AST 309 Lecture Notes - Lecture 6: Orion Molecular Cloud Complex, Molecular Cloud, Solar Mass
Document Summary
Regions where intense uv radiation from massive hot o and b stars is able to ionize even the denser parts of the interstellar clouds. Also called h ii regions = regions made of ionized hydrogen. The glowing clouds seen as h ii regions are usually the clouds from which these massive stars have been born from. H ii regions are referred to as signposts where active star formation is occurring. Hydrogen is the most abundant element in the universe and makes up majority of material in clouds. When hydrogen is in the molecular state, it is so cold that it cant transition up to a higher energy state, and thus wont radiate when it transitions back down. So hydrogen is stuck in the ground state at these temperatures. It can be observed directly, not just through its obscuring effect on light. Because molecular clouds are very cloud, the dust grains are also very cold.
