Structure of the ISM
•HI clouds:
•Hot intercloud medium:
The ISM
occurs in two main types of clouds:
Cold (T ~ 100 K)
clouds of neutral hydrogen (HI);
moderate density (n ~
10 – a few hundred atoms/cm3);
size: ~ 100 pc
Hot (T
~ a few 1000 K), ionized hydrogen (HII);
low
density (n ~ 0.1 atom/cm3);
gas can
remain ionized because of very low density.
