Interstellar Medium and Star Formation
Core concepts
Know the phases of the interstellar medium (molecular clouds, cold and warm neutral gas, ionized regions, hot corona), dust extinction and reddening, and the Jeans criterion for collapse. Understand molecular cloud structure, turbulence, magnetic support, and the stages from core collapse to protostar, disk, and outflow. Cover HII regions, the initial mass function, and feedback.
Traps
Do not confuse extinction with reddening; extinction dims, reddening changes color. Remember that the Jeans mass depends on temperature and density. Watch for confusion between free-fall time and Kelvin-Helmholtz time. Distinguish emission, reflection, and dark nebulae.
Worked example
A cloud at 10 K with number density 10^4 cm^-3. Jeans mass scales as T^(3/2) n^(-1/2). Relative to a reference cloud at 100 K and 100 cm^-3, the ratio is (10/100)^1.5 × (100/10^4)^-0.5 = 0.0316 × 10 ≈ 0.32, so this cold dense core has a lower Jeans mass and is more prone to collapse. Rule of thumb: colder and denser means easier collapse.
Rule of thumb
Dust reddening follows an extinction curve steepest in the blue and ultraviolet.