4
Collapse
of
Molecular Clouds
Molecular Clouds
•Barely
stable against collapse:
•Imagine
slightly compressing cloud
–Gravity
goes up because material is packed more tightly (R in 1/R2 is smaller)
•Tends
to make cloud want to collapse
–Pressure
goes up because material is packed more tightly (P µ rT) and r higher
•Tends
to make cloud want to expand
–For
smaller clouds Pressure wins (stable)
–For
larger clouds Gravity wins (collapse)
•As it
collapses and becomes denser, smaller and smaller parts become unstable
•Shock wave can trigger collapse
From our text: Horizons, by Sees