AST 112 Study Guide - Midterm Guide: Molecular Cloud, Hydrogen Atom, Orbital Period
Document Summary
Interstellar medium is gas and dust, primarily atomic hydrogen gas (90%), helium (10%) other elements (~1%) 99% of the mass of interstellar medium is gas, remainder is dust: cold clouds of atomic/molecular hydrogen gas, hot ionized hydrogen near young hot stars. Very low density; 1 hydrogen atom per cubic centimeter. Molecular clouds: regions where hydrogen gas is denser (10^4 molecules per cubic centimeter), also 10% he, ~1% h2o, co carbon monoxide, dust etc. Most of the volume is warm atomic gas. Most of the mass is in the cold molecular clouds, in regions not occupied by atomic gas. Visible light is blocked by the dust/density of molecular clouds. Dust has two effects on starlight: extinction: completely blocks shorter-wavelength light, reddening: transmitted light has longer wavelengths so more gets through, but it appears redder than it already is. Infrared light penetrates the thick molecular clouds where visible light fails.
