The interstellar medium is the gas, dust, magnetic field, and energetic-particle population between stars in a galaxy.
A molecular cloud is a cold dense region of the interstellar medium in which hydrogen is predominantly molecular and stars can form.
Rotational carbon-monoxide line emission traces cold molecular clouds because abundant molecular hydrogen is difficult to observe directly. A conversion factor relates integrated CO intensity to molecular-hydrogen column density.
Interstellar dust absorbs and scatters short-wavelength starlight and reradiates the absorbed energy thermally in the infrared.
The neutral-hydrogen 21-centimeter line is the ground-state hyperfine transition of atomic hydrogen. Its brightness and Doppler shift map atomic-gas column density and line-of-sight velocity.
A cosmic ray is a high-energy charged particle propagating through space. Galactic cosmic-ray electrons emit synchrotron and inverse-Compton radiation, while cosmic-ray nuclei produce gamma rays through collisions with gas.
