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.
Galactic multiwavelength astronomy combines spectral bands that respond differently to stars, gas, dust, magnetic fields, temperature, and energetic particles.
Galactic synchrotron emission is nonthermal radio radiation from relativistic electrons spiralling in the Galactic magnetic field. Its broad latitude distribution reflects cosmic-ray transport and the magnetic halo.
Diffuse Galactic gamma rays arise mainly from neutral-pion decay after cosmic-ray nuclear collisions, electron bremsstrahlung in gas, and inverse-Compton scattering from radiation fields.
The Galactic scale height measures the vertical thickness of a tracer. Gravity, velocity dispersion, thermal pressure, magnetic and cosmic-ray support, particle transport, and source lifetime determine its value.
The Galactic bar is the elongated central stellar structure of the Milky Way. Extinction-corrected near-infrared standard candles and gas position--velocity maps constrain its distance, orientation, and three-dimensional shape.
