中性氢

• the neutral hydrogen (H i) in the interstellar medium (ISM) is in two phases - a warm and a cold phase.

• The warm neutral medium (WNM) has typical temperatures of ∼ 10000 K
• he cold neutral medium (CNM) has typical temperatures < 300 K (20-50%)
• The WNM is mostly detected in emission and the CNM in absorption.
• H i self-absorption (HISA) clouds are unique places where we can study the structure and the physical properties of the CNM at the same time.(mixed with H2分子氢