Interstellar matter

Abstract

Interstellar space is filled with interstellar matter (ISM) composed of various kinds of gases from low temperature (molecules, HI gas, and dust) to high temperature (ionized gas, HII regions), high-energy particles (cosmic rays), and magnetic fields. They are embedded in the radiation field of stars. The ISM makes up approximately 10% of the mass of the galactic disk, and the majority is HI and H2. The remaining 90% is shared by stars and dark matter, which determine the dynamical properties of the galaxy. The IMS is deeply coupled to activities such as the star formation near the galactic plane, and is categorized as younger-generation matter in the galaxy, called Population I.