Do free protons and neutrons absorb much radiation? To affect astronomers' observations? If so, at what wavelength(s)?
Why is the H-alpha line slightly shorter in wavelength (656.28 nm) in air than in vacuum (656.46 nm)? Shouldn't it be longer?
How is the H II 'region' directly detectable? By Compton or Thomson free-particle scattering? At what wavelengths?
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