S8 - Observed properties of molecular clouds

Question 1

What is the typical shape of molecular clouds?

Answer 1

Filamentary, not spherical.

Question 2

What assumption causes the combination of Jeans mass and free-fall time to give inaccurate star formation rates?

Answer 2

The assumption that thermal pressure is the only mechanism acting against gravity to resist collapse.

Question 3

What are cloud lifetimes estimated to be?

Answer 3

≥10^7 years (10 million years)

Question 4

What other sources than thermal pressure provide structural support against gravitational collapse?

Answer 4

(1) Rotation due to galactic rotation or cloud-cloud collisions (although relatively small contribution).
(2) Magnetohydrodynamic waves called Alfven waves created by perturbations in a magnetised cloud (if magnetic field strength is sufficiently high).
(3) Turbulence.