecular)
Hot and Warm ISM fill most of the volume
Cold ISM fills about 1-5% of volume
HII regions and molecular clouds are distinct
“clouds” that fill <<1% of volume
Stars are born in molecular clouds
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II : Gravitational collapse and the
Jeans Mass
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Star formation is driven by the gravitational
collapse of interstellar gas clouds
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A gas cloud of density ρ and temperature T collapses
if its mass is above a critical amount given by…
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This is called the Jeans Mass
A very large gas cloud will fragment into small
pieces, each of which is equal to the Jeans Mass
As a gas cloud collapses and increases in density,
the Jeans Mass decreases… the gas fragments into
yet smaller pieces
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III : Protostars
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Gravitational collapse leads to formation of a
protostar
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Gravitational contraction releases energy
Energy is transported to surface via convection
Energy then radiated as IR radiation
Protostar evolves into a main sequence star
T-Tauri stars…
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So we have a young star surrounded by a remnant
accretion disk
Disk (and star) sheds excess angular momentum via
formation of a jet
Disk is the site for planet formation
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