Galactic multiwavelength astronomy compares observations of the Milky Way in different parts of the electromagnetic spectrum to distinguish its overlapping physical components. The neutral-hydrogen 21-centimeter line traces neutral atomic gas, carbon monoxide spectral lines trace molecular clouds, and infrared emission reveals warm interstellar dust and stars hidden at visible wavelengths. Radio waves from synchrotron radiation trace energetic electrons in magnetic fields, while X-rays reveal hot plasma and compact sources. Maps of the same sky region can therefore test whether a feature is a stellar concentration, an obscuring cloud or shocked gas. Comparing them also requires attention to differing angular resolution and foreground absorption.[ref]
Multiwavelength Milky Way by NASA. Map descriptions: atomic hydrogen, molecular hydrogen, infrared, radio continuum and X-ray images
Interstellar grains heated by starlight emit a modified blackbody spectrum, with a frequency-dependent emissivity multiplying the Planck law. This Galactic thermal dust emission rises across much of the microwave/submillimeter band and contaminates high-frequency cosmic microwave background measurements. Its strength and effective emissivity vary across the sky, so the frequency at which it exceeds cosmological anisotropy is not a universal constant.
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.
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