When analyzing stellar and galactic spectra with spectrographs, the spectral lines get broadened from the instrument. Why do the spectral lines get broadened after the light moves through the instrument? I understand that it is some sort of filter and you can mathematically describe it as a convolution of a delta function (approximately, neglecting pressure and rotational broadening) with a broadening function from the instrument (usually Gaussian). But I don't quite understand the physical process behind it.

The second question would be about the scales of the different types of broadening: My guess would be something like this:

natural linewidth (uncertainty principle) < pressure broadening (interactions of particles in stellar atmospheres) < rotational broadening (superposition of stellar spectra in galaxy spectra with individual orbital velocity components along the line of sight) < instrumental broadening

Would this be accurate, also some numers (size scales) would be interesting to me. Maybe somebody can help me with an approximate estimation of the different types of broadening? Rotational broadening also occurs when observing only stellar spectra, due to the proper motion of the particles in the star's atmosphere, right? It is just much smaller than the rotational broadening in galaxy spectra, due to the proper motions of the stars.

Would appreciate it if somebody could help me put this in perspective. I am analyzing the kinematics of a galaxy, so I assume that natural linewidth, pressure broadening, and rotational broadening due to the proper motions of particles in the stellar atmospheres are negligible.

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    $\begingroup$ Second question. You can't generalise |(although natural linewidth would usually be smallest).. The rest depends on the galaxy and the spectrograph. First question - start with en.wikipedia.org/wiki/Fraunhofer_diffraction $\endgroup$
    – ProfRob
    Nov 9, 2022 at 12:07
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    $\begingroup$ Thanks I vaguely remember it from optics course and I will revise it. Are the types of broadening I listed described/reasoned for correctly? $\endgroup$
    – trynerror
    Nov 9, 2022 at 12:53


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