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Should the tags stellar-classification and spectral-type be merged? has been asked in meta two weeks ago. It requires some careful consideration but so far no response has been forthcoming, so I'm asking this question here to get a definitive answer that can then be applied there.

Wikipedia's Stellar classification begins

In astronomy, stellar classification is the classification of stars based on their spectral characteristics.

and while there is one subsection labeled simply Spectral types to my untrained eye it looks like there may be other classification schemes besides O, B, A, F, G, K, M, L, T.

Question: Do the astronomical terms "spectral type" and "stellar classification" refer to the same thing? Or at least always when applied to stars? Might "spectral types" also refer to a way to classify asteroids?

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    $\begingroup$ Well, the same Wikipedia article also mentions (third paragraph): “In the MK [Morgan–Keenan] system, a luminosity class is added to the spectral class using Roman numerals.” For example, the Sun is a G2 V (yellow main-sequence) star, as opposed to, say, Sadalmelik (α Aqr), which is a G2 Ib (yellow supergiant) star. So stellar classification is more than just the spectral type; it’s also the luminosity class. $\endgroup$
    – Pierre Paquette
    Commented Dec 15, 2020 at 1:48
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    $\begingroup$ "Stellar classification" can mean anything you like, even if based on spectra. You could divide things into metal-rich and metal-poor; lithium-rich and lithium-poor; hot and cold; with or without forbidden emission lines; none of which is encapsulated in the basic MK system. $\endgroup$
    – ProfRob
    Commented Feb 10 at 16:19

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"Spectral Class" System of classification, including O, B, A, F, G, K, M. Stellar classification includes the Harvard system, and also other methods of classification including the Yerkes Classification. https://en.wikipedia.org/wiki/Stellar_classification#

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There are all kinds of ways to classify stars. Betelgeuse and Barnard's star are both spectral class M, but one is a red giant while the other is a red dwarf. "Red giant" and "red dwarf" are classes of stars. Then there are Wolf-Rayet stars, a special class of stars with their own spectral classification system, different from OBAFGKM. We sometimes divide stars into two classes: population I and population II based on "metal" content. There are lots of special classes: Cepheids, T-tauri stars, white dwarfs, ...

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  • $\begingroup$ The spectral type is usually a 2-D classification. i.e. Betelgeuse is a M2I supergiant, while Barnard's star is an M4V dwarf. $\endgroup$
    – ProfRob
    Commented Feb 10 at 16:18
  • $\begingroup$ @ProfRob The terminology is fluid. I-V are often called "luminosity" classes (but it's true that they are determined from the spectrum). "Red giant" and "red dwarf" are broader classes than M2I and M4V, and other classifications don't fit the MK system at all. $\endgroup$
    – John Doty
    Commented Feb 10 at 16:48
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Spectral type is only based on the star's color. You need additional the mass or luminosity to make a stellar classification form it. Example: Giants may belong to the same spectral class as main sequence stars.

Of course, if you don't simply look at the color but measure the spectra you see the differences. Spectroscopy is one of the most important tools for astronomers.

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    $\begingroup$ Spectral type is based on a spectrum. Quoting a spectral type based on a colour is fraught with uncertainty. Neither the mass or luminosity are required to assign a spectral type. $\endgroup$
    – ProfRob
    Commented Dec 15, 2020 at 8:01

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