Answer to Question #202920 in Astronomy | Astrophysics for Prachi

Question

What are the characteristic features of the H II region of ISM? How is it different from the molecular cloud?

Answer 1

H II regions are the regions of ionized hydrogen. They vary in size, from ultra-compact (~ 1 pc) to giant (~ 100 pc). Their size depends strongly on the intensity of emission of a young bright star, which ionizes the hydrogen and the temperature of gas becomes 10000 K; the strongest line is H-alpha. There may be up to thousand young stars in such regions.

Almost always H II regions are associated with a cold molecular gas, which also originates from the same giant molecular cloud. The molecular cloud is cold (so molecules can exist) and dense, it is very large (up to kpc) and massive, and when it suffers from a shock wave, the star formation becomes more intensive. The most massive stars reach such high temperatures, that allow their emission to ionize the surrounding gas, and H II regions are born.