Continuing the tutorial on how astronomers distinguish between various causes (or combos thereof) of redshift, to explain the redshifts they infer in the spectra of astronomical objects they observe.
I said I would next take a look at spectra, and how 'redshift' could be estimated, building partly on what Skwinty wrote about the line spectra of elements*.
However, I'm going to leave that for now, and take a look at the sky.
With our eyes - unaided by telescopes, glasses, etc - we can see, on a dark cloudless night, twinkling points of light and fuzzy patches of light (and the Moon); technically, "point sources" and "extended sources". With telescopes and better detectors than our eyes, we can see that some of the point sources can be "resolved", into lots of other point sources and/or extended sources. With various detectors, we can extend the range of wavelengths we can 'see', through telescopes here on the surface of the Earth, from the UV (~300 nm) through to the IR (~1 µ), with small windows at longer wavelengths in the IR.
Well over a century ago a simple explanation for the sources of light in the night sky which moved, relative to the others, over periods of minutes to hours to (sometimes) days was accepted - they are bodies in motion within the solar system: planets, dwarf planets, satellites/moons, comets, asteroids, TNOs, ... Two kinds of extended source are included: zodiacal light and gegenschein.
When radio 'telescopes' were turned to the sky, and when 'telescopes' tuned to other parts of the electromagnetic (EM) spectrum - from TeV gammas to x-rays to UV and IR that the atmosphere blocked - started observing above the atmosphere, point sources and extended sources were also found, and once again solar system sources (objects) could be easily distinguished from all other sources.
Of course, the sky looks quite different in other parts of EM spectrum than it does in the visual (or optical) waveband, but otherwise ...
... well, not quite. In the gamma region (and to a lesser extent the hard x-ray), the sky is dominated by flashes, sometimes intensely bright bursts that last mere milliseconds to perhaps a second or two, and it took many years before these could be shown to be sources way, way beyond the solar system (do you recall how this conclusion became firm, Skwinty?). In the radio region, some point sources are periodic flashes, of such extreme perfection that they are more accurate than even the most expensive of wrist watches (of course there are periodic variables - point sources - in the visual waveband too, but these pulsars - as they were quickly called - are qualitatively different).
The superb pattern-detection machine that is the human brain sorted all the non-solar system sources into neat boxes, based on the characteristics of the sources: stars, galaxies, planetary nebulae (PNe), GRBs, quasars, pulsars, and so on. The important thing to note about these classifications is that they are made on the basis of criteria such as whether they are point sources or not, how bright they are in one waveband vs another (e.g. 'quasar' originally meant 'quasi-stellar radio source'), the type of time-variability they exhibit, their shape and key features in their spectra (e.g. galaxies vs PNe), and so on.
Of course, humans being what they are, simply having a nice label for something - 'star', for example - is not enough; we want to what these things 'really' are!
And as physics developed - from Newton to Maxwell to Einstein to the founders of quantum mechanics to ... - once a really good answer to 'what X really is' was to hand, the purely empirical meaning of a classification came to take on aspects of the explanation, to the point where distinguishing between pure description (features of what is observed) and explanation (what we conclude it 'really is') is often blurred and sometimes difficult to do. For example 'pulsar' is BOTH {insert description, to do with radio flashes} AND a neutron star {insert qualifiers about the radio flashes and their observability}.
With me so far, Skwinty (and PS, and Z??)?
The classification of 'things we see in the sky' is critical to the answer to the question of how astronomers can distinguish one source of redshift - in the spectrum of an astronomical object (source) - from another.
And classifications, and their bases, are also key to understanding why much of what you read on crackpot websites is nonsense (or just plain wrong); for example, there is, very often, muddle-headedness and confusion over what 'a quasar' is (to take just one example), however, because few laypersons understand the bases of classifications (and, in general, the key role of consistent definitions in science), they are easily fooled by seemingly plausible strings of words.
And yeah, at times this can seem like pedantry gone mad, but it's very important that you grok this ... if only because getting your definitions clean and consistent, and using your terms consistently, etc is an important part of what doing science is all about**.
* this is what he actually wrote: "the spectra in question are compared to spectra obtained in a laboratory so that one would know the element or elements that the spectra are showing, otherwise the spectra would be meaningless. These spectra could be either emission or absorption spectra."
** for an example of just how many knots you can tie yourself into if you muddle definitions and terms up, you have to look no further than MM's posts (e.g. what is "EU/PC theory"? "General Relativity as taught by Einstein"? "negative pressure"? "Dark Energy"? ...)
) once I've finished the astronomy tutorial (that which relevant to your original question anyway); OK?